diff --git a/polygerrit-ui/app/elements/change/gr-change-view/gr-change-view.js b/polygerrit-ui/app/elements/change/gr-change-view/gr-change-view.js
index e5b3daf..20fe1a6 100644
--- a/polygerrit-ui/app/elements/change/gr-change-view/gr-change-view.js
+++ b/polygerrit-ui/app/elements/change/gr-change-view/gr-change-view.js
@@ -37,7 +37,6 @@
 import '../gr-commit-info/gr-commit-info.js';
 import '../gr-download-dialog/gr-download-dialog.js';
 import '../gr-file-list-header/gr-file-list-header.js';
-import '../gr-file-list/gr-file-list.js';
 import '../gr-included-in-dialog/gr-included-in-dialog.js';
 import '../gr-messages-list/gr-messages-list.js';
 import '../gr-related-changes-list/gr-related-changes-list.js';
@@ -73,6 +72,7 @@
 } from '../../../utils/patch-set-util.js';
 import {changeStatuses, changeStatusString} from '../../../utils/change-util.js';
 import {EventType} from '../../plugins/gr-plugin-types.js';
+import {DEFAULT_NUM_FILES_SHOWN} from '../gr-file-list/gr-file-list.js';
 
 const CHANGE_ID_ERROR = {
   MISMATCH: 'mismatch',
@@ -82,7 +82,6 @@
   /^(Change-Id\:\s|Link:.*\/id\/)(I[0-9a-f]{8,40})/gm;
 
 const MIN_LINES_FOR_COMMIT_COLLAPSE = 30;
-const DEFAULT_NUM_FILES_SHOWN = 200;
 
 const REVIEWERS_REGEX = /^(R|CC)=/gm;
 const MIN_CHECK_INTERVAL_SECS = 0;
diff --git a/polygerrit-ui/app/elements/change/gr-file-list/gr-file-list.ts b/polygerrit-ui/app/elements/change/gr-file-list/gr-file-list.ts
index bf45eb6..3a2c858 100644
--- a/polygerrit-ui/app/elements/change/gr-file-list/gr-file-list.ts
+++ b/polygerrit-ui/app/elements/change/gr-file-list/gr-file-list.ts
@@ -14,41 +14,71 @@
  * See the License for the specific language governing permissions and
  * limitations under the License.
  */
-import '../../../styles/shared-styles.js';
-import '../../diff/gr-diff-cursor/gr-diff-cursor.js';
-import '../../diff/gr-diff-host/gr-diff-host.js';
-import '../../diff/gr-diff-preferences-dialog/gr-diff-preferences-dialog.js';
-import '../../edit/gr-edit-file-controls/gr-edit-file-controls.js';
-import '../../shared/gr-button/gr-button.js';
-import '../../shared/gr-cursor-manager/gr-cursor-manager.js';
-import '../../shared/gr-icons/gr-icons.js';
-import '../../shared/gr-linked-text/gr-linked-text.js';
-import '../../shared/gr-rest-api-interface/gr-rest-api-interface.js';
-import '../../shared/gr-select/gr-select.js';
-import '../../shared/gr-tooltip-content/gr-tooltip-content.js';
-import '../../shared/gr-copy-clipboard/gr-copy-clipboard.js';
-import {flush} from '@polymer/polymer/lib/legacy/polymer.dom.js';
-import {GestureEventListeners} from '@polymer/polymer/lib/mixins/gesture-event-listeners.js';
-import {LegacyElementMixin} from '@polymer/polymer/lib/legacy/legacy-element-mixin.js';
-import {PolymerElement} from '@polymer/polymer/polymer-element.js';
-import {htmlTemplate} from './gr-file-list_html.js';
-import {asyncForeach} from '../../../utils/async-util.js';
-import {KeyboardShortcutMixin, Shortcut} from '../../../mixins/keyboard-shortcut-mixin/keyboard-shortcut-mixin.js';
-import {FilesExpandedState} from '../gr-file-list-constants.js';
-import {GrCountStringFormatter} from '../../shared/gr-count-string-formatter/gr-count-string-formatter.js';
-import {GerritNav} from '../../core/gr-navigation/gr-navigation.js';
-import {getPluginEndpoints} from '../../shared/gr-js-api-interface/gr-plugin-endpoints.js';
-import {getPluginLoader} from '../../shared/gr-js-api-interface/gr-plugin-loader.js';
-import {appContext} from '../../../services/app-context.js';
-import {SpecialFilePath} from '../../../constants/constants.js';
-import {descendedFromClass} from '../../../utils/dom-util.js';
+import '../../../styles/shared-styles';
+import '../../diff/gr-diff-cursor/gr-diff-cursor';
+import '../../diff/gr-diff-host/gr-diff-host';
+import '../../diff/gr-diff-preferences-dialog/gr-diff-preferences-dialog';
+import '../../edit/gr-edit-file-controls/gr-edit-file-controls';
+import '../../shared/gr-button/gr-button';
+import '../../shared/gr-cursor-manager/gr-cursor-manager';
+import '../../shared/gr-icons/gr-icons';
+import '../../shared/gr-linked-text/gr-linked-text';
+import '../../shared/gr-rest-api-interface/gr-rest-api-interface';
+import '../../shared/gr-select/gr-select';
+import '../../shared/gr-tooltip-content/gr-tooltip-content';
+import '../../shared/gr-copy-clipboard/gr-copy-clipboard';
+import {flush} from '@polymer/polymer/lib/legacy/polymer.dom';
+import {GestureEventListeners} from '@polymer/polymer/lib/mixins/gesture-event-listeners';
+import {LegacyElementMixin} from '@polymer/polymer/lib/legacy/legacy-element-mixin';
+import {PolymerElement} from '@polymer/polymer/polymer-element';
+import {htmlTemplate} from './gr-file-list_html';
+import {asyncForeach} from '../../../utils/async-util';
+import {
+  CustomKeyboardEvent,
+  KeyboardShortcutMixin,
+  Modifier,
+  Shortcut,
+} from '../../../mixins/keyboard-shortcut-mixin/keyboard-shortcut-mixin';
+import {FilesExpandedState} from '../gr-file-list-constants';
+import {GrCountStringFormatter} from '../../shared/gr-count-string-formatter/gr-count-string-formatter';
+import {GerritNav} from '../../core/gr-navigation/gr-navigation';
+import {getPluginEndpoints} from '../../shared/gr-js-api-interface/gr-plugin-endpoints';
+import {getPluginLoader} from '../../shared/gr-js-api-interface/gr-plugin-loader';
+import {appContext} from '../../../services/app-context';
+import {DiffViewMode, SpecialFilePath} from '../../../constants/constants';
+import {descendedFromClass} from '../../../utils/dom-util';
 import {
   addUnmodifiedFiles,
   computeDisplayPath,
   computeTruncatedPath,
   isMagicPath,
   specialFilePathCompare,
-} from '../../../utils/path-list-util.js';
+} from '../../../utils/path-list-util';
+import {customElement, observe, property} from '@polymer/decorators';
+import {RestApiService} from '../../../services/services/gr-rest-api/gr-rest-api';
+import {
+  ConfigInfo,
+  DiffPreferencesInfo,
+  ElementPropertyDeepChange,
+  FileInfo,
+  FileNameToFileInfoMap,
+  NumericChangeId,
+  PatchRange,
+  PreferencesInfo,
+  RevisionInfo,
+  UrlEncodedCommentId,
+} from '../../../types/common';
+import {GrDiffHost} from '../../diff/gr-diff-host/gr-diff-host';
+import {GrDiffPreferencesDialog} from '../../diff/gr-diff-preferences-dialog/gr-diff-preferences-dialog';
+import {hasOwnProperty} from '../../../utils/common-util';
+import {GrDiffCursor} from '../../diff/gr-diff-cursor/gr-diff-cursor';
+import {GrCursorManager} from '../../shared/gr-cursor-manager/gr-cursor-manager';
+import {PolymerSpliceChange} from '@polymer/polymer/interfaces';
+import {ChangeComments} from '../../diff/gr-comment-api/gr-comment-api';
+import {PatchSetFile, UIDraft} from '../../../utils/comment-util';
+import {ParsedChangeInfo} from '../../shared/gr-rest-api-interface/gr-reviewer-updates-parser';
+
+export const DEFAULT_NUM_FILES_SHOWN = 200;
 
 const WARN_SHOW_ALL_THRESHOLD = 1000;
 const LOADING_DEBOUNCE_INTERVAL = 100;
@@ -74,6 +104,69 @@
 
 const FILE_ROW_CLASS = 'file-row';
 
+export interface GrFileList {
+  $: {
+    restAPI: RestApiService & Element;
+    diffPreferencesDialog: GrDiffPreferencesDialog;
+    diffCursor: GrDiffCursor;
+    fileCursor: GrCursorManager;
+  };
+}
+
+interface ReviewedFileInfo extends FileInfo {
+  isReviewed?: boolean;
+}
+interface NormalizedFileInfo extends ReviewedFileInfo {
+  __path: string;
+}
+
+interface PatchChange {
+  inserted: number;
+  deleted: number;
+  size_delta_inserted: number;
+  size_delta_deleted: number;
+  total_size: number;
+}
+
+function createDefaultPatchChange(): PatchChange {
+  // Use function instead of const to prevent unexpected changes in the default
+  // values.
+  return {
+    inserted: 0,
+    deleted: 0,
+    size_delta_inserted: 0,
+    size_delta_deleted: 0,
+    total_size: 0,
+  };
+}
+
+interface SizeBarLayout {
+  maxInserted: number;
+  maxDeleted: number;
+  maxAdditionWidth: number;
+  maxDeletionWidth: number;
+  deletionOffset: number;
+}
+
+function createDefaultSizeBarLayout(): SizeBarLayout {
+  // Use function instead of const to prevent unexpected changes in the default
+  // values.
+  return {
+    maxInserted: 0,
+    maxDeleted: 0,
+    maxAdditionWidth: 0,
+    maxDeletionWidth: 0,
+    deletionOffset: 0,
+  };
+}
+
+interface FileRow {
+  file: PatchSetFile;
+  element: HTMLElement;
+}
+
+export type FileNameToReviewedFileInfoMap = {[name: string]: ReviewedFileInfo};
+
 /**
  * Type for FileInfo
  *
@@ -89,175 +182,156 @@
  * @property {number} lines_inserted - fallback to 0 if not present in api
  */
 
-/**
- * @extends PolymerElement
- */
-class GrFileList extends KeyboardShortcutMixin(
-    GestureEventListeners(
-        LegacyElementMixin(PolymerElement))) {
-  static get template() { return htmlTemplate; }
+@customElement('gr-file-list')
+export class GrFileList extends KeyboardShortcutMixin(
+  GestureEventListeners(LegacyElementMixin(PolymerElement))
+) {
+  static get template() {
+    return htmlTemplate;
+  }
 
-  static get is() { return 'gr-file-list'; }
   /**
    * Fired when a draft refresh should get triggered
    *
    * @event reload-drafts
    */
 
-  static get properties() {
-    return {
-    /** @type {?} */
-      patchRange: Object,
-      patchNum: String,
-      changeNum: String,
-      /** @type {?} */
-      changeComments: Object,
-      drafts: Object,
-      revisions: Array,
-      projectConfig: Object,
-      selectedIndex: {
-        type: Number,
-        notify: true,
-      },
-      keyEventTarget: {
-        type: Object,
-        value() { return document.body; },
-      },
-      /** @type {?} */
-      change: Object,
-      diffViewMode: {
-        type: String,
-        notify: true,
-        observer: '_updateDiffPreferences',
-      },
-      editMode: {
-        type: Boolean,
-        observer: '_editModeChanged',
-      },
-      filesExpanded: {
-        type: String,
-        value: FilesExpandedState.NONE,
-        notify: true,
-      },
-      _filesByPath: Object,
+  @property({type: Object})
+  patchRange?: PatchRange;
 
