diff --git a/polygerrit-ui/app/elements/change/gr-change-view/gr-change-view.ts b/polygerrit-ui/app/elements/change/gr-change-view/gr-change-view.ts
index c33eb26..8f74f76 100644
--- a/polygerrit-ui/app/elements/change/gr-change-view/gr-change-view.ts
+++ b/polygerrit-ui/app/elements/change/gr-change-view/gr-change-view.ts
@@ -118,7 +118,7 @@
   GrCommentApi,
   ChangeComments,
 } from '../../diff/gr-comment-api/gr-comment-api';
-import {hasOwnProperty} from '../../../utils/common-util';
+import {assertIsDefined, hasOwnProperty} from '../../../utils/common-util';
 import {GrEditControls} from '../../edit/gr-edit-controls/gr-edit-controls';
 import {
   CommentThread,
@@ -883,7 +883,7 @@
   }
 
   _handleCommitMessageSave(e: EditableContentSaveEvent) {
-    if (!this._change) throw new Error('missing required change property');
+    assertIsDefined(this._change, '_change');
     if (!this._changeNum)
       throw new Error('missing required changeNum property');
     // Trim trailing whitespace from each line.
@@ -1424,7 +1424,7 @@
   }
 
   _handleMessageAnchorTap(e: CustomEvent<{id: string}>) {
-    if (!this._change) throw new Error('missing required change property');
+    assertIsDefined(this._change, '_change');
     if (!this._patchRange)
       throw new Error('missing required _patchRange property');
     const hash = MSG_PREFIX + e.detail.id;
@@ -1706,7 +1706,7 @@
     if (this.shouldSuppressKeyboardShortcut(e)) {
       return;
     }
-    if (!this._change) throw new Error('missing required change property');
+    assertIsDefined(this._change, '_change');
     if (!this._patchRange)
       throw new Error('missing required _patchRange property');
     if (this._patchRange.basePatchNum === ParentPatchSetNum) {
@@ -1720,7 +1720,7 @@
     if (this.shouldSuppressKeyboardShortcut(e)) {
       return;
     }
-    if (!this._change) throw new Error('missing required change property');
+    assertIsDefined(this._change, '_change');
     if (!this._patchRange)
       throw new Error('missing required _patchRange property');
     if (this._patchRange.basePatchNum === ParentPatchSetNum) {
@@ -1734,7 +1734,7 @@
     if (this.shouldSuppressKeyboardShortcut(e)) {
       return;
     }
-    if (!this._change) throw new Error('missing required change property');
+    assertIsDefined(this._change, '_change');
     if (!this._patchRange)
       throw new Error('missing required _patchRange property');
     const latestPatchNum = computeLatestPatchNum(this._allPatchSets);
@@ -1753,7 +1753,7 @@
     if (this.shouldSuppressKeyboardShortcut(e)) {
       return;
     }
-    if (!this._change) throw new Error('missing required change property');
+    assertIsDefined(this._change, '_change');
     const latestPatchNum = computeLatestPatchNum(this._allPatchSets);
     if (!this._patchRange)
       throw new Error('missing required _patchRange property');
@@ -1772,7 +1772,7 @@
     if (this.shouldSuppressKeyboardShortcut(e)) {
       return;
     }
-    if (!this._change) throw new Error('missing required change property');
+    assertIsDefined(this._change, '_change');
     if (!this._patchRange)
       throw new Error('missing required _patchRange property');
     const latestPatchNum = computeLatestPatchNum(this._allPatchSets);
@@ -1918,7 +1918,7 @@
   }
 
