diff --git a/polygerrit-ui/app/embed/diff/gr-diff/gr-diff-group.ts b/polygerrit-ui/app/embed/diff/gr-diff/gr-diff-group.ts
index 6d80d78..771e298 100644
--- a/polygerrit-ui/app/embed/diff/gr-diff/gr-diff-group.ts
+++ b/polygerrit-ui/app/embed/diff/gr-diff/gr-diff-group.ts
@@ -3,9 +3,8 @@
  * Copyright 2016 Google LLC
  * SPDX-License-Identifier: Apache-2.0
  */
-import {BLANK_LINE, GrDiffLine, GrDiffLineType} from './gr-diff-line';
-import {LineRange, Side} from '../../../api/diff';
-import {LineNumber} from './gr-diff-line';
+import {BLANK_LINE, GrDiffLine} from './gr-diff-line';
+import {GrDiffLineType, LineNumber, LineRange, Side} from '../../../api/diff';
 import {assertIsDefined, assert} from '../../../utils/common-util';
 import {untilRendered} from '../../../utils/dom-util';
 import {isDefined} from '../../../types/types';
@@ -133,12 +132,10 @@
     for (const line of group.lines) {
       if (
         (line.beforeNumber &&
-          line.beforeNumber !== 'FILE' &&
-          line.beforeNumber !== 'LOST' &&
+          typeof line.beforeNumber === 'number' &&
           line.beforeNumber < leftSplit) ||
         (line.afterNumber &&
-          line.afterNumber !== 'FILE' &&
-          line.afterNumber !== 'LOST' &&
+          typeof line.afterNumber === 'number' &&
           line.afterNumber < rightSplit)
       ) {
         before.push(line);
@@ -435,7 +432,7 @@
   }
 
   containsLine(side: Side, line: LineNumber) {
-    if (line === 'FILE' || line === 'LOST') {
+    if (typeof line !== 'number') {
       // For FILE and LOST, beforeNumber and afterNumber are the same
       return this.lines[0]?.beforeNumber === line;
     }
@@ -462,14 +459,8 @@
   }
 
   private _updateRangeWithNewLine(line: GrDiffLine) {
-    if (
-      line.beforeNumber === 'FILE' ||
-      line.afterNumber === 'FILE' ||
-      line.beforeNumber === 'LOST' ||
-      line.afterNumber === 'LOST'
-    ) {
-      return;
-    }
+    if (typeof line.beforeNumber !== 'number') return;
+    if (typeof line.afterNumber !== 'number') return;
 
     if (line.type === GrDiffLineType.ADD || line.type === GrDiffLineType.BOTH) {
       if (
@@ -505,8 +496,7 @@
     // untilRendered() promise.
     if (
       this.skip !== undefined ||
-      lineNumber === 'LOST' ||
-      lineNumber === 'FILE' ||
+      typeof lineNumber !== 'number' ||
       this.type === GrDiffGroupType.CONTEXT_CONTROL
     ) {
       return Promise.resolve();
diff --git a/polygerrit-ui/app/embed/diff/gr-diff/gr-diff-group_test.ts b/polygerrit-ui/app/embed/diff/gr-diff/gr-diff-group_test.ts
index 7ead68f..bbbb4ad 100644
--- a/polygerrit-ui/app/embed/diff/gr-diff/gr-diff-group_test.ts
+++ b/polygerrit-ui/app/embed/diff/gr-diff/gr-diff-group_test.ts
@@ -4,14 +4,14 @@
  * SPDX-License-Identifier: Apache-2.0
  */
 import '../../../test/common-test-setup';
-import {GrDiffLine, GrDiffLineType, BLANK_LINE} from './gr-diff-line';
+import {GrDiffLine, BLANK_LINE} from './gr-diff-line';
 import {
   GrDiffGroup,
   GrDiffGroupType,
   hideInContextControl,
 } from './gr-diff-group';
 import {assert} from '@open-wc/testing';
-import {Side} from '../../../api/diff';
+import {FILE, GrDiffLineType, LOST, Side} from '../../../api/diff';
 
 suite('gr-diff-group tests', () => {
   test('delta line pairs', () => {
@@ -297,18 +297,18 @@
 
     test('FILE', () => {
       const lines: GrDiffLine[] = [];
-      lines.push(new GrDiffLine(GrDiffLineType.BOTH, 'FILE', 'FILE'));
+      lines.push(new GrDiffLine(GrDiffLineType.BOTH, FILE, FILE));
       const group = new GrDiffGroup({type: GrDiffGroupType.DELTA, lines});
-      assert.equal(group.startLine(Side.LEFT), 'FILE');
-      assert.equal(group.startLine(Side.RIGHT), 'FILE');
+      assert.equal(group.startLine(Side.LEFT), FILE);
+      assert.equal(group.startLine(Side.RIGHT), FILE);
     });
 