-      /** @type {!Array<FileInfo>} */
-      _files: {
-        type: Array,
-        observer: '_filesChanged',
-        value() { return []; },
-      },
-      _loggedIn: {
-        type: Boolean,
-        value: false,
-      },
-      _reviewed: {
-        type: Array,
-        value() { return []; },
-      },
-      diffPrefs: {
-        type: Object,
-        notify: true,
-        observer: '_updateDiffPreferences',
-      },
-      /** @type {?} */
-      _userPrefs: Object,
-      _showInlineDiffs: Boolean,
-      numFilesShown: {
-        type: Number,
-        notify: true,
-      },
-      /** @type {?} */
-      _patchChange: {
-        type: Object,
-        computed: '_calculatePatchChange(_files)',
-      },
-      fileListIncrement: Number,
-      _hideChangeTotals: {
-        type: Boolean,
-        computed: '_shouldHideChangeTotals(_patchChange)',
-      },
-      _hideBinaryChangeTotals: {
-        type: Boolean,
-        computed: '_shouldHideBinaryChangeTotals(_patchChange)',
-      },
+  @property({type: String})
+  patchNum?: string;
 
-      _shownFiles: {
-        type: Array,
-        computed: '_computeFilesShown(numFilesShown, _files)',
-      },
+  @property({type: Number})
+  changeNum?: NumericChangeId;
 
-      /**
-       * The amount of files added to the shown files list the last time it was
-       * updated. This is used for reporting the average render time.
-       */
-      _reportinShownFilesIncrement: Number,
+  @property({type: Object})
+  changeComments?: ChangeComments;
 
-      /** @type {!Array<Gerrit.FileRange>} */
-      _expandedFiles: {
-        type: Array,
-        value() { return []; },
-      },
-      _displayLine: Boolean,
-      _loading: {
-        type: Boolean,
-        observer: '_loadingChanged',
-      },
-      /** @type {Gerrit.LayoutStats|undefined} */
-      _sizeBarLayout: {
-        type: Object,
-        computed: '_computeSizeBarLayout(_shownFiles.*)',
-      },
+  @property({type: Object})
+  drafts?: {[path: string]: UIDraft[]};
 
-      _showSizeBars: {
-        type: Boolean,
-        value: true,
-        computed: '_computeShowSizeBars(_userPrefs)',
-      },
+  @property({type: Array})
+  revisions?: {[revisionId: string]: RevisionInfo};
 
-      /** @type {Function} */
-      _cancelForEachDiff: Function,
+  @property({type: Object})
+  projectConfig?: ConfigInfo;
 
-      _showDynamicColumns: {
-        type: Boolean,
-        computed: '_computeShowDynamicColumns(_dynamicHeaderEndpoints, ' +
-                '_dynamicContentEndpoints, _dynamicSummaryEndpoints)',
-      },
-      _showPrependedDynamicColumns: {
-        type: Boolean,
-        computed: '_computeShowPrependedDynamicColumns(' +
-        '_dynamicPrependedHeaderEndpoints, _dynamicPrependedContentEndpoints)',
-      },
-      /** @type {Array<string>} */
-      _dynamicHeaderEndpoints: {
-        type: Array,
-      },
-      /** @type {Array<string>} */
-      _dynamicContentEndpoints: {
-        type: Array,
-      },
-      /** @type {Array<string>} */
-      _dynamicSummaryEndpoints: {
-        type: Array,
-      },
-      /** @type {Array<string>} */
-      _dynamicPrependedHeaderEndpoints: {
-        type: Array,
-      },
-      /** @type {Array<string>} */
-      _dynamicPrependedContentEndpoints: {
-        type: Array,
-      },
-    };
-  }
+  @property({type: Number, notify: true})
+  selectedIndex = -1;
 
-  static get observers() {
-    return [
-      '_expandedFilesChanged(_expandedFiles.splices)',
-      '_computeFiles(_filesByPath, changeComments, patchRange, _reviewed, ' +
-        '_loading)',
-    ];
-  }
+  @property({type: Object})
+  keyEventTarget = document.body;
+
+  @property({type: Object})
+  change?: ParsedChangeInfo;
+
+  @property({type: String, notify: true, observer: '_updateDiffPreferences'})
+  diffViewMode?: DiffViewMode;
+
+  @property({type: Boolean, observer: '_editModeChanged'})
+  editMode?: boolean;
+
+  @property({type: String, notify: true})
+  filesExpanded = FilesExpandedState.NONE;
+
+  @property({type: Object})
+  _filesByPath?: FileNameToFileInfoMap;
+
+  @property({type: Array, observer: '_filesChanged'})
+  _files: NormalizedFileInfo[] = [];
+
+  @property({type: Boolean})
+  _loggedIn = false;
+
+  @property({type: Array})
+  _reviewed?: string[] = [];
+
+  @property({type: Object, notify: true, observer: '_updateDiffPreferences'})
+  diffPrefs?: DiffPreferencesInfo;
+
+  @property({type: Object})
+  _userPrefs?: PreferencesInfo;
+
+  @property({type: Boolean})
+  _showInlineDiffs?: boolean;
+
+  @property({type: Number, notify: true})
+  numFilesShown: number = DEFAULT_NUM_FILES_SHOWN;
+
+  @property({type: Object, computed: '_calculatePatchChange(_files)'})
+  _patchChange: PatchChange = createDefaultPatchChange();
+
+  @property({type: Number})
+  fileListIncrement: number = DEFAULT_NUM_FILES_SHOWN;
+
+  @property({type: Boolean, computed: '_shouldHideChangeTotals(_patchChange)'})
+  _hideChangeTotals = true;
+
+  @property({
+    type: Boolean,
+    computed: '_shouldHideBinaryChangeTotals(_patchChange)',
+  })
+  _hideBinaryChangeTotals = true;
+
+  @property({
+    type: Array,
+    computed: '_computeFilesShown(numFilesShown, _files)',
+  })
+  _shownFiles: NormalizedFileInfo[] = [];
+
+  @property({type: Number})
+  _reportinShownFilesIncrement = 0;
+
+  @property({type: Array})
+  _expandedFiles: PatchSetFile[] = [];
+
+  @property({type: Boolean})
+  _displayLine?: boolean;
+
+  @property({type: Boolean, observer: '_loadingChanged'})
+  _loading?: boolean;
+
+  @property({type: Object, computed: '_computeSizeBarLayout(_shownFiles.*)'})
+  _sizeBarLayout: SizeBarLayout = createDefaultSizeBarLayout();
+
+  @property({type: Boolean, computed: '_computeShowSizeBars(_userPrefs)'})
+  _showSizeBars = true;
+
+  private _cancelForEachDiff?: () => void;
+
+  @property({
+    type: Boolean,
+    computed:
+      '_computeShowDynamicColumns(_dynamicHeaderEndpoints, ' +
+      '_dynamicContentEndpoints, _dynamicSummaryEndpoints)',
+  })
+  _showDynamicColumns = false;
+
+  @property({
+    type: Boolean,
+    computed:
+      '_computeShowPrependedDynamicColumns(' +
+      '_dynamicPrependedHeaderEndpoints, _dynamicPrependedContentEndpoints)',
+  })
+  _showPrependedDynamicColumns = false;
+
+  @property({type: Array})
+  _dynamicHeaderEndpoints?: string[];
+
+  @property({type: Array})
+  _dynamicContentEndpoints?: string[];
+
+  @property({type: Array})
+  _dynamicSummaryEndpoints?: string[];
+
+  @property({type: Array})
+  _dynamicPrependedHeaderEndpoints?: string[];
+
+  @property({type: Array})
+  _dynamicPrependedContentEndpoints?: string[];
+
+  private readonly reporting = appContext.reportingService;
 
   get keyBindings() {
     return {
@@ -292,50 +366,59 @@
     };
   }
 
-  constructor() {
-    super();
-    this.reporting = appContext.reportingService;
-  }
-
   /** @override */
   created() {
     super.created();
-    this.addEventListener('keydown',
-        e => this._scopedKeydownHandler(e));
+    this.addEventListener('keydown', e => this._scopedKeydownHandler(e));
   }
 
   /** @override */
   attached() {
     super.attached();
-    getPluginLoader().awaitPluginsLoaded()
-        .then(() => {
-          this._dynamicHeaderEndpoints = getPluginEndpoints()
-              .getDynamicEndpoints('change-view-file-list-header');
-          this._dynamicContentEndpoints = getPluginEndpoints()
-              .getDynamicEndpoints('change-view-file-list-content');
-          this._dynamicPrependedHeaderEndpoints = getPluginEndpoints()
-              .getDynamicEndpoints('change-view-file-list-header-prepend');
-          this._dynamicPrependedContentEndpoints = getPluginEndpoints()
-              .getDynamicEndpoints('change-view-file-list-content-prepend');
-          this._dynamicSummaryEndpoints = getPluginEndpoints()
-              .getDynamicEndpoints('change-view-file-list-summary');
+    getPluginLoader()
+      .awaitPluginsLoaded()
+      .then(() => {
+        this._dynamicHeaderEndpoints = getPluginEndpoints().getDynamicEndpoints(
+          'change-view-file-list-header'
+        );
+        this._dynamicContentEndpoints = getPluginEndpoints().getDynamicEndpoints(
+          'change-view-file-list-content'
+        );
+        this._dynamicPrependedHeaderEndpoints = getPluginEndpoints().getDynamicEndpoints(
+          'change-view-file-list-header-prepend'
+        );
+        this._dynamicPrependedContentEndpoints = getPluginEndpoints().getDynamicEndpoints(
+          'change-view-file-list-content-prepend'
+        );
+        this._dynamicSummaryEndpoints = getPluginEndpoints().getDynamicEndpoints(
+          'change-view-file-list-summary'
+        );
 
-          if (this._dynamicHeaderEndpoints.length !==
-          this._dynamicContentEndpoints.length) {
-            console.warn(
-                'Different number of dynamic file-list header and content.');
-          }
-          if (this._dynamicPrependedHeaderEndpoints.length !==
-        this._dynamicPrependedContentEndpoints.length) {
-            console.warn(
-                'Different number of dynamic file-list header and content.');
-          }
-          if (this._dynamicHeaderEndpoints.length !==
-          this._dynamicSummaryEndpoints.length) {
-            console.warn(
-                'Different number of dynamic file-list headers and summary.');
-          }
-        });
+        if (
+          this._dynamicHeaderEndpoints.length !==
+          this._dynamicContentEndpoints.length
+        ) {
+          console.warn(
+            'Different number of dynamic file-list header and content.'
+          );
+        }
+        if (
+          this._dynamicPrependedHeaderEndpoints.length !==
+          this._dynamicPrependedContentEndpoints.length
+        ) {
+          console.warn(
+            'Different number of dynamic file-list header and content.'
+          );
+        }
+        if (
+          this._dynamicHeaderEndpoints.length !==
+          this._dynamicSummaryEndpoints.length
+        ) {
+          console.warn(
+            'Different number of dynamic file-list headers and summary.'
+          );
+        }
+      });
   }
 
   /** @override */
@@ -351,43 +434,61 @@
    *
    * Context: Issue 7277
    */
-  _scopedKeydownHandler(e) {
+  _scopedKeydownHandler(e: KeyboardEvent) {
     if (e.keyCode === 13) {
-      // Enter.
-      this._handleOpenFile(e);
+      // TODO(TS): e is not an instance of CustomKeyboardEvent.
+      // However, to fix it we should fix keyboard-shortcut-mixin first
+      // The keyboard-shortcut-mixin will be updated in a separate change
+      this._handleOpenFile((e as unknown) as CustomKeyboardEvent);
     }
   }
 
   reload() {
-    if (!this.changeNum || !this.patchRange.patchNum) {
+    if (!this.changeNum || !this.patchRange?.patchNum) {
       return Promise.resolve();
     }
+    const changeNum = this.changeNum;
+    const patchRange = this.patchRange;
 
     this._loading = true;
 
     this.collapseAllDiffs();
     const promises = [];
 