   _getProjectConfig() {
-    if (!this._change) throw new Error('missing required change property');
+    assertIsDefined(this._change, '_change');
     return this.restApiService
       .getProjectConfig(this._change.project)
       .then(config => {
@@ -2314,7 +2314,7 @@
    * (`this._patchRange`) being defined.
    */
   _reloadPatchNumDependentResources(rightPatchNumChanged?: boolean) {
-    if (!this._changeNum) throw new Error('missing changeNum');
+    assertIsDefined(this._changeNum, '_changeNum');
     if (!this._patchRange?.patchNum) throw new Error('missing patchNum');
     const promises = [this._getCommitInfo(), this.$.fileList.reload()];
     if (rightPatchNumChanged)
@@ -2554,7 +2554,7 @@
     }
 
     this._updateCheckTimerHandle = this.async(() => {
-      if (!this._change) throw new Error('missing required change property');
+      assertIsDefined(this._change, '_change');
       const change = this._change;
       fetchChangeUpdates(change, this.restApiService).then(result => {
         let toastMessage = null;
@@ -2662,7 +2662,7 @@
       GrEditControls
     >('#editControls');
     if (!controls) throw new Error('Missing edit controls');
-    if (!this._change) throw new Error('missing required change property');
+    assertIsDefined(this._change, '_change');
     if (!this._patchRange)
       throw new Error('missing required _patchRange property');
     const path = e.detail.path;
@@ -2697,7 +2697,7 @@
     if (!this._selectedRevision) {
       return;
     }
-    if (!this._change) throw new Error('missing required change property');
+    assertIsDefined(this._change, '_change');
 
     let patchNum: PatchSetNum;
     if (patchNumStr === 'edit') {
@@ -2745,7 +2745,7 @@
   }
 
   _handleStopEditTap() {
-    if (!this._change) throw new Error('missing required change property');
+    assertIsDefined(this._change, '_change');
     if (!this._patchRange)
       throw new Error('missing required _patchRange property');
     GerritNav.navigateToChange(this._change, this._patchRange.patchNum);
diff --git a/polygerrit-ui/app/elements/change/gr-reply-dialog/gr-reply-dialog.ts b/polygerrit-ui/app/elements/change/gr-reply-dialog/gr-reply-dialog.ts
index 36764f3..705a402 100644
--- a/polygerrit-ui/app/elements/change/gr-reply-dialog/gr-reply-dialog.ts
+++ b/polygerrit-ui/app/elements/change/gr-reply-dialog/gr-reply-dialog.ts
@@ -92,6 +92,7 @@
 } from '@polymer/polymer/interfaces';
 import {
   areSetsEqual,
+  assertIsDefined,
   assertNever,
   containsAll,
 } from '../../../utils/common-util';
@@ -433,7 +434,7 @@
   }
 
   open(focusTarget?: FocusTarget) {
-    if (!this.change) throw new Error('missing required change property');
+    assertIsDefined(this.change, 'change');
     this.knownLatestState = LatestPatchState.CHECKING;
     fetchChangeUpdates(this.change, this.restApiService).then(result => {
       this.knownLatestState = result.isLatest
@@ -605,7 +606,7 @@
     account: AccountInfoInput | GroupInfoInput,
     type: ReviewerType
   ) {
-    if (!this.change) throw new Error('missing required change property');
+    assertIsDefined(this.change, 'change');
     if (account._pendingAdd || !isAccount(account)) {
       return;
     }
@@ -1213,7 +1214,7 @@
   }
 
   cancel() {
-    if (!this.change) throw new Error('missing required change property');
+    assertIsDefined(this.change, 'change');
     if (!this._owner) throw new Error('missing required _owner property');
     this.dispatchEvent(
       new CustomEvent('cancel', {
@@ -1269,8 +1270,8 @@
   }
 
   _saveReview(review: ReviewInput, errFn?: ErrorCallback) {
-    if (!this.change) throw new Error('missing required change property');
-    if (!this.patchNum) throw new Error('missing required patchNum property');
+    assertIsDefined(this.change, 'change');
+    assertIsDefined(this.patchNum, 'patchNum');
     return this.restApiService.saveChangeReview(
       this.change._number,
       this.patchNum,
@@ -1316,7 +1317,7 @@
   }
 