     test('LOST', () => {
       const lines: GrDiffLine[] = [];
-      lines.push(new GrDiffLine(GrDiffLineType.BOTH, 'LOST', 'LOST'));
+      lines.push(new GrDiffLine(GrDiffLineType.BOTH, LOST, LOST));
       const group = new GrDiffGroup({type: GrDiffGroupType.DELTA, lines});
-      assert.equal(group.startLine(Side.LEFT), 'LOST');
-      assert.equal(group.startLine(Side.RIGHT), 'LOST');
+      assert.equal(group.startLine(Side.LEFT), LOST);
+      assert.equal(group.startLine(Side.RIGHT), LOST);
     });
   });
 });
diff --git a/polygerrit-ui/app/embed/diff/gr-diff/gr-diff-line.ts b/polygerrit-ui/app/embed/diff/gr-diff/gr-diff-line.ts
index 338a275..1a89207 100644
--- a/polygerrit-ui/app/embed/diff/gr-diff/gr-diff-line.ts
+++ b/polygerrit-ui/app/embed/diff/gr-diff/gr-diff-line.ts
@@ -4,17 +4,13 @@
  * SPDX-License-Identifier: Apache-2.0
  */
 import {
+  FILE,
   GrDiffLine as GrDiffLineApi,
   GrDiffLineType,
   LineNumber,
   Side,
 } from '../../../api/diff';
 
-export {GrDiffLineType};
-export type {LineNumber};
-
-export const FILE = 'FILE';
-
 export class GrDiffLine implements GrDiffLineApi {
   constructor(
     readonly type: GrDiffLineType,
diff --git a/polygerrit-ui/app/embed/diff/gr-diff/gr-diff-utils.ts b/polygerrit-ui/app/embed/diff/gr-diff/gr-diff-utils.ts
index 669537e..927943c 100644
--- a/polygerrit-ui/app/embed/diff/gr-diff/gr-diff-utils.ts
+++ b/polygerrit-ui/app/embed/diff/gr-diff/gr-diff-utils.ts
@@ -4,12 +4,14 @@
  * SPDX-License-Identifier: Apache-2.0
  */
 import {BlameInfo, CommentRange} from '../../../types/common';
-import {FILE, LineNumber} from './gr-diff-line';
 import {Side} from '../../../constants/constants';
 import {DiffInfo} from '../../../types/diff';
 import {
   DiffPreferencesInfo,
   DiffResponsiveMode,
+  FILE,
+  LOST,
+  LineNumber,
   RenderPreferences,
 } from '../../../api/diff';
 import {getBaseUrl} from '../../../utils/url-util';
@@ -103,9 +105,7 @@
 }
 
 export function lineNumberToNumber(lineNumber?: LineNumber | null): number {
-  if (!lineNumber) return 0;
-  if (lineNumber === 'LOST') return 0;
-  if (lineNumber === 'FILE') return 0;
+  if (typeof lineNumber !== 'number') return 0;
   return lineNumber;
 }
 
@@ -138,15 +138,15 @@
   const lineNumberStr = lineEl.getAttribute('data-value');
   if (!lineNumberStr) return null;
   if (lineNumberStr === FILE) return FILE;
-  if (lineNumberStr === 'LOST') return 'LOST';
+  if (lineNumberStr === LOST) return LOST;
   const lineNumber = Number(lineNumberStr);
   return Number.isInteger(lineNumber) ? lineNumber : null;
 }
 
 export function getLine(threadEl: HTMLElement): LineNumber {
   const lineAtt = threadEl.getAttribute('line-num');
-  if (lineAtt === 'LOST') return lineAtt;
-  if (!lineAtt || lineAtt === 'FILE') return FILE;
+  if (lineAtt === LOST) return lineAtt;
+  if (!lineAtt || lineAtt === FILE) return FILE;
   const line = Number(lineAtt);
   if (isNaN(line)) throw new Error(`cannot parse line number: ${lineAtt}`);
   if (line < 1) throw new Error(`line number smaller than 1: ${line}`);
diff --git a/polygerrit-ui/app/embed/diff/gr-diff/gr-diff.ts b/polygerrit-ui/app/embed/diff/gr-diff/gr-diff.ts
index 3929330..38cb2ba 100644
--- a/polygerrit-ui/app/embed/diff/gr-diff/gr-diff.ts
+++ b/polygerrit-ui/app/embed/diff/gr-diff/gr-diff.ts
@@ -12,7 +12,6 @@
 import '../gr-syntax-themes/gr-syntax-theme';
 import '../gr-ranged-comment-themes/gr-ranged-comment-theme';
 import '../gr-ranged-comment-hint/gr-ranged-comment-hint';
-import {LineNumber} from './gr-diff-line';
 import {
   getLine,
   getLineElByChild,
@@ -54,6 +53,8 @@
   RenderPreferences,
   GrDiff as GrDiffApi,
   DisplayLine,
+  LineNumber,
+  LOST,
 } from '../../../api/diff';
 import {isSafari, toggleClass} from '../../../utils/dom-util';
 import {assertIsDefined} from '../../../utils/common-util';
@@ -617,7 +618,7 @@
     const el = e.target as Element;
 
     if (
-      el.getAttribute('data-value') !== 'LOST' &&
+      el.getAttribute('data-value') !== LOST &&
       (el.classList.contains('lineNum') ||
         el.classList.contains('lineNumButton'))
     ) {
@@ -980,7 +981,7 @@
       }
       const contentEl = this.diffBuilder.getContentTdByLineEl(lineEl);
       if (!contentEl) continue;
-      if (lineNum === 'LOST') {
+      if (lineNum === LOST) {
         this.insertPortedCommentsWithoutRangeMessage(contentEl);
       }