-    promises.push(this._getFiles().then(filesByPath => {
-      this._filesByPath = filesByPath;
-    }));
-    promises.push(this._getLoggedIn()
-        .then(loggedIn => this._loggedIn = loggedIn)
+    promises.push(
+      this.$.restAPI
+        .getChangeOrEditFiles(changeNum, patchRange)
+        .then(filesByPath => {
+          this._filesByPath = filesByPath;
+        })
+    );
+    promises.push(
+      this._getLoggedIn()
+        .then(loggedIn => (this._loggedIn = loggedIn))
         .then(loggedIn => {
-          if (!loggedIn) { return; }
+          if (!loggedIn) {
+            return;
+          }
 
-          return this._getReviewedFiles().then(reviewed => {
-            this._reviewed = reviewed;
-          });
-        }));
+          return this._getReviewedFiles(changeNum, patchRange).then(
+            reviewed => {
+              this._reviewed = reviewed;
+            }
+          );
+        })
+    );
 
-    promises.push(this._getDiffPreferences().then(prefs => {
-      this.diffPrefs = prefs;
-    }));
+    promises.push(
+      this._getDiffPreferences().then(prefs => {
+        this.diffPrefs = prefs;
+      })
+    );
 
-    promises.push(this._getPreferences().then(prefs => {
-      this._userPrefs = prefs;
-    }));
+    promises.push(
+      this._getPreferences().then(prefs => {
+        this._userPrefs = prefs;
+      })
+    );
 
     return Promise.all(promises).then(() => {
       this._loading = false;
@@ -403,32 +504,33 @@
     }
   }
 
-  get diffs() {
-    const diffs = this.root.querySelectorAll('gr-diff-host');
+  get diffs(): GrDiffHost[] {
+    const diffs = this.root!.querySelectorAll('gr-diff-host');
     // It is possible that a bogus diff element is hanging around invisibly
     // from earlier with a different patch set choice and associated with a
     // different entry in the files array. So filter on visible items only.
     return Array.from(diffs).filter(
-        el => !!el && !!el.style && el.style.display !== 'none');
+      el => !!el && !!el.style && el.style.display !== 'none'
+    );
   }
 
   openDiffPrefs() {
     this.$.diffPreferencesDialog.open();
   }
 
-  _calculatePatchChange(files) {
-    const magicFilesExcluded = files.filter(files =>
-      !isMagicPath(files.__path)
+  _calculatePatchChange(files: NormalizedFileInfo[]): PatchChange {
+    const magicFilesExcluded = files.filter(
+      files => !isMagicPath(files.__path)
     );
 
     return magicFilesExcluded.reduce((acc, obj) => {
       const inserted = obj.lines_inserted ? obj.lines_inserted : 0;
       const deleted = obj.lines_deleted ? obj.lines_deleted : 0;
-      const total_size = (obj.size && obj.binary) ? obj.size : 0;
+      const total_size = obj.size && obj.binary ? obj.size : 0;
       const size_delta_inserted =
-          obj.binary && obj.size_delta > 0 ? obj.size_delta : 0;
+        obj.binary && obj.size_delta > 0 ? obj.size_delta : 0;
       const size_delta_deleted =
-          obj.binary && obj.size_delta < 0 ? obj.size_delta : 0;
+        obj.binary && obj.size_delta < 0 ? obj.size_delta : 0;
 
       return {
         inserted: acc.inserted + inserted,
@@ -437,8 +539,7 @@
         size_delta_deleted: acc.size_delta_deleted + size_delta_deleted,
         total_size: acc.total_size + total_size,
       };
-    }, {inserted: 0, deleted: 0, size_delta_inserted: 0,
-      size_delta_deleted: 0, total_size: 0});
+    }, createDefaultPatchChange());
   }
 
   _getDiffPreferences() {
@@ -449,7 +550,7 @@
     return this.$.restAPI.getPreferences();
   }
 
-  _toggleFileExpanded(file) {
+  private _toggleFileExpanded(file: PatchSetFile) {
     // Is the path in the list of expanded diffs? IF so remove it, otherwise
     // add it to the list.
     const pathIndex = this._expandedFiles.findIndex(f => f.path === file.path);
@@ -460,19 +561,24 @@
     }
   }
 
-  _toggleFileExpandedByIndex(index) {
-    this._toggleFileExpanded(this._computeFileRange(this._files[index]));
+  _toggleFileExpandedByIndex(index: number) {
+    this._toggleFileExpanded(this._computePatchSetFile(this._files[index]));
   }
 
   _updateDiffPreferences() {
-    if (!this.diffs.length) { return; }
+    if (!this.diffs.length) {
+      return;
+    }
     // Re-render all expanded diffs sequentially.
     this.reporting.time(EXPAND_ALL_TIMING_LABEL);
-    this._renderInOrder(this._expandedFiles, this.diffs,
-        this._expandedFiles.length);
+    this._renderInOrder(
+      this._expandedFiles,
+      this.diffs,
+      this._expandedFiles.length
+    );
   }
 
-  _forEachDiff(fn) {
+  _forEachDiff(fn: (host: GrDiffHost) => void) {
     const diffs = this.diffs;
     for (let i = 0; i < diffs.length; i++) {
       fn(diffs[i]);
@@ -484,187 +590,207 @@
 
     // Find the list of paths that are in the file list, but not in the
     // expanded list.
-    const newFiles = [];
-    let path;
+    const newFiles: PatchSetFile[] = [];
+    let path: string;
     for (let i = 0; i < this._shownFiles.length; i++) {
       path = this._shownFiles[i].__path;
       if (!this._expandedFiles.some(f => f.path === path)) {
-        newFiles.push(this._computeFileRange(this._shownFiles[i]));
+        newFiles.push(this._computePatchSetFile(this._shownFiles[i]));
       }
     }
 
-    this.splice(...['_expandedFiles', 0, 0].concat(newFiles));
+    this.splice('_expandedFiles', 0, 0, ...newFiles);
   }
 
   collapseAllDiffs() {
     this._showInlineDiffs = false;
     this._expandedFiles = [];
     this.filesExpanded = this._computeExpandedFiles(
-        this._expandedFiles.length, this._files.length);
+      this._expandedFiles.length,
+      this._files.length
+    );
     this.$.diffCursor.handleDiffUpdate();
   }
 
   /**
    * Computes a string with the number of comments and unresolved comments.
-   *
-   * @param {!Object} changeComments
-   * @param {!Object} patchRange
-   * @param {string} path
-   * @return {string}
    */
-  _computeCommentsString(changeComments, patchRange, path) {
-    if ([changeComments, patchRange, path].includes(undefined)) {
+  _computeCommentsString(
+    changeComments?: ChangeComments,
+    patchRange?: PatchRange,
+    path?: string
+  ) {
+    if (
+      changeComments === undefined ||
+      patchRange === undefined ||
+      path === undefined
+    ) {
       return '';
     }
     const unresolvedCount =
-        changeComments.computeUnresolvedNum({
-          patchNum: patchRange.basePatchNum,
-          path,
-        }) +
-        changeComments.computeUnresolvedNum({
-          patchNum: patchRange.patchNum,
-          path,
-        });
+      changeComments.computeUnresolvedNum({
+        patchNum: patchRange.basePatchNum,
+        path,
+      }) +
+      changeComments.computeUnresolvedNum({
+        patchNum: patchRange.patchNum,
+        path,
+      });
     const commentCount =
-        changeComments.computeCommentCount({
-          patchNum: patchRange.basePatchNum,
-          path,
-        }) +
-        changeComments.computeCommentCount({
-          patchNum: patchRange.patchNum,
-          path,
-        });
+      changeComments.computeCommentCount({
+        patchNum: patchRange.basePatchNum,
+        path,
+      }) +
+      changeComments.computeCommentCount({
+        patchNum: patchRange.patchNum,
+        path,
+      });
     const commentString = GrCountStringFormatter.computePluralString(
-        commentCount, 'comment');
+      commentCount,
+      'comment'
+    );
     const unresolvedString = GrCountStringFormatter.computeString(
-        unresolvedCount, 'unresolved');
+      unresolvedCount,
+      'unresolved'
+    );
 
-    return commentString +
-        // Add a space if both comments and unresolved
-        (commentString && unresolvedString ? ' ' : '') +
-        // Add parentheses around unresolved if it exists.
-        (unresolvedString ? `(${unresolvedString})` : '');
+    return (
+      commentString +
+      // Add a space if both comments and unresolved
+      (commentString && unresolvedString ? ' ' : '') +
+      // Add parentheses around unresolved if it exists.
+      (unresolvedString ? `(${unresolvedString})` : '')
+    );
   }
 
   /**
    * Computes a string with the number of drafts.
-   *
-   * @param {!Object} changeComments
-   * @param {!Object} patchRange
-   * @param {string} path
-   * @return {string}
    */
-  _computeDraftsString(changeComments, patchRange, path) {
-    if ([changeComments, patchRange, path].includes(undefined)) {
+  _computeDraftsString(
+    changeComments?: ChangeComments,
+    patchRange?: PatchRange,
+    path?: string
+  ) {
+    if (
+      changeComments === undefined ||
+      patchRange === undefined ||
+      path === undefined
+    ) {
       return '';
     }
     const draftCount =
-        changeComments.computeDraftCount({
-          patchNum: patchRange.basePatchNum,
-          path,
-        }) +
-        changeComments.computeDraftCount({
-          patchNum: patchRange.patchNum,
-          path,
-        });
+      changeComments.computeDraftCount({
+        patchNum: patchRange.basePatchNum,
+        path,
+      }) +
+      changeComments.computeDraftCount({
+        patchNum: patchRange.patchNum,
+        path,
+      });
     return GrCountStringFormatter.computePluralString(draftCount, 'draft');
   }
 
   /**
    * Computes a shortened string with the number of drafts.
-   *
-   * @param {!Object} changeComments
-   * @param {!Object} patchRange
-   * @param {string} path
-   * @return {string}
    */
-  _computeDraftsStringMobile(changeComments, patchRange, path) {
-    if ([changeComments, patchRange, path].includes(undefined)) {
+  _computeDraftsStringMobile(
+    changeComments?: ChangeComments,
+    patchRange?: PatchRange,
+    path?: string
+  ) {
+    if (
+      changeComments === undefined ||
+      patchRange === undefined ||
+      path === undefined
+    ) {
       return '';
     }
     const draftCount =
-        changeComments.computeDraftCount({
-          patchNum: patchRange.basePatchNum,
-          path,
-        }) +
-        changeComments.computeDraftCount({
-          patchNum: patchRange.patchNum,
-          path,
-        });
+      changeComments.computeDraftCount({
+        patchNum: patchRange.basePatchNum,
+        path,
+      }) +
+      changeComments.computeDraftCount({
+        patchNum: patchRange.patchNum,
+        path,
+      });
     return GrCountStringFormatter.computeShortString(draftCount, 'd');
   }
 
   /**
    * Computes a shortened string with the number of comments.
-   *
-   * @param {!Object} changeComments
-   * @param {!Object} patchRange
-   * @param {string} path
-   * @return {string}
    */
-  _computeCommentsStringMobile(changeComments, patchRange, path) {
-    if ([changeComments, patchRange, path].includes(undefined)) {
+  _computeCommentsStringMobile(
+    changeComments?: ChangeComments,
+    patchRange?: PatchRange,
+    path?: string
+  ) {
+    if (
+      changeComments === undefined ||
+      patchRange === undefined ||
+      path === undefined
+    ) {
       return '';
     }
     const commentCount =
-        changeComments.computeCommentCount({
-          patchNum: patchRange.basePatchNum,
-          path,
-        }) +
-        changeComments.computeCommentCount({
-          patchNum: patchRange.patchNum,
-          path,
-        });
+      changeComments.computeCommentCount({
+        patchNum: patchRange.basePatchNum,
+        path,
+      }) +
+      changeComments.computeCommentCount({
+        patchNum: patchRange.patchNum,
+        path,
+      });
     return GrCountStringFormatter.computeShortString(commentCount, 'c');
   }
 
-  /**
-   * @param {string} path
-   * @param {boolean=} opt_reviewed
-   */
-  _reviewFile(path, opt_reviewed) {
-    if (this.editMode) { return; }
+  private _reviewFile(path: string, reviewed?: boolean) {
+    if (this.editMode) {
+      return Promise.resolve();
+    }
     const index = this._files.findIndex(file => file.__path === path);
-    const reviewed = opt_reviewed || !this._files[index].isReviewed;
+    reviewed = reviewed || !this._files[index].isReviewed;
 
     this.set(['_files', index, 'isReviewed'], reviewed);
     if (index < this._shownFiles.length) {
       this.notifyPath(`_shownFiles.${index}.isReviewed`);
     }
 