   _getStorageLocation(): StorageLocation {
-    if (!this.change) throw new Error('missing required change property');
+    assertIsDefined(this.change, 'change');
     return {
       changeNum: this.change._number,
       patchNum: '@change',
diff --git a/polygerrit-ui/app/elements/diff/gr-diff-host/gr-diff-host.ts b/polygerrit-ui/app/elements/diff/gr-diff-host/gr-diff-host.ts
index ff60531..47b4a1f 100644
--- a/polygerrit-ui/app/elements/diff/gr-diff-host/gr-diff-host.ts
+++ b/polygerrit-ui/app/elements/diff/gr-diff-host/gr-diff-host.ts
@@ -79,6 +79,7 @@
   fireEvent,
 } from '../../../utils/event-util';
 import {getPluginLoader} from '../../shared/gr-js-api-interface/gr-plugin-loader';
+import {assertIsDefined} from '../../../utils/common-util';
 
 const MSG_EMPTY_BLAME = 'No blame information for this diff.';
 
@@ -324,8 +325,8 @@
     return getPluginLoader()
       .awaitPluginsLoaded()
       .then(() => {
-        if (!this.path) throw new Error('Missing required "path" property.');
-        if (!this.changeNum) throw new Error('Missing required "changeNum".');
+        assertIsDefined(this.path, 'path');
+        assertIsDefined(this.changeNum, 'changeNum');
         this._layers = this._getLayers(this.path, this.changeNum);
         this._coverageRanges = [];
         // We kick off fetching the data here, but we don't return the promise,
@@ -341,8 +342,8 @@
    */
   async reload(shouldReportMetric?: boolean) {
     this.clear();
-    if (!this.path) throw new Error('Missing required "path" property.');
-    if (!this.changeNum) throw new Error('Missing required "changeNum" prop.');
+    assertIsDefined(this.path, 'path');
+    assertIsDefined(this.changeNum, 'changeNum');
     this.diff = undefined;
     this._errorMessage = null;
     const whitespaceLevel = this._getIgnoreWhitespace();
@@ -420,10 +421,10 @@
   }
 
   _getCoverageData() {
-    if (!this.changeNum) throw new Error('Missing required "changeNum" prop.');
-    if (!this.change) throw new Error('Missing required "change" prop.');
-    if (!this.path) throw new Error('Missing required "path" prop.');
-    if (!this.patchRange) throw new Error('Missing required "patchRange".');
+    assertIsDefined(this.changeNum, 'changeNum');
+    assertIsDefined(this.change, 'change');
+    assertIsDefined(this.path, 'path');
+    assertIsDefined(this.patchRange, 'patchRange');
     const changeNum = this.changeNum;
     const change = this.change;
     const path = this.path;
@@ -442,7 +443,7 @@
           if (!provider) return;
           provider(changeNum, path, basePatchNum, patchNum, change)
             .then(coverageRanges => {
-              if (!this.patchRange) throw new Error('Missing "patchRange".');
+              assertIsDefined(this.patchRange, 'patchRange');
               if (
                 !coverageRanges ||
                 changeNum !== this.changeNum ||
@@ -532,9 +533,9 @@
    * Load and display blame information for the base of the diff.
    */
   loadBlame(): Promise<BlameInfo[]> {
-    if (!this.changeNum) throw new Error('Missing required "changeNum" prop.');
-    if (!this.patchRange) throw new Error('Missing required "patchRange".');
-    if (!this.path) throw new Error('Missing required "path" property.');
+    assertIsDefined(this.changeNum, 'changeNum');
+    assertIsDefined(this.patchRange, 'patchRange');
+    assertIsDefined(this.path, 'path');
     return this.restApiService
       .getBlame(this.changeNum, this.patchRange.patchNum, this.path, true)
       .then(blame => {
@@ -599,9 +600,9 @@
     // Wrap the diff request in a new promise so that the error handler
     // rejects the promise, allowing the error to be handled in the .catch.
     return new Promise((resolve, reject) => {
-      if (!this.changeNum) throw new Error('Missing required "changeNum".');
-      if (!this.patchRange) throw new Error('Missing required "patchRange".');
-      if (!this.path) throw new Error('Missing required "path" property.');
+      assertIsDefined(this.changeNum, 'changeNum');
+      assertIsDefined(this.patchRange, 'patchRange');
+      assertIsDefined(this.path, 'path');
       this.restApiService
         .getDiff(
           this.changeNum,
@@ -669,7 +670,7 @@
 
