Handle surrogate pairs in jest-diff's diffStrings

CHANGELOG.md

@@ -1,5 +1,9 @@
 ## main
 
+### Fixes
+
+- `[jest-diff]` Fix `diffStrings()` to not break apart surrogate pairs ([#15812](https://github.com/jestjs/jest/pull/15812))
+
 ## 30.1.3
 
 ### Fixes

packages/jest-diff/src/__tests__/diffStringsRaw.test.ts

@@ -31,4 +31,40 @@ describe('diffStringsRaw', () => {
 
     expect(received).toEqual(expected);
   });
+
+  describe('unicode', () => {
+    test('surrogate pairs', () => {
+      const expected: Array<Diff> = [
+        new Diff(DIFF_DELETE, '😞'),
+        new Diff(DIFF_INSERT, '😄'),
+      ];
+      const received = diffStringsRaw('😞', '😄', false);
+
+      expect(received).toEqual(expected);
+    });
+    test('grapheme clusters', () => {
+      const expected: Array<Diff> = [
+        new Diff(DIFF_DELETE, '👩‍👩‍'),
+        new Diff(DIFF_EQUAL, '👧'),
+        new Diff(DIFF_DELETE, '‍👦'),
+        new Diff(DIFF_EQUAL, ' 🇺'),
+        new Diff(DIFF_DELETE, '🇸'),
+        new Diff(DIFF_INSERT, '🇦'),
+      ];
+      const received = diffStringsRaw('👩‍👩‍👧‍👦 🇺🇸', '👧 🇺🇦', false);
+
+      expect(received).toEqual(expected);
+    });
+    test('normalization', () => {
+      const expected: Array<Diff> = [
+        new Diff(DIFF_EQUAL, 'ma'),
+        new Diff(DIFF_DELETE, 'n\u0303'),
+        new Diff(DIFF_INSERT, 'ñ'),
+        new Diff(DIFF_EQUAL, 'ana'),
+      ];
+      const received = diffStringsRaw('man\u0303ana', 'mañana', false);
+
+      expect(received).toEqual(expected);
+    });
+  });
 });

packages/jest-diff/src/diffStrings.ts

@@ -9,7 +9,11 @@ import diffSequences from '@jest/diff-sequences';
 import {DIFF_DELETE, DIFF_EQUAL, DIFF_INSERT, Diff} from './cleanupSemantic';
 
 const diffStrings = (a: string, b: string): Array<Diff> => {
-  const isCommon = (aIndex: number, bIndex: number) => a[aIndex] === b[bIndex];
+  // Split strings into code points to handle surrogate pairs.
+  const aCodepoints = [...a];
+  const bCodepoints = [...b];
+  const isCommon = (aIndex: number, bIndex: number) =>
+    aCodepoints[aIndex] === bCodepoints[bIndex];
 
   let aIndex = 0;
   let bIndex = 0;
@@ -21,25 +25,36 @@ const diffStrings = (a: string, b: string): Array<Diff> => {
     bCommon: number,
   ) => {
     if (aIndex !== aCommon) {
-      diffs.push(new Diff(DIFF_DELETE, a.slice(aIndex, aCommon)));
+      diffs.push(
+        new Diff(DIFF_DELETE, aCodepoints.slice(aIndex, aCommon).join('')),
+      );
     }
     if (bIndex !== bCommon) {
-      diffs.push(new Diff(DIFF_INSERT, b.slice(bIndex, bCommon)));
+      diffs.push(
+        new Diff(DIFF_INSERT, bCodepoints.slice(bIndex, bCommon).join('')),
+      );
     }
 
     aIndex = aCommon + nCommon; // number of characters compared in a
     bIndex = bCommon + nCommon; // number of characters compared in b
-    diffs.push(new Diff(DIFF_EQUAL, b.slice(bCommon, bIndex)));
+    diffs.push(
+      new Diff(DIFF_EQUAL, bCodepoints.slice(bCommon, bIndex).join('')),
+    );
   };
 
-  diffSequences(a.length, b.length, isCommon, foundSubsequence);
+  diffSequences(
+    aCodepoints.length,
+    bCodepoints.length,
+    isCommon,
+    foundSubsequence,
+  );
 
   // After the last common subsequence, push remaining change items.
-  if (aIndex !== a.length) {
-    diffs.push(new Diff(DIFF_DELETE, a.slice(aIndex)));
+  if (aIndex !== aCodepoints.length) {
+    diffs.push(new Diff(DIFF_DELETE, aCodepoints.slice(aIndex).join('')));
   }
-  if (bIndex !== b.length) {
-    diffs.push(new Diff(DIFF_INSERT, b.slice(bIndex)));
+  if (bIndex !== bCodepoints.length) {
+    diffs.push(new Diff(DIFF_INSERT, bCodepoints.slice(bIndex).join('')));
   }
 
   return diffs;