-    this._saveReviewedState(path, reviewed);
+    return this._saveReviewedState(path, reviewed);
   }
 
-  _saveReviewedState(path, reviewed) {
-    return this.$.restAPI.saveFileReviewed(this.changeNum,
-        this.patchRange.patchNum, path, reviewed);
+  _saveReviewedState(path: string, reviewed: boolean) {
+    if (!this.changeNum || !this.patchRange) {
+      throw new Error('changeNum and patchRange must be set');
+    }
+
+    return this.$.restAPI.saveFileReviewed(
+      this.changeNum,
+      this.patchRange.patchNum,
+      path,
+      reviewed
+    );
   }
 
   _getLoggedIn() {
     return this.$.restAPI.getLoggedIn();
   }
 
-  _getReviewedFiles() {
-    if (this.editMode) { return Promise.resolve([]); }
-    return this.$.restAPI.getReviewedFiles(this.changeNum,
-        this.patchRange.patchNum);
+  _getReviewedFiles(changeNum: NumericChangeId, patchRange: PatchRange) {
+    if (this.editMode) {
+      return Promise.resolve([]);
+    }
+    return this.$.restAPI.getReviewedFiles(changeNum, patchRange.patchNum);
   }
 
-  _getFiles() {
-    return this.$.restAPI.getChangeOrEditFiles(
-        this.changeNum, this.patchRange);
-  }
-
-  /**
-   *
-   * @returns {!Array<FileInfo>}
-   */
-  _normalizeChangeFilesResponse(response) {
-    if (!response) { return []; }
+  _normalizeChangeFilesResponse(
+    response: FileNameToReviewedFileInfoMap
+  ): NormalizedFileInfo[] {
     const paths = Object.keys(response).sort(specialFilePathCompare);
-    const files = [];
+    const files: NormalizedFileInfo[] = [];
     for (let i = 0; i < paths.length; i++) {
-      const info = response[paths[i]];
+      // TODO(TS): make copy instead of as NormalizedFileInfo
+      const info = response[paths[i]] as NormalizedFileInfo;
       info.__path = paths[i];
       info.lines_inserted = info.lines_inserted || 0;
       info.lines_deleted = info.lines_deleted || 0;
@@ -680,15 +806,19 @@
    * The click is: mouse click or pressing Enter or Space key
    * P.S> Screen readers sends click event as well
    */
-  _isClickEvent(e) {
+  _isClickEvent(e: MouseEvent | KeyboardEvent) {
     if (e.type === 'click') {
       return true;
     }
-    const isSpaceOrEnter = (e.key === 'Enter' || e.key === ' ');
-    return e.type === 'keydown' && isSpaceOrEnter;
+    const ke = e as KeyboardEvent;
+    const isSpaceOrEnter = ke.key === 'Enter' || ke.key === ' ';
+    return ke.type === 'keydown' && isSpaceOrEnter;
   }
 
-  _fileActionClick(e, fileAction) {
+  _fileActionClick(
+    e: MouseEvent | KeyboardEvent,
+    fileAction: (file: PatchSetFile) => void
+  ) {
     if (this._isClickEvent(e)) {
       const fileRow = this._getFileRowFromEvent(e);
       if (!fileRow) {
@@ -703,21 +833,22 @@
     }
   }
 
-  _reviewedClick(e) {
-    this._fileActionClick(e,
-        file => this._reviewFile(file.path));
+  _reviewedClick(e: MouseEvent | KeyboardEvent) {
+    this._fileActionClick(e, file => this._reviewFile(file.path));
   }
 
-  _expandedClick(e) {
-    this._fileActionClick(e,
-        file => this._toggleFileExpanded(file));
+  _expandedClick(e: MouseEvent | KeyboardEvent) {
+    this._fileActionClick(e, file => this._toggleFileExpanded(file));
   }
 
   /**
    * Handle all events from the file list dom-repeat so event handleers don't
    * have to get registered for potentially very long lists.
    */
-  _handleFileListClick(e) {
+  _handleFileListClick(e: MouseEvent) {
+    if (!e.target) {
+      return;
+    }
     const fileRow = this._getFileRowFromEvent(e);
     if (!fileRow) {
       return;
@@ -727,19 +858,23 @@
     // If a path cannot be interpreted from the click target (meaning it's not
     // somewhere in the row, e.g. diff content) or if the user clicked the
     // link, defer to the native behavior.
-    if (!path || descendedFromClass(e.target, 'pathLink')) { return; }
+    if (!path || descendedFromClass(e.target as Element, 'pathLink')) {
+      return;
+    }
 
     // Disregard the event if the click target is in the edit controls.
-    if (descendedFromClass(e.target, 'editFileControls')) { return; }
+    if (descendedFromClass(e.target as Element, 'editFileControls')) {
+      return;
+    }
 
     e.preventDefault();
     this.$.fileCursor.setCursor(fileRow.element);
     this._toggleFileExpanded(file);
   }
 
-  _getFileRowFromEvent(e) {
+  _getFileRowFromEvent(e: Event): FileRow | null {
     // Traverse upwards to find the row element if the target is not the row.
-    let row = e.target;
+    let row = e.target as HTMLElement;
     while (!row.classList.contains(FILE_ROW_CLASS) && row.parentElement) {
       row = row.parentElement;
     }
@@ -750,19 +885,16 @@
     }
 
     return {
-      file: JSON.parse(row.dataset['file']),
+      file: JSON.parse(row.dataset['file']) as PatchSetFile,
       element: row,
     };
   }
 
   /**
    * Generates file range from file info object.
-   *
-   * @param {FileInfo} file
-   * @returns {Gerrit.FileRange}
    */
-  _computeFileRange(file) {
-    const fileData = {
+  _computePatchSetFile(file: NormalizedFileInfo): PatchSetFile {
+    const fileData: PatchSetFile = {
       path: file.__path,
     };
     if (file.old_path) {
@@ -771,7 +903,7 @@
     return fileData;
   }
 
-  _handleLeftPane(e) {
+  _handleLeftPane(e: CustomKeyboardEvent) {
     if (this.shouldSuppressKeyboardShortcut(e) || this._noDiffsExpanded()) {
       return;
     }
@@ -780,7 +912,7 @@
     this.$.diffCursor.moveLeft();
   }
 
-  _handleRightPane(e) {
+  _handleRightPane(e: CustomKeyboardEvent) {
     if (this.shouldSuppressKeyboardShortcut(e) || this._noDiffsExpanded()) {
       return;
     }
@@ -789,23 +921,29 @@
     this.$.diffCursor.moveRight();
   }
 
-  _handleToggleInlineDiff(e) {
-    if (this.shouldSuppressKeyboardShortcut(e) ||
-        this.modifierPressed(e) ||
-        this.$.fileCursor.index === -1) { return; }
+  _handleToggleInlineDiff(e: CustomKeyboardEvent) {
+    if (
+      this.shouldSuppressKeyboardShortcut(e) ||
+      this.modifierPressed(e) ||
+      this.$.fileCursor.index === -1
+    ) {
+      return;
+    }
 
     e.preventDefault();
     this._toggleFileExpandedByIndex(this.$.fileCursor.index);
   }
 
-  _handleToggleAllInlineDiffs(e) {
-    if (this.shouldSuppressKeyboardShortcut(e)) { return; }
+  _handleToggleAllInlineDiffs(e: CustomKeyboardEvent) {
+    if (this.shouldSuppressKeyboardShortcut(e)) {
+      return;
+    }
 
     e.preventDefault();
     this._toggleInlineDiffs();
   }
 
-  _handleToggleHideAllCommentThreads(e) {
+  _handleToggleHideAllCommentThreads(e: CustomKeyboardEvent) {
     if (this.shouldSuppressKeyboardShortcut(e) || this.modifierPressed(e)) {
       return;
     }
@@ -814,7 +952,7 @@
     this.toggleClass('hideComments');
   }
 
-  _handleCursorNext(e) {
+  _handleCursorNext(e: CustomKeyboardEvent) {
     if (this.shouldSuppressKeyboardShortcut(e) || this.modifierPressed(e)) {
       return;
     }
@@ -825,14 +963,16 @@
       this._displayLine = true;
     } else {
       // Down key
-      if (this.getKeyboardEvent(e).keyCode === 40) { return; }
+      if (this.getKeyboardEvent(e).keyCode === 40) {
+        return;
+      }
       e.preventDefault();
       this.$.fileCursor.next();
       this.selectedIndex = this.$.fileCursor.index;
     }
   }
 
-  _handleCursorPrev(e) {
+  _handleCursorPrev(e: CustomKeyboardEvent) {
     if (this.shouldSuppressKeyboardShortcut(e) || this.modifierPressed(e)) {
       return;
     }
@@ -843,41 +983,53 @@
       this._displayLine = true;
     } else {
       // Up key
-      if (this.getKeyboardEvent(e).keyCode === 38) { return; }
+      if (this.getKeyboardEvent(e).keyCode === 38) {
+        return;
+      }
       e.preventDefault();
       this.$.fileCursor.previous();
       this.selectedIndex = this.$.fileCursor.index;
     }
   }
 
-  _handleNewComment(e) {
-    if (this.shouldSuppressKeyboardShortcut(e) ||
-        this.modifierPressed(e)) { return; }
+  _handleNewComment(e: CustomKeyboardEvent) {
+    if (this.shouldSuppressKeyboardShortcut(e) || this.modifierPressed(e)) {
+      return;
+    }
     e.preventDefault();
     this.$.diffCursor.createCommentInPlace();
   }
 
-  _handleOpenLastFile(e) {
+  _handleOpenLastFile(e: CustomKeyboardEvent) {
     // Check for meta key to avoid overriding native chrome shortcut.
-    if (this.shouldSuppressKeyboardShortcut(e) ||
-        this.getKeyboardEvent(e).metaKey) { return; }
+    if (
+      this.shouldSuppressKeyboardShortcut(e) ||
+      this.getKeyboardEvent(e).metaKey
+    ) {
+      return;
+    }
 
     e.preventDefault();
     this._openSelectedFile(this._files.length - 1);
   }
 
-  _handleOpenFirstFile(e) {
+  _handleOpenFirstFile(e: CustomKeyboardEvent) {
     // Check for meta key to avoid overriding native chrome shortcut.
-    if (this.shouldSuppressKeyboardShortcut(e) ||
-        this.getKeyboardEvent(e).metaKey) { return; }
+    if (
+      this.shouldSuppressKeyboardShortcut(e) ||
+      this.getKeyboardEvent(e).metaKey
+    ) {
+      return;
+    }
 
     e.preventDefault();
     this._openSelectedFile(0);
   }
 
-  _handleOpenFile(e) {
-    if (this.shouldSuppressKeyboardShortcut(e) ||
-        this.modifierPressed(e)) { return; }
+  _handleOpenFile(e: CustomKeyboardEvent) {
+    if (this.shouldSuppressKeyboardShortcut(e) || this.modifierPressed(e)) {
+      return;
+    }
     e.preventDefault();
 
     if (this._showInlineDiffs) {
@@ -888,48 +1040,58 @@
     this._openSelectedFile();
   }
 
-  _handleNextChunk(e) {
-    if (this.shouldSuppressKeyboardShortcut(e) ||
-        (this.modifierPressed(e) && !this.isModifierPressed(e, 'shiftKey')) ||
-        this._noDiffsExpanded()) {
+  _handleNextChunk(e: CustomKeyboardEvent) {
+    if (
+      this.shouldSuppressKeyboardShortcut(e) ||
+      (this.modifierPressed(e) &&
+        !this.isModifierPressed(e, Modifier.SHIFT_KEY)) ||
+      this._noDiffsExpanded()
+    ) {
       return;
     }
 
     e.preventDefault();
-    if (this.isModifierPressed(e, 'shiftKey')) {
+    if (this.isModifierPressed(e, Modifier.SHIFT_KEY)) {
       this.$.diffCursor.moveToNextCommentThread();
     } else {
       this.$.diffCursor.moveToNextChunk();
     }
   }
 