     // Report the due_to_rebase percentage in the "diff" category when
     // applicable.
-    if (!this.patchRange) throw new Error('Missing required "patchRange".');
+    assertIsDefined(this.patchRange, 'patchRange');
     if (this.patchRange.basePatchNum === 'PARENT') {
       this.reporting.reportInteraction(EVENT_AGAINST_PARENT);
     } else if (percentRebaseDelta === 0) {
@@ -726,8 +727,8 @@
   }
 
   _getImages(diff: DiffInfo) {
-    if (!this.changeNum) throw new Error('Missing required "changeNum" prop.');
-    if (!this.patchRange) throw new Error('Missing required "patchRange".');
+    assertIsDefined(this.changeNum, 'changeNum');
+    assertIsDefined(this.patchRange, 'patchRange');
     return this.restApiService.getImagesForDiff(
       this.changeNum,
       diff,
diff --git a/polygerrit-ui/app/elements/diff/gr-diff-view/gr-diff-view.ts b/polygerrit-ui/app/elements/diff/gr-diff-view/gr-diff-view.ts
index b31333c..088d9cf 100644
--- a/polygerrit-ui/app/elements/diff/gr-diff-view/gr-diff-view.ts
+++ b/polygerrit-ui/app/elements/diff/gr-diff-view/gr-diff-view.ts
@@ -97,6 +97,7 @@
 import {CustomKeyboardEvent, OpenFixPreviewEvent} from '../../../types/events';
 import {fireAlert, fireTitleChange} from '../../../utils/event-util';
 import {GerritView} from '../../../services/router/router-model';
+import {assertIsDefined} from '../../../utils/common-util';
 const ERR_REVIEW_STATUS = 'Couldn’t change file review status.';
 const MSG_LOADING_BLAME = 'Loading blame...';
 const MSG_LOADED_BLAME = 'Blame loaded';
@@ -370,7 +371,7 @@
   }
 
   _getChangeEdit() {
-    if (!this._changeNum) throw new Error('Missing this._changeNum');
+    assertIsDefined(this._changeNum, '_changeNum');
     return this.restApiService.getChangeEdit(this._changeNum);
   }
 
@@ -980,7 +981,7 @@
         leftSide = !!this.params.leftSide;
       }
     }
-    if (!this._patchRange) throw new Error('Failed to initialize patchRange.');
+    assertIsDefined(this._patchRange, '_patchRange');
     this._initLineOfInterestAndCursor(leftSide);
 
     if (this.params?.commentId) {
@@ -1052,10 +1053,10 @@
         this._initPatchRange();
         this._initCommitRange();
 
-        if (!this._path) throw new Error('path must be defined');
+        assertIsDefined(this._path, '_path');
         if (!this._changeComments)
           throw new Error('change comments must be defined');
-        if (!this._patchRange) throw new Error('patch range must be defined');
+        assertIsDefined(this._patchRange, '_patchRange');
 
         // TODO(dhruvsri): check if basePath should be set here
         this.$.diffHost.threads = this._changeComments.getThreadsBySideForFile(
@@ -1082,9 +1083,9 @@
         if (!this._diff) throw new Error('Missing this._diff');
         const fileUnchanged = this._isFileUnchanged(this._diff);
         if (fileUnchanged && value.commentLink) {
-          if (!this._change) throw new Error('Missing this._change');
-          if (!this._path) throw new Error('Missing this._path');
-          if (!this._patchRange) throw new Error('Missing this._patchRange');
+          assertIsDefined(this._change, '_change');
+          assertIsDefined(this._path, '_path');
+          assertIsDefined(this._patchRange, '_patchRange');
 
           if (this._patchRange.basePatchNum === ParentPatchSetNum) {
             // file is unchanged between Base vs X
@@ -1493,7 +1494,7 @@
   }
 
   _loadComments(patchSet?: PatchSetNum) {
-    if (!this._changeNum) throw new Error('Missing this._changeNum');
+    assertIsDefined(this._changeNum, '_changeNum');
     return this.$.commentAPI
       .loadAll(this._changeNum, patchSet)
       .then(comments => {
@@ -1529,7 +1530,7 @@
   }
 