-  _handlePrevChunk(e) {
-    if (this.shouldSuppressKeyboardShortcut(e) ||
-        (this.modifierPressed(e) && !this.isModifierPressed(e, 'shiftKey')) ||
-        this._noDiffsExpanded()) {
+  _handlePrevChunk(e: CustomKeyboardEvent) {
+    if (
+      this.shouldSuppressKeyboardShortcut(e) ||
+      (this.modifierPressed(e) &&
+        !this.isModifierPressed(e, Modifier.SHIFT_KEY)) ||
+      this._noDiffsExpanded()
+    ) {
       return;
     }
 
     e.preventDefault();
-    if (this.isModifierPressed(e, 'shiftKey')) {
+    if (this.isModifierPressed(e, Modifier.SHIFT_KEY)) {
       this.$.diffCursor.moveToPreviousCommentThread();
     } else {
       this.$.diffCursor.moveToPreviousChunk();
     }
   }
 
-  _handleToggleFileReviewed(e) {
+  _handleToggleFileReviewed(e: CustomKeyboardEvent) {
     if (this.shouldSuppressKeyboardShortcut(e) || this.modifierPressed(e)) {
       return;
     }
 
     e.preventDefault();
-    if (!this._files[this.$.fileCursor.index]) { return; }
+    if (!this._files[this.$.fileCursor.index]) {
+      return;
+    }
     this._reviewFile(this._files[this.$.fileCursor.index].__path);
   }
 
-  _handleToggleLeftPane(e) {
-    if (this.shouldSuppressKeyboardShortcut(e)) { return; }
+  _handleToggleLeftPane(e: CustomKeyboardEvent) {
+    if (this.shouldSuppressKeyboardShortcut(e)) {
+      return;
+    }
 
     e.preventDefault();
     this._forEachDiff(diff => {
@@ -947,21 +1109,39 @@
 
   _openCursorFile() {
     const diff = this.$.diffCursor.getTargetDiffElement();
-    GerritNav.navigateToDiff(this.change, diff.path,
-        diff.patchRange.patchNum, this.patchRange.basePatchNum);
+    if (
+      !this.change ||
+      !diff ||
+      !this.patchRange ||
+      !diff.path ||
+      !diff.patchRange
+    ) {
+      throw new Error('change, diff and patchRange must be all set and valid');
+    }
+    GerritNav.navigateToDiff(
+      this.change,
+      diff.path,
+      diff.patchRange.patchNum,
+      this.patchRange.basePatchNum
+    );
   }
 
-  /**
-   * @param {number=} opt_index
-   */
-  _openSelectedFile(opt_index) {
-    if (opt_index != null) {
-      this.$.fileCursor.setCursorAtIndex(opt_index);
+  _openSelectedFile(index?: number) {
+    if (index !== undefined) {
+      this.$.fileCursor.setCursorAtIndex(index);
     }
-    if (!this._files[this.$.fileCursor.index]) { return; }
-    GerritNav.navigateToDiff(this.change,
-        this._files[this.$.fileCursor.index].__path, this.patchRange.patchNum,
-        this.patchRange.basePatchNum);
+    if (!this._files[this.$.fileCursor.index]) {
+      return;
+    }
+    if (!this.change || !this.patchRange) {
+      throw new Error('change and patchRange must be set');
+    }
+    GerritNav.navigateToDiff(
+      this.change,
+      this._files[this.$.fileCursor.index].__path,
+      this.patchRange.patchNum,
+      this.patchRange.basePatchNum
+    );
   }
 
   _addDraftAtTarget() {
@@ -972,130 +1152,182 @@
     }
   }
 
-  _shouldHideChangeTotals(_patchChange) {
+  _shouldHideChangeTotals(_patchChange: PatchChange): boolean {
     return _patchChange.inserted === 0 && _patchChange.deleted === 0;
   }
 
-  _shouldHideBinaryChangeTotals(_patchChange) {
-    return _patchChange.size_delta_inserted === 0 &&
-        _patchChange.size_delta_deleted === 0;
+  _shouldHideBinaryChangeTotals(_patchChange: PatchChange) {
+    return (
+      _patchChange.size_delta_inserted === 0 &&
+      _patchChange.size_delta_deleted === 0
+    );
   }
 
-  _computeFileStatus(status) {
+  _computeFileStatus(
+    status?: keyof typeof FileStatus
+  ): keyof typeof FileStatus {
     return status || 'M';
   }
 
-  _computeDiffURL(change, patchRange, path, editMode) {
+  _computeDiffURL(
+    change?: ParsedChangeInfo,
+    patchRange?: PatchRange,
+    path?: string,
+    editMode?: boolean
+  ) {
     // Polymer 2: check for undefined
-    if ([change, patchRange, path, editMode]
-        .some(arg => arg === undefined)) {
+    if (
+      change === undefined ||
+      patchRange === undefined ||
+      path === undefined ||
+      editMode === undefined
+    ) {
       return;
     }
     if (editMode && path !== SpecialFilePath.MERGE_LIST) {
-      return GerritNav.getEditUrlForDiff(change, path, patchRange.patchNum,
-          patchRange.basePatchNum);
+      return GerritNav.getEditUrlForDiff(change, path, patchRange.patchNum);
     }
-    return GerritNav.getUrlForDiff(change, path, patchRange.patchNum,
-        patchRange.basePatchNum);
+    return GerritNav.getUrlForDiff(
+      change,
+      path,
+      patchRange.patchNum,
+      patchRange.basePatchNum
+    );
   }
 
-  _formatBytes(bytes) {
-    if (bytes == 0) return '+/-0 B';
+  _formatBytes(bytes?: number) {
+    if (!bytes) return '+/-0 B';
     const bits = 1024;
     const decimals = 1;
-    const sizes =
-        ['B', 'KiB', 'MiB', 'GiB', 'TiB', 'PiB', 'EiB', 'ZiB', 'YiB'];
+    const sizes = ['B', 'KiB', 'MiB', 'GiB', 'TiB', 'PiB', 'EiB', 'ZiB', 'YiB'];
     const exponent = Math.floor(Math.log(Math.abs(bytes)) / Math.log(bits));
     const prepend = bytes > 0 ? '+' : '';
-    return prepend + parseFloat((bytes / Math.pow(bits, exponent))
-        .toFixed(decimals)) + ' ' + sizes[exponent];
+    const value = parseFloat(
+      (bytes / Math.pow(bits, exponent)).toFixed(decimals)
+    );
+    return `${prepend}${value} ${sizes[exponent]}`;
   }
 
-  _formatPercentage(size, delta) {
+  _formatPercentage(size?: number, delta?: number) {
+    if (size === undefined || delta === undefined) {
+      return '';
+    }
     const oldSize = size - delta;
 
-    if (oldSize === 0) { return ''; }
+    if (oldSize === 0) {
+      return '';
+    }
 
-    const percentage = Math.round(Math.abs(delta * 100 / oldSize));
-    return '(' + (delta > 0 ? '+' : '-') + percentage + '%)';
+    const percentage = Math.round(Math.abs((delta * 100) / oldSize));
+    return `(${delta > 0 ? '+' : '-'}${percentage}%)`;
   }
 
-  _computeBinaryClass(delta) {
-    if (delta === 0) { return; }
-    return delta >= 0 ? 'added' : 'removed';
+  _computeBinaryClass(delta?: number) {
+    if (!delta) {
+      return;
+    }
+    return delta > 0 ? 'added' : 'removed';
   }
 
-  /**
-   * @param {string} baseClass
-   * @param {string} path
-   */
-  _computeClass(baseClass, path) {
+  _computeClass(baseClass?: string, path?: string) {
     const classes = [];
     if (baseClass) {
       classes.push(baseClass);
     }
-    if (path === SpecialFilePath.COMMIT_MESSAGE ||
-      path === SpecialFilePath.MERGE_LIST) {
+    if (
+      path === SpecialFilePath.COMMIT_MESSAGE ||
+      path === SpecialFilePath.MERGE_LIST
+    ) {
       classes.push('invisible');
     }
     return classes.join(' ');
   }
 
-  _computeStatusClass(file) {
+  _computeStatusClass(file?: NormalizedFileInfo) {
+    if (!file) return '';
     const classStr = this._computeClass('status', file.__path);
     return `${classStr} ${this._computeFileStatus(file.status)}`;
   }
 
-  _computePathClass(path, expandedFilesRecord) {
+  _computePathClass(
+    path: string | undefined,
+    expandedFilesRecord: ElementPropertyDeepChange<GrFileList, '_expandedFiles'>
+  ) {
     return this._isFileExpanded(path, expandedFilesRecord) ? 'expanded' : '';
   }
 
-  _computeShowHideIcon(path, expandedFilesRecord) {
-    return this._isFileExpanded(path, expandedFilesRecord) ?
-      'gr-icons:expand-less' : 'gr-icons:expand-more';
+  _computeShowHideIcon(
+    path: string | undefined,
+    expandedFilesRecord: ElementPropertyDeepChange<GrFileList, '_expandedFiles'>
+  ) {
+    return this._isFileExpanded(path, expandedFilesRecord)
+      ? 'gr-icons:expand-less'
+      : 'gr-icons:expand-more';
   }
 
-  _computeFiles(filesByPath, changeComments, patchRange, reviewed, loading) {
+  @observe(
+    '_filesByPath',
+    'changeComments',
+    'patchRange',
+    '_reviewed',
+    '_loading'
+  )
+  _computeFiles(
+    filesByPath?: FileNameToFileInfoMap,
+    changeComments?: ChangeComments,
+    patchRange?: PatchRange,
+    reviewed?: string[],
+    loading?: boolean
+  ) {
     // Polymer 2: check for undefined
-    if ([
-      filesByPath,
-      changeComments,
-      patchRange,
-      reviewed,
-      loading,
-    ].includes(undefined)) {
+    if (
+      filesByPath === undefined ||
+      changeComments === undefined ||
+      patchRange === undefined ||
+      reviewed === undefined ||
+      loading === undefined
+    ) {
       return;
     }
 
     // Await all promises resolving from reload. @See Issue 9057
-    if (loading || !changeComments) { return; }
+    if (loading || !changeComments) {
+      return;
+    }
 
     const commentedPaths = changeComments.getPaths(patchRange);
-    const files = {...filesByPath};
+    const files: FileNameToReviewedFileInfoMap = {...filesByPath};
     addUnmodifiedFiles(files, commentedPaths);
     const reviewedSet = new Set(reviewed || []);
     for (const filePath in files) {
-      if (!files.hasOwnProperty(filePath)) { continue; }
+      if (!hasOwnProperty(files, filePath)) {
+        continue;
+      }
       files[filePath].isReviewed = reviewedSet.has(filePath);
     }
 
     this._files = this._normalizeChangeFilesResponse(files);
   }
 
-  _computeFilesShown(numFilesShown, files) {
+  _computeFilesShown(
+    numFilesShown: number,
+    files: NormalizedFileInfo[]
+  ): NormalizedFileInfo[] | undefined {
     // Polymer 2: check for undefined
-    if ([numFilesShown, files].includes(undefined)) {
-      return undefined;
-    }
+    if (numFilesShown === undefined || files === undefined) return undefined;
 
-    const previousNumFilesShown = this._shownFiles ?
-      this._shownFiles.length : 0;
+    const previousNumFilesShown = this._shownFiles
+      ? this._shownFiles.length
+      : 0;
 
     const filesShown = files.slice(0, numFilesShown);
-    this.dispatchEvent(new CustomEvent('files-shown-changed', {
-      detail: {length: filesShown.length},
-      composed: true, bubbles: true,
-    }));
+    this.dispatchEvent(
+      new CustomEvent('files-shown-changed', {
+        detail: {length: filesShown.length},
+        composed: true,
+        bubbles: true,
+      })
+    );
 
     // Start the timer for the rendering work hwere because this is where the
     // _shownFiles property is being set, and _shownFiles is used in the
@@ -1103,8 +1335,10 @@
     this.reporting.time(RENDER_TIMING_LABEL);
 
     // How many more files are being shown (if it's an increase).
-    this._reportinShownFilesIncrement =
-        Math.max(0, filesShown.length - previousNumFilesShown);
+    this._reportinShownFilesIncrement = Math.max(
+      0,
+      filesShown.length - previousNumFilesShown
+    );
 
     return filesShown;
   }
@@ -1112,14 +1346,19 @@
   _updateDiffCursor() {
     // Overwrite the cursor's list of diffs:
     this.$.diffCursor.splice(
-        ...['diffs', 0, this.$.diffCursor.diffs.length].concat(this.diffs));
+      'diffs',
+      0,
+      this.$.diffCursor.diffs.length,
+      ...this.diffs
+    );
   }
 
   _filesChanged() {
     if (this._files && this._files.length > 0) {
       flush();
       this.$.fileCursor.stops = Array.from(
-          this.root.querySelectorAll(`.${FILE_ROW_CLASS}`));
+        this.root!.querySelectorAll(`.${FILE_ROW_CLASS}`)
+      );
       this.$.fileCursor.setCursorAtIndex(this.selectedIndex, true);
     }
   }
@@ -1128,30 +1367,36 @@
     this.numFilesShown += this.fileListIncrement;
   }
 
-  _computeFileListControlClass(numFilesShown, files) {
+  _computeFileListControlClass(
+    numFilesShown?: number,
+    files?: NormalizedFileInfo[]
+  ) {
+    if (numFilesShown === undefined || files === undefined) return 'invisible';
     return numFilesShown >= files.length ? 'invisible' : '';
   }
 
-  _computeIncrementText(numFilesShown, files) {
-    if (!files) { return ''; }
-    const text =
-        Math.min(this.fileListIncrement, files.length - numFilesShown);
-    return 'Show ' + text + ' more';
+  _computeIncrementText(numFilesShown?: number, files?: NormalizedFileInfo[]) {
+    if (numFilesShown === undefined || files === undefined) return '';
+    const text = Math.min(this.fileListIncrement, files.length - numFilesShown);
+    return `Show ${text} more`;
   }
 
-  _computeShowAllText(files) {
-    if (!files) { return ''; }
-    return 'Show all ' + files.length + ' files';
+  _computeShowAllText(files: NormalizedFileInfo[]) {
+    if (!files) {
+      return '';
+    }
+    return `Show all ${files.length} files`;
   }
 
-  _computeWarnShowAll(files) {
+  _computeWarnShowAll(files: NormalizedFileInfo[]) {
     return files.length > WARN_SHOW_ALL_THRESHOLD;
   }
 
-  _computeShowAllWarning(files) {
-    if (!this._computeWarnShowAll(files)) { return ''; }
-    return 'Warning: showing all ' + files.length +
-        ' files may take several seconds.';
+  _computeShowAllWarning(files: NormalizedFileInfo[]) {
+    if (!this._computeWarnShowAll(files)) {
+      return '';
+    }
+    return `Warning: showing all ${files.length} files may take several seconds.`;
   }
 
   _showAllFiles() {
@@ -1161,14 +1406,12 @@
   /**
    * Get a descriptive label for use in the status indicator's tooltip and
    * ARIA label.
-   *
-   * @param {string} status
-   * @return {string}
    */
-  _computeFileStatusLabel(status) {
+  _computeFileStatusLabel(status?: keyof typeof FileStatus) {
     const statusCode = this._computeFileStatus(status);
-    return FileStatus.hasOwnProperty(statusCode) ?
-      FileStatus[statusCode] : 'Status Unknown';
+    return hasOwnProperty(FileStatus, statusCode)
+      ? FileStatus[statusCode]
+      : 'Status Unknown';
   }
 
   /**
@@ -1178,23 +1421,32 @@
    * value. The aria-checked attribute is string attribute. Binding directly
    * to boolean variable causes problem on gerrit-CI.
    *
-   * @param {object} val
-   * @return {string} 'true' if val is true-like, otherwise false
+   * @return 'true' if val is true-like, otherwise false
    */
-  _booleanToString(val) {
+  _booleanToString(val?: unknown) {
     return val ? 'true' : 'false';
   }
 
-  _isFileExpanded(path, expandedFilesRecord) {
+  _isFileExpanded(
+    path: string | undefined,
+    expandedFilesRecord: ElementPropertyDeepChange<GrFileList, '_expandedFiles'>
+  ) {
     return expandedFilesRecord.base.some(f => f.path === path);
   }
 
-  _isFileExpandedStr(path, expandedFilesRecord) {
+  _isFileExpandedStr(
+    path: string | undefined,
+    expandedFilesRecord: ElementPropertyDeepChange<GrFileList, '_expandedFiles'>
+  ) {
     return this._booleanToString(
-        this._isFileExpanded(path, expandedFilesRecord));
+      this._isFileExpanded(path, expandedFilesRecord)
+    );
   }
 
-  _computeExpandedFiles(expandedCount, totalCount) {
+  private _computeExpandedFiles(
+    expandedCount: number,
+    totalCount: number
+  ): FilesExpandedState {
     if (expandedCount === 0) {
       return FilesExpandedState.NONE;
     } else if (expandedCount === totalCount) {
@@ -1209,25 +1461,32 @@
    * order by waiting for the previous diff to finish before starting the next
    * one.
    *
-   * @param {!Array} record The splice record in the expanded paths list.
+   * @param record The splice record in the expanded paths list.
    */
-  _expandedFilesChanged(record) {
+  @observe('_expandedFiles.splices')
+  _expandedFilesChanged(record?: PolymerSpliceChange<PatchSetFile[]>) {
     // Clear content for any diffs that are not open so if they get re-opened
     // the stale content does not flash before it is cleared and reloaded.
-    const collapsedDiffs = this.diffs.filter(diff =>
-      this._expandedFiles.findIndex(f => f.path === diff.path) === -1);
+    const collapsedDiffs = this.diffs.filter(
+      diff => this._expandedFiles.findIndex(f => f.path === diff.path) === -1
+    );
     this._clearCollapsedDiffs(collapsedDiffs);
 
-    if (!record) { return; } // Happens after "Collapse all" clicked.
+    if (!record) {
+      return;
+    } // Happens after "Collapse all" clicked.
 
     this.filesExpanded = this._computeExpandedFiles(
-        this._expandedFiles.length, this._files.length);
+      this._expandedFiles.length,
+      this._files.length
+    );
 
     // Find the paths introduced by the new index splices:
     const newFiles = record.indexSplices
-        .map(splice => splice.object.slice(
-            splice.index, splice.index + splice.addedCount))
-        .reduce((acc, paths) => acc.concat(paths), []);
+      .map(splice =>
+        splice.object.slice(splice.index, splice.index + splice.addedCount)
+      )
+      .reduce((acc, paths) => acc.concat(paths), []);
 
     // Required so that the newly created diff view is included in this.diffs.
     flush();
@@ -1242,7 +1501,7 @@
     this.$.diffCursor.reInitAndUpdateStops();
   }
 
-  _clearCollapsedDiffs(collapsedDiffs) {
+  private _clearCollapsedDiffs(collapsedDiffs: GrDiffHost[]) {
     for (const diff of collapsedDiffs) {
       diff.cancel();
       diff.clearDiffContent();
@@ -1254,13 +1513,14 @@
    * for each path in order, awaiting the previous render to complete before
    * continuing.
    *
-   * @param  {!Array<Gerrit.FileRange>} files
-   * @param  {!NodeList<!Object>} diffElements (GrDiffHostElement)
-   * @param  {number} initialCount The total number of paths in the pass. This
-   *   is used to generate log messages.
-   * @return {!Promise}
+   * @param initialCount The total number of paths in the pass. This
+   * is used to generate log messages.
    */
-  _renderInOrder(files, diffElements, initialCount) {
+  private _renderInOrder(
+    files: PatchSetFile[],
+    diffElements: GrDiffHost[],
+    initialCount: number
+  ) {
     let iter = 0;
 
     for (const file of files) {
@@ -1271,37 +1531,50 @@
       }
     }
 
-    return (new Promise(resolve => {
-      this.dispatchEvent(new CustomEvent('reload-drafts', {
-        detail: {resolve},
-        composed: true, bubbles: true,
-      }));
-    })).then(() => asyncForeach(files, (file, cancel) => {
-      const path = file.path;
-      this._cancelForEachDiff = cancel;
+    return new Promise(resolve => {
+      this.dispatchEvent(
+        new CustomEvent('reload-drafts', {
+          detail: {resolve},
+          composed: true,
+          bubbles: true,
+        })
+      );
+    }).then(() =>
+      asyncForeach(files, (file, cancel) => {
+        const path = file.path;
+        this._cancelForEachDiff = cancel;
 
-      iter++;
-      console.info('Expanding diff', iter, 'of', initialCount, ':',
-          path);
-      const diffElem = this._findDiffByPath(path, diffElements);
-      if (!diffElem) {
-        console.warn(`Did not find <gr-diff-host> element for ${path}`);
-        return Promise.resolve();
-      }
-      diffElem.comments = this.changeComments.getCommentsBySideForFile(
-          file, this.patchRange, this.projectConfig);
-      const promises = [diffElem.reload()];
-      if (this._loggedIn && !this.diffPrefs.manual_review) {
-        promises.push(this._reviewFile(path, true));
-      }
-      return Promise.all(promises);
-    }).then(() => {
-      this._cancelForEachDiff = null;
-      this._nextRenderParams = null;
-      console.info('Finished expanding', initialCount, 'diff(s)');
-      this.reporting.timeEndWithAverage(EXPAND_ALL_TIMING_LABEL,
-          EXPAND_ALL_AVG_TIMING_LABEL, initialCount);
-      /* Block diff cursor from auto scrolling after files are done rendering.
+        iter++;
+        console.info('Expanding diff', iter, 'of', initialCount, ':', path);
+        const diffElem = this._findDiffByPath(path, diffElements);
+        if (!diffElem) {
+          console.warn(`Did not find <gr-diff-host> element for ${path}`);
+          return Promise.resolve();
+        }
+        if (!this.changeComments || !this.patchRange || !this.diffPrefs) {
+          throw new Error(
+            'changeComments, patchRange and diffPrefs must be set'
+          );
+        }
+        diffElem.comments = this.changeComments.getCommentsBySideForFile(
+          file,
+          this.patchRange,
+          this.projectConfig
+        );
+        const promises: Array<Promise<unknown>> = [diffElem.reload()];
+        if (this._loggedIn && !this.diffPrefs.manual_review) {
+          promises.push(this._reviewFile(path, true));
+        }
+        return Promise.all(promises);
+      }).then(() => {
+        this._cancelForEachDiff = undefined;
+        console.info('Finished expanding', initialCount, 'diff(s)');
+        this.reporting.timeEndWithAverage(
+          EXPAND_ALL_TIMING_LABEL,
+          EXPAND_ALL_AVG_TIMING_LABEL,
+          initialCount
+        );
+        /* Block diff cursor from auto scrolling after files are done rendering.
        * This prevents the bug where the screen jumps to the first diff chunk
        * after files are done being rendered after the user has already begun
        * scrolling.
@@ -1313,45 +1586,54 @@
        * prevented the issue of scrolling to top when we expand the second
        * file individually.
        */
-      this.$.diffCursor.reInitAndUpdateStops();
-    }));
+        this.$.diffCursor.reInitAndUpdateStops();
+      })
+    );
   }
 
   /** Cancel the rendering work of every diff in the list */
   _cancelDiffs() {
-    if (this._cancelForEachDiff) { this._cancelForEachDiff(); }
+    if (this._cancelForEachDiff) {
+      this._cancelForEachDiff();
+    }
     this._forEachDiff(d => d.cancel());
   }
 
   /**
    * In the given NodeList of diff elements, find the diff for the given path.
-   *
-   * @param  {string} path
-   * @param  {!NodeList<!Object>} diffElements (GrDiffElement)
-   * @return {!Object|undefined} (GrDiffElement)
    */
-  _findDiffByPath(path, diffElements) {
+  private _findDiffByPath(path: string, diffElements: GrDiffHost[]) {
     for (let i = 0; i < diffElements.length; i++) {
       if (diffElements[i].path === path) {
         return diffElements[i];
       }
     }
+    return undefined;
   }
 