   _getDiffDrafts() {
-    if (!this._changeNum) throw new Error('Missing this._changeNum');
+    assertIsDefined(this._changeNum, '_changeNum');
 
     return this.restApiService.getDiffDrafts(this._changeNum);
   }
diff --git a/polygerrit-ui/app/elements/diff/gr-diff/gr-diff.ts b/polygerrit-ui/app/elements/diff/gr-diff/gr-diff.ts
index 189bac7..6974a76 100644
--- a/polygerrit-ui/app/elements/diff/gr-diff/gr-diff.ts
+++ b/polygerrit-ui/app/elements/diff/gr-diff/gr-diff.ts
@@ -75,6 +75,7 @@
   RenderPreferences,
 } from '../../../api/diff';
 import {isSafari} from '../../../utils/dom-util';
+import {assertIsDefined} from '../../../utils/common-util';
 
 const NO_NEWLINE_BASE = 'No newline at end of base file.';
 const NO_NEWLINE_REVISION = 'No newline at end of revision file.';
@@ -629,7 +630,7 @@
     const contentEl = this.$.diffBuilder.getContentTdByLineEl(lineEl);
     if (!contentEl) throw new Error('content el not found for line el');
     side = side ?? this._getCommentSideByLineAndContent(lineEl, contentEl);
-    if (!this.path) throw new Error('must have a path to create comments');
+    assertIsDefined(this.path, 'path');
     this.dispatchEvent(
       new CustomEvent<CreateCommentEventDetail>('create-comment', {
         bubbles: true,
diff --git a/polygerrit-ui/app/elements/edit/gr-editor-view/gr-editor-view.ts b/polygerrit-ui/app/elements/edit/gr-editor-view/gr-editor-view.ts
index f6f4395..1864598 100644
--- a/polygerrit-ui/app/elements/edit/gr-editor-view/gr-editor-view.ts
+++ b/polygerrit-ui/app/elements/edit/gr-editor-view/gr-editor-view.ts
@@ -45,6 +45,7 @@
 import {fireAlert, fireTitleChange} from '../../../utils/event-util';
 import {appContext} from '../../../services/app-context';
 import {ErrorCallback} from '../../../api/rest';
+import {assertIsDefined} from '../../../utils/common-util';
 
 const RESTORED_MESSAGE = 'Content restored from a previous edit.';
 const SAVING_MESSAGE = 'Saving changes...';
@@ -326,7 +327,7 @@
   }
 
   _handlePublishTap() {
-    if (!this._changeNum) throw new Error('missing changeNum');
+    assertIsDefined(this._changeNum, '_changeNum');
 
     const changeNum = this._changeNum;
     this._saveEdit().then(() => {
@@ -347,7 +348,7 @@
           handleError
         )
         .then(() => {
-          if (!this._change) throw new Error('missing change');
+          assertIsDefined(this._change, '_change');
           GerritNav.navigateToChange(this._change);
         });
     });
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 a9b910d..a5b7df7 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
@@ -60,7 +60,7 @@
 import {KnownExperimentId} from '../../../services/flags/flags';
 import {DiffInfo, DiffPreferencesInfo} from '../../../types/diff';
 import {RenderPreferences} from '../../../api/diff';
-import {check, checkProperty} from '../../../utils/common-util';
+import {check, assertIsDefined} from '../../../utils/common-util';
 
 const UNRESOLVED_EXPAND_COUNT = 5;
 const NEWLINE_PATTERN = /\n/g;
@@ -334,8 +334,8 @@
   }
 
   _getUrlForViewDiff(comments: UIComment[]) {
-    checkProperty(!!this.changeNum, 'changeNum');
-    checkProperty(!!this.projectName, 'projectName');
+    assertIsDefined(this.changeNum, 'changeNum');
+    assertIsDefined(this.projectName, 'projectName');
     check(comments.length > 0, 'comment not found');
     return GerritNav.getUrlForComment(
       this.changeNum,
@@ -633,8 +633,8 @@
   }
 