   /**
    * Reset the comments of a modified thread
-   *
-   * @param  {string} rootId
-   * @param  {string} path
    */
-  reloadCommentsForThreadWithRootId(rootId, path) {
+  reloadCommentsForThreadWithRootId(rootId: UrlEncodedCommentId, path: string) {
     // Don't bother continuing if we already know that the path that contains
     // the updated comment thread is not expanded.
-    if (!this._expandedFiles.some(f => f.path === path)) { return; }
+    if (!this._expandedFiles.some(f => f.path === path)) {
+      return;
+    }
     const diff = this.diffs.find(d => d.path === path);
 
+    if (!diff) {
+      throw new Error("Can't find diff by path");
+    }
+
     const threadEl = diff.getThreadEls().find(t => t.rootId === rootId);
-    if (!threadEl) { return; }
+    if (!threadEl) {
+      return;
+    }
+
+    if (!this.changeComments) {
+      throw new Error('changeComments must be set');
+    }
 
     const newComments = this.changeComments.getCommentsForThread(rootId);
 
@@ -1369,15 +1651,16 @@
     // comments due to use in the _handleCommentUpdate function.
     // The comment thread already has a side associated with it, so
     // set the comment's side to match.
-    threadEl.comments = newComments.map(c => Object.assign(
-        c, {__commentSide: threadEl.commentSide}
-    ));
+    threadEl.comments = newComments.map(c =>
+      Object.assign(c, {__commentSide: threadEl.commentSide})
+    );
     flush();
   }
 
-  _handleEscKey(e) {
-    if (this.shouldSuppressKeyboardShortcut(e) ||
-        this.modifierPressed(e)) { return; }
+  _handleEscKey(e: CustomKeyboardEvent) {
+    if (this.shouldSuppressKeyboardShortcut(e) || this.modifierPressed(e)) {
+      return;
+    }
     e.preventDefault();
     this._displayLine = false;
   }
@@ -1386,72 +1669,68 @@
    * Update the loading class for the file list rows. The update is inside a
    * debouncer so that the file list doesn't flash gray when the API requests
    * are reasonably fast.
-   *
-   * @param {boolean} loading
    */
-  _loadingChanged(loading) {
-    this.debounce('loading-change', () => {
-      // Only show set the loading if there have been files loaded to show. In
-      // this way, the gray loading style is not shown on initial loads.
-      this.classList.toggle('loading', loading && !!this._files.length);
-    }, LOADING_DEBOUNCE_INTERVAL);
+  _loadingChanged(loading?: boolean) {
+    this.debounce(
+      'loading-change',
+      () => {
+        // Only show set the loading if there have been files loaded to show. In
+        // this way, the gray loading style is not shown on initial loads.
+        this.classList.toggle('loading', loading && !!this._files.length);
+      },
+      LOADING_DEBOUNCE_INTERVAL
+    );
   }
 
-  _editModeChanged(editMode) {
+  _editModeChanged(editMode?: boolean) {
     this.classList.toggle('editMode', editMode);
   }
 
-  _computeReviewedClass(isReviewed) {
+  _computeReviewedClass(isReviewed?: boolean) {
     return isReviewed ? 'isReviewed' : '';
   }
 
-  _computeReviewedText(isReviewed) {
+  _computeReviewedText(isReviewed?: boolean) {
     return isReviewed ? 'MARK UNREVIEWED' : 'MARK REVIEWED';
   }
 
   /**
    * Given a file path, return whether that path should have visible size bars
    * and be included in the size bars calculation.
-   *
-   * @param {string} path
-   * @return {boolean}
    */
-  _showBarsForPath(path) {
-    return path !== SpecialFilePath.COMMIT_MESSAGE &&
-      path !== SpecialFilePath.MERGE_LIST;
+  _showBarsForPath(path?: string) {
+    return (
+      path !== SpecialFilePath.COMMIT_MESSAGE &&
+      path !== SpecialFilePath.MERGE_LIST
+    );
   }
 
   /**
    * Compute size bar layout values from the file list.
-   *
-   * @return {Gerrit.LayoutStats|undefined}
-   *
    */
-  _computeSizeBarLayout(shownFilesRecord) {
-    if (!shownFilesRecord || !shownFilesRecord.base) { return undefined; }
-    const stats = {
-      maxInserted: 0,
-      maxDeleted: 0,
-      maxAdditionWidth: 0,
-      maxDeletionWidth: 0,
-      deletionOffset: 0,
-    };
+  _computeSizeBarLayout(
+    shownFilesRecord?: ElementPropertyDeepChange<GrFileList, '_shownFiles'>
+  ) {
+    const stats: SizeBarLayout = createDefaultSizeBarLayout();
+    if (!shownFilesRecord || !shownFilesRecord.base) {
+      return stats;
+    }
     shownFilesRecord.base
-        .filter(f => this._showBarsForPath(f.__path))
-        .forEach(f => {
-          if (f.lines_inserted) {
-            stats.maxInserted = Math.max(stats.maxInserted, f.lines_inserted);
-          }
-          if (f.lines_deleted) {
-            stats.maxDeleted = Math.max(stats.maxDeleted, f.lines_deleted);
-          }
-        });
+      .filter(f => this._showBarsForPath(f.__path))
+      .forEach(f => {
+        if (f.lines_inserted) {
+          stats.maxInserted = Math.max(stats.maxInserted, f.lines_inserted);
+        }
+        if (f.lines_deleted) {
+          stats.maxDeleted = Math.max(stats.maxDeleted, f.lines_deleted);
+        }
+      });
     const ratio = stats.maxInserted / (stats.maxInserted + stats.maxDeleted);
     if (!isNaN(ratio)) {
       stats.maxAdditionWidth =
-          (SIZE_BAR_MAX_WIDTH - SIZE_BAR_GAP_WIDTH) * ratio;
+        (SIZE_BAR_MAX_WIDTH - SIZE_BAR_GAP_WIDTH) * ratio;
       stats.maxDeletionWidth =
-          SIZE_BAR_MAX_WIDTH - SIZE_BAR_GAP_WIDTH - stats.maxAdditionWidth;
+        SIZE_BAR_MAX_WIDTH - SIZE_BAR_GAP_WIDTH - stats.maxAdditionWidth;
       stats.deletionOffset = stats.maxAdditionWidth + SIZE_BAR_GAP_WIDTH;
     }
     return stats;
@@ -1459,68 +1738,60 @@
 
   /**
    * Get the width of the addition bar for a file.
-   *
-   * @param {Object} file
-   * @param {Gerrit.LayoutStats} stats
-   * @return {number}
    */
-  _computeBarAdditionWidth(file, stats) {
-    if (stats.maxInserted === 0 ||
-        !file.lines_inserted ||
-        !this._showBarsForPath(file.__path)) {
+  _computeBarAdditionWidth(file?: NormalizedFileInfo, stats?: SizeBarLayout) {
+    if (
+      !file ||
+      !stats ||
+      stats.maxInserted === 0 ||
+      !file.lines_inserted ||
+      !this._showBarsForPath(file.__path)
+    ) {
       return 0;
     }
     const width =
-        stats.maxAdditionWidth * file.lines_inserted / stats.maxInserted;
+      (stats.maxAdditionWidth * file.lines_inserted) / stats.maxInserted;
     return width === 0 ? 0 : Math.max(SIZE_BAR_MIN_WIDTH, width);
   }
 
   /**
    * Get the x-offset of the addition bar for a file.
-   *
-   * @param {Object} file
-   * @param {Gerrit.LayoutStats} stats
-   * @return {number}
    */
-  _computeBarAdditionX(file, stats) {
-    return stats.maxAdditionWidth -
-        this._computeBarAdditionWidth(file, stats);
+  _computeBarAdditionX(file?: NormalizedFileInfo, stats?: SizeBarLayout) {
+    if (!file || !stats) return;
+    return stats.maxAdditionWidth - this._computeBarAdditionWidth(file, stats);
   }
 
   /**
    * Get the width of the deletion bar for a file.
-   *
-   * @param {Object} file
-   * @param {Gerrit.LayoutStats} stats
-   * @return {number}
    */
-  _computeBarDeletionWidth(file, stats) {
-    if (stats.maxDeleted === 0 ||
-        !file.lines_deleted ||
-        !this._showBarsForPath(file.__path)) {
+  _computeBarDeletionWidth(file?: NormalizedFileInfo, stats?: SizeBarLayout) {
+    if (
+      !file ||
+      !stats ||
+      stats.maxDeleted === 0 ||
+      !file.lines_deleted ||
+      !this._showBarsForPath(file.__path)
+    ) {
       return 0;
     }
     const width =
-        stats.maxDeletionWidth * file.lines_deleted / stats.maxDeleted;
+      (stats.maxDeletionWidth * file.lines_deleted) / stats.maxDeleted;
     return width === 0 ? 0 : Math.max(SIZE_BAR_MIN_WIDTH, width);
   }
 
   /**
    * Get the x-offset of the deletion bar for a file.
-   *
-   * @param {Gerrit.LayoutStats} stats
-   *
-   * @return {number}
    */
-  _computeBarDeletionX(stats) {
+  _computeBarDeletionX(stats: SizeBarLayout) {
     return stats.deletionOffset;
   }
 
-  _computeShowSizeBars(userPrefs) {
-    return !!userPrefs.size_bar_in_change_table;
+  _computeShowSizeBars(userPrefs?: PreferencesInfo) {
+    return !!userPrefs?.size_bar_in_change_table;
   }
 
-  _computeSizeBarsClass(showSizeBars, path) {
+  _computeSizeBarsClass(showSizeBars?: boolean, path?: string) {
     let hideClass = '';
     if (!showSizeBars) {
       hideClass = 'hide';
@@ -1537,11 +1808,18 @@
    * dependencies between dynamic endpoints.
    */
   _computeShowDynamicColumns(
-      headerEndpoints, contentEndpoints, summaryEndpoints) {
-    return headerEndpoints && contentEndpoints && summaryEndpoints &&
-           headerEndpoints.length &&
-           headerEndpoints.length === contentEndpoints.length &&
-           headerEndpoints.length === summaryEndpoints.length;
+    headerEndpoints?: string,
+    contentEndpoints?: string,
+    summaryEndpoints?: string
+  ) {
+    return (
+      headerEndpoints &&
+      contentEndpoints &&
+      summaryEndpoints &&
+      headerEndpoints.length &&
+      headerEndpoints.length === contentEndpoints.length &&
+      headerEndpoints.length === summaryEndpoints.length
+    );
   }
 
   /**
@@ -1549,16 +1827,19 @@
    * endpoints are registered the exact same number of times.
    */
   _computeShowPrependedDynamicColumns(
-      headerEndpoints, contentEndpoints) {
-    return headerEndpoints && contentEndpoints &&
-           headerEndpoints.length &&
-           headerEndpoints.length === contentEndpoints.length;
+    headerEndpoints?: string,
+    contentEndpoints?: string
+  ) {
+    return (
+      headerEndpoints &&
+      contentEndpoints &&
+      headerEndpoints.length &&
+      headerEndpoints.length === contentEndpoints.length
+    );
   }
 
   /**
    * Returns true if none of the inline diffs have been expanded.
-   *
-   * @return {boolean}
    */
   _noDiffsExpanded() {
     return this.filesExpanded === FilesExpandedState.NONE;
@@ -1569,20 +1850,22 @@
    * allows approximate detection of when the dom-repeat has completed
    * rendering.
    *
-   * @param {number} index The index of the row being rendered.
-   * @return {string} an empty string.
+   * @param index The index of the row being rendered.
    */
-  _reportRenderedRow(index) {
+  _reportRenderedRow(index: number) {
     if (index === this._shownFiles.length - 1) {
       this.async(() => {
-        this.reporting.timeEndWithAverage(RENDER_TIMING_LABEL,
-            RENDER_AVG_TIMING_LABEL, this._reportinShownFilesIncrement);
+        this.reporting.timeEndWithAverage(
+          RENDER_TIMING_LABEL,
+          RENDER_AVG_TIMING_LABEL,
+          this._reportinShownFilesIncrement
+        );
       }, 1);
     }
     return '';
   }
 
-  _reviewedTitle(reviewed) {
+  _reviewedTitle(reviewed?: boolean) {
     if (reviewed) {
       return 'Mark as not reviewed (shortcut: r)';
     }
@@ -1599,16 +1882,20 @@
   /**
    * Wrapper for using in the element template and computed properties
    */
-  _computeDisplayPath(path) {
+  _computeDisplayPath(path: string) {
     return computeDisplayPath(path);
   }
 
   /**
    * Wrapper for using in the element template and computed properties
    */
-  _computeTruncatedPath(path) {
+  _computeTruncatedPath(path: string) {
     return computeTruncatedPath(path);
   }
 }
 
-customElements.define(GrFileList.is, GrFileList);
+declare global {
+  interface HTMLElementTagNameMap {
+    'gr-file-list': GrFileList;
+  }
+}
diff --git a/polygerrit-ui/app/elements/change/gr-file-list/gr-file-list_html.ts b/polygerrit-ui/app/elements/change/gr-file-list/gr-file-list_html.ts
index a577eb6..1c30a65 100644
--- a/polygerrit-ui/app/elements/change/gr-file-list/gr-file-list_html.ts
+++ b/polygerrit-ui/app/elements/change/gr-file-list/gr-file-list_html.ts
@@ -375,7 +375,7 @@
       <div class="stickyArea">
         <div
           class$="file-row row [[_computePathClass(file.__path, _expandedFiles.*)]]"
-          data-file$="[[_computeFileRange(file)]]"
+          data-file$="[[_computePatchSetFile(file)]]"
           tabindex="-1"
           role="row"
         >
@@ -657,7 +657,7 @@
             hidden="[[!_isFileExpanded(file.__path, _expandedFiles.*)]]"
             change-num="[[changeNum]]"
             patch-range="[[patchRange]]"
-            file="[[_computeFileRange(file)]]"
+            file="[[_computePatchSetFile(file)]]"
             path="[[file.__path]]"
             prefs="[[diffPrefs]]"
             project-name="[[change.project]]"
diff --git a/polygerrit-ui/app/elements/change/gr-file-list/gr-file-list_test.js b/polygerrit-ui/app/elements/change/gr-file-list/gr-file-list_test.js
index 8aebf9b..129b65e 100644
--- a/polygerrit-ui/app/elements/change/gr-file-list/gr-file-list_test.js
+++ b/polygerrit-ui/app/elements/change/gr-file-list/gr-file-list_test.js
@@ -312,7 +312,7 @@
 
       for (const bytes in table) {
         if (table.hasOwnProperty(bytes)) {
-          assert.equal(element._formatBytes(bytes), table[bytes]);
+          assert.equal(element._formatBytes(Number(bytes)), table[bytes]);
         }
       }
     });
@@ -1329,15 +1329,23 @@
 
   suite('size bars', () => {
     test('_computeSizeBarLayout', () => {
-      assert.isUndefined(element._computeSizeBarLayout(null));
-      assert.isUndefined(element._computeSizeBarLayout({}));
-      assert.deepEqual(element._computeSizeBarLayout({base: []}), {
+      const defaultSizeBarLayout = {
         maxInserted: 0,
         maxDeleted: 0,
         maxAdditionWidth: 0,
         maxDeletionWidth: 0,
         deletionOffset: 0,
-      });
+      };
+
+      assert.deepEqual(
+          element._computeSizeBarLayout(null),
+          defaultSizeBarLayout);
+      assert.deepEqual(
+          element._computeSizeBarLayout({}),
+          defaultSizeBarLayout);
+      assert.deepEqual(
+          element._computeSizeBarLayout({base: []}),
+          defaultSizeBarLayout);
 
       const files = [
         {__path: '/COMMIT_MSG', lines_inserted: 10000},
diff --git a/polygerrit-ui/app/elements/core/gr-navigation/gr-navigation.ts b/polygerrit-ui/app/elements/core/gr-navigation/gr-navigation.ts
index 0bb334a..8470611 100644
--- a/polygerrit-ui/app/elements/core/gr-navigation/gr-navigation.ts
+++ b/polygerrit-ui/app/elements/core/gr-navigation/gr-navigation.ts
@@ -32,6 +32,7 @@
   ParentPatchSetNum,
   ServerInfo,
 } from '../../../types/common';
+import {ParsedChangeInfo} from '../../shared/gr-rest-api-interface/gr-reviewer-updates-parser';
 
 // Navigation parameters object format:
 //
@@ -664,7 +665,7 @@
    * @param basePatchNum The string 'PARENT' can be used for none.
    */
   getUrlForDiff(
-    change: ChangeInfo,
+    change: ChangeInfo | ParsedChangeInfo,
     filePath: string,
     patchNum?: PatchSetNum,
     basePatchNum?: PatchSetNum,
@@ -723,7 +724,7 @@
   },
 
   getEditUrlForDiff(
-    change: ChangeInfo,
+    change: ChangeInfo | ParsedChangeInfo,
     filePath: string,
     patchNum?: PatchSetNum,
     lineNum?: number
@@ -763,7 +764,7 @@
    * @param basePatchNum The string 'PARENT' can be used for none.
    */
   navigateToDiff(
-    change: ChangeInfo,
+    change: ChangeInfo | ParsedChangeInfo,
     filePath: string,
     patchNum?: PatchSetNum,
     basePatchNum?: PatchSetNum,
diff --git a/polygerrit-ui/app/elements/shared/gr-comment-thread/gr-comment-thread.ts b/polygerrit-ui/app/elements/shared/gr-comment-thread/gr-comment-thread.ts
index de283d8..a9df3f9 100644
--- a/polygerrit-ui/app/elements/shared/gr-comment-thread/gr-comment-thread.ts
+++ b/polygerrit-ui/app/elements/shared/gr-comment-thread/gr-comment-thread.ts
@@ -143,7 +143,7 @@
     notify: true,
     computed: '_computeRootId(comments.*)',
   })
-  rootId?: string;
+  rootId?: UrlEncodedCommentId;
 
   @property({type: Boolean})
   showFilePath = false;
diff --git a/polygerrit-ui/app/elements/shared/gr-rest-api-interface/gr-rest-api-interface.ts b/polygerrit-ui/app/elements/shared/gr-rest-api-interface/gr-rest-api-interface.ts
index d1459a3..70a1ace 100644
--- a/polygerrit-ui/app/elements/shared/gr-rest-api-interface/gr-rest-api-interface.ts
+++ b/polygerrit-ui/app/elements/shared/gr-rest-api-interface/gr-rest-api-interface.ts
@@ -1603,7 +1603,10 @@
     }) as Promise<string[] | undefined>;
   }
 
-  getChangeOrEditFiles(changeNum: NumericChangeId, patchRange: PatchRange) {
+  getChangeOrEditFiles(
+    changeNum: NumericChangeId,
+    patchRange: PatchRange
+  ): Promise<FileNameToFileInfoMap | undefined> {
     if (patchNumEquals(patchRange.patchNum, EditPatchSetNum)) {
       return this.getChangeEditFiles(changeNum, patchRange).then(
         res => res && res.files
@@ -2086,13 +2089,16 @@
     }) as Promise<ChangeInfo[] | undefined>;
   }
 
-  getReviewedFiles(changeNum: NumericChangeId, patchNum: PatchSetNum) {
+  getReviewedFiles(
+    changeNum: NumericChangeId,
+    patchNum: PatchSetNum
+  ): Promise<string[] | undefined> {
     return this._getChangeURLAndFetch({
       changeNum,
       endpoint: '/files?reviewed',
       patchNum,
       reportEndpointAsIs: true,
-    });
+    }) as Promise<string[] | undefined>;
   }
 
   saveFileReviewed(
diff --git a/polygerrit-ui/app/mixins/keyboard-shortcut-mixin/keyboard-shortcut-mixin.ts b/polygerrit-ui/app/mixins/keyboard-shortcut-mixin/keyboard-shortcut-mixin.ts
index 80df630..b255ea5 100644
--- a/polygerrit-ui/app/mixins/keyboard-shortcut-mixin/keyboard-shortcut-mixin.ts
+++ b/polygerrit-ui/app/mixins/keyboard-shortcut-mixin/keyboard-shortcut-mixin.ts
@@ -1082,7 +1082,7 @@
   _shortcut_v_key_last_pressed: number | null;
   _shortcut_go_table: Map<string, string>;
   _shortcut_v_table: Map<string, string>;
-  keyboardShortcuts(): {[key: string]: string};
+  keyboardShortcuts(): {[key: string]: string | null};
   createTitle(name: Shortcut, section: ShortcutSection): string;
   bindShortcut(shortcut: Shortcut, ...bindings: string[]): void;
   shouldSuppressKeyboardShortcut(event: CustomKeyboardEvent): boolean;
diff --git a/polygerrit-ui/app/services/services/gr-rest-api/gr-rest-api.ts b/polygerrit-ui/app/services/services/gr-rest-api/gr-rest-api.ts
index 3082aab..bf8b25a 100644
--- a/polygerrit-ui/app/services/services/gr-rest-api/gr-rest-api.ts
+++ b/polygerrit-ui/app/services/services/gr-rest-api/gr-rest-api.ts
@@ -96,6 +96,7 @@
   DashboardId,
   HashtagsInput,
   Hashtag,
+  FileNameToFileInfoMap,
 } from '../../../types/common';
 import {ParsedChangeInfo} from '../../../elements/shared/gr-rest-api-interface/gr-reviewer-updates-parser';
 import {HttpMethod, IgnoreWhitespaceType} from '../../../constants/constants';
@@ -815,4 +816,29 @@
     changeNum: NumericChangeId,
     topic: string | null
   ): Promise<string>;
+
+  getChangeOrEditFiles(
+    changeNum: NumericChangeId,
+    patchRange: PatchRange
+  ): Promise<FileNameToFileInfoMap | undefined>;
+
+  getReviewedFiles(
+    changeNum: NumericChangeId,
+    patchNum: PatchSetNum
+  ): Promise<string[] | undefined>;
+
+  saveFileReviewed(
+    changeNum: NumericChangeId,
+    patchNum: PatchSetNum,
+    path: string,
+    reviewed: boolean
+  ): Promise<Response>;
+
+  saveFileReviewed(
+    changeNum: NumericChangeId,
+    patchNum: PatchSetNum,
+    path: string,
+    reviewed: boolean,
+    errFn: ErrorCallback
+  ): Promise<Response | undefined>;
 }
diff --git a/polygerrit-ui/app/utils/async-util.ts b/polygerrit-ui/app/utils/async-util.ts
index 7e63f70..119b09b 100644
--- a/polygerrit-ui/app/utils/async-util.ts
+++ b/polygerrit-ui/app/utils/async-util.ts
@@ -26,7 +26,7 @@
  */
 export function asyncForeach<T>(
   array: T[],
-  fn: (item: T, stopCallback: () => void) => Promise<T>
+  fn: (item: T, stopCallback: () => void) => Promise<unknown>
 ): Promise<T | void> {
   if (!array.length) {
     return Promise.resolve();
diff --git a/polygerrit-ui/app/utils/dom-util.ts b/polygerrit-ui/app/utils/dom-util.ts
index ae6d616..76db40b 100644
--- a/polygerrit-ui/app/utils/dom-util.ts
+++ b/polygerrit-ui/app/utils/dom-util.ts
@@ -211,13 +211,13 @@
 export function descendedFromClass(
   element: Element,
   className: string,
-  opt_stopElement: Element
+  stopElement?: Element
 ) {
   let isDescendant = element.classList.contains(className);
   while (
     !isDescendant &&
     element.parentElement &&
-    (!opt_stopElement || element.parentElement !== opt_stopElement)
+    (!stopElement || element.parentElement !== stopElement)
   ) {
     isDescendant = element.classList.contains(className);
     element = element.parentElement;