   _handleCommentDiscard(e: Event) {
-    if (!this.changeNum) throw new Error('changeNum is missing');
-    if (!this.patchNum) throw new Error('patchNum is missing');
+    assertIsDefined(this.changeNum, 'changeNum');
+    assertIsDefined(this.patchNum, 'patchNum');
     const diffCommentEl = (dom(e) as EventApi).rootTarget as GrComment;
     const comment = diffCommentEl.comment;
     const idx = this._indexOf(comment, this.comments);
diff --git a/polygerrit-ui/app/elements/shared/gr-comment/gr-comment.ts b/polygerrit-ui/app/elements/shared/gr-comment/gr-comment.ts
index 898aff3..bf376f6 100644
--- a/polygerrit-ui/app/elements/shared/gr-comment/gr-comment.ts
+++ b/polygerrit-ui/app/elements/shared/gr-comment/gr-comment.ts
@@ -61,6 +61,7 @@
 import {OpenFixPreviewEventDetail} from '../../../types/events';
 import {fireAlert} from '../../../utils/event-util';
 import {pluralize} from '../../../utils/string-util';
+import {assertIsDefined} from '../../../utils/common-util';
 
 const STORAGE_DEBOUNCE_INTERVAL = 400;
 const TOAST_DEBOUNCE_INTERVAL = 200;
@@ -322,7 +323,7 @@
   }
 
   _handlePortedMessageClick() {
-    if (!this.comment) throw new Error('comment not set');
+    assertIsDefined(this.comment, 'comment');
     this.reporting.reportInteraction('navigate-to-original-comment', {
       line: this.comment.line,
       range: this.comment.range,
@@ -496,10 +497,8 @@
     // prior to it being saved.
     this.cancelDebouncer(DEBOUNCER_STORE);
 
-    if (!this.comment?.path) throw new Error('Cannot erase Draft Comment');
-    if (this.changeNum === undefined) {
-      throw new Error('undefined changeNum');
-    }
+    assertIsDefined(this.comment?.path, 'comment.path');
+    assertIsDefined(this.changeNum, 'changeNum');
     this.storage.eraseDraftComment({
       changeNum: this.changeNum,
       patchNum: this._getPatchNum(),
diff --git a/polygerrit-ui/app/elements/shared/gr-hovercard-account/gr-hovercard-account.ts b/polygerrit-ui/app/elements/shared/gr-hovercard-account/gr-hovercard-account.ts
index 592f7903..ddae8ea 100644
--- a/polygerrit-ui/app/elements/shared/gr-hovercard-account/gr-hovercard-account.ts
+++ b/polygerrit-ui/app/elements/shared/gr-hovercard-account/gr-hovercard-account.ts
@@ -45,6 +45,7 @@
 import {ReviewerState} from '../../../constants/constants';
 import {CURRENT} from '../../../utils/patch-set-util';
 import {isInvolved, isRemovableReviewer} from '../../../utils/change-util';
+import {assertIsDefined} from '../../../utils/common-util';
 
 @customElement('gr-hovercard-account')
 export class GrHovercardAccount extends GestureEventListeners(
@@ -163,7 +164,7 @@
   }
 
   _handleChangeReviewerOrCCStatus() {
-    if (!this.change) throw new Error('expected change object to be present');
+    assertIsDefined(this.change, 'change');
     // accountKey() throws an error if _account_id & email is not found, which
     // we want to check before showing reloading toast
     const _accountKey = accountKey(this.account);
diff --git a/polygerrit-ui/app/utils/common-util.ts b/polygerrit-ui/app/utils/common-util.ts
index ad76b79..f95105d 100644
--- a/polygerrit-ui/app/utils/common-util.ts
+++ b/polygerrit-ui/app/utils/common-util.ts
@@ -67,6 +67,18 @@
 }
 
 /**
+ * Throws an error if the property is not defined.
+ */
+export function assertIsDefined<T>(
+  val: T,
+  variableName = 'variable'
+): asserts val is NonNullable<T> {
+  if (val === undefined || val === null) {
+    throw new Error(`${variableName} is not defined`);
+  }
+}
+
+/**
  * Returns true, if both sets contain the same members.
  */
 export function areSetsEqual<T>(a: Set<T>, b: Set<T>): boolean {