feat: Show non-printable control characters to diff output

CHANGELOG.md

@@ -2,6 +2,10 @@
 
 ## 30.0.2
 
+### Features
+
+- `[jest-diff]` Show non-printable control characters to diffs ([#15696](https://github.com/facebook/jest/pull/15696))
+
 ### Fixes
 
 - `[jest-matcher-utils]` Make 'deepCyclicCopyObject' safer by setting descriptors to a null-prototype object ([#15689](https://github.com/jestjs/jest/pull/15689))

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

@@ -0,0 +1,115 @@
+/**
+ * Copyright (c) Meta Platforms, Inc. and affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+import {escapeControlCharacters} from '../escapeControlCharacters';
+
+describe('escapeControlCharacters', () => {
+  test('preserves regular printable characters', () => {
+    const input = 'Jest 123!@#$%^&*()';
+    expect(escapeControlCharacters(input)).toBe(input);
+  });
+
+  test('preserves whitespace characters that are meaningful for formatting', () => {
+    const input = 'line1\nline2\tindented\rcarriage';
+    expect(escapeControlCharacters(input)).toBe(input);
+  });
+
+  test('escapes NULL character', () => {
+    const input = 'before\u0000after';
+    expect(escapeControlCharacters(input)).toBe('before\\x00after');
+  });
+
+  test('escapes SOH (Start of Heading) character', () => {
+    const input = 'before\u0001after';
+    expect(escapeControlCharacters(input)).toBe('before\\x01after');
+  });
+
+  test('escapes backspace character to \\b', () => {
+    const input = 'before\u0008after';
+    expect(escapeControlCharacters(input)).toBe('before\\bafter');
+  });
+
+  test('escapes vertical tab character to \\v', () => {
+    const input = 'before\u000Bafter';
+    expect(escapeControlCharacters(input)).toBe('before\\vafter');
+  });
+
+  test('escapes form feed character to \\f', () => {
+    const input = 'before\u000Cafter';
+    expect(escapeControlCharacters(input)).toBe('before\\fafter');
+  });
+
+  test('escapes ESC (Escape) character', () => {
+    const input = 'before\u001Bafter';
+    expect(escapeControlCharacters(input)).toBe('before\\x1bafter');
+  });
+
+  test('escapes DEL character', () => {
+    const input = 'before\u007Fafter';
+    expect(escapeControlCharacters(input)).toBe('before\\x7fafter');
+  });
+
+  test('escapes C1 control characters', () => {
+    const input = 'before\u0080\u0081\u009Fafter';
+    expect(escapeControlCharacters(input)).toBe('before\\x80\\x81\\x9fafter');
+  });
+
+  test('handles mixed control characters and regular text', () => {
+    const input = 'FIX\u00014.4\u00019=68\u00135=A\u0001MSG_TYPE=D';
+    expect(escapeControlCharacters(input)).toBe(
+      'FIX\\x014.4\\x019=68\\x135=A\\x01MSG_TYPE=D',
+    );
+  });
+
+  test('handles financial message protocol string with control characters', () => {
+    const input = '8=FIXT.1.1\u00019=68\u00135=A\u00134=1\u00149=ISLD';
+    expect(escapeControlCharacters(input)).toBe(
+      '8=FIXT.1.1\\x019=68\\x135=A\\x134=1\\x149=ISLD',
+    );
+  });
+
+  test('preserves empty string', () => {
+    expect(escapeControlCharacters('')).toBe('');
+  });
+
+  test('handles string with only control characters', () => {
+    const input = '\u0000\u0001\u0002\u0003';
+    expect(escapeControlCharacters(input)).toBe('\\x00\\x01\\x02\\x03');
+  });
+
+  test('preserves Unicode characters that are not control characters', () => {
+    const input = 'café 中文 🚀 αβγ';
+    expect(escapeControlCharacters(input)).toBe(input);
+  });
+
+  test('handles BEL (Bell) character', () => {
+    const input = 'alert\u0007sound';
+    expect(escapeControlCharacters(input)).toBe('alert\\x07sound');
+  });
+
+  test('preserves newlines in multiline strings', () => {
+    const input = 'line 1\nline 2\nline 3';
+    expect(escapeControlCharacters(input)).toBe(input);
+  });
+
+  test('preserves tabs for code formatting', () => {
+    const input = 'function() {\n\treturn true;\n}';
+    expect(escapeControlCharacters(input)).toBe(input);
+  });
+
+  test('escapes multiple consecutive control characters', () => {
+    const input = 'data\u0001\u0002\u0003separator';
+    expect(escapeControlCharacters(input)).toBe('data\\x01\\x02\\x03separator');
+  });
+
+  test('handles control characters at string boundaries', () => {
+    const startControl = '\u0001start';
+    const endControl = 'end\u0001';
+    expect(escapeControlCharacters(startControl)).toBe('\\x01start');
+    expect(escapeControlCharacters(endControl)).toBe('end\\x01');
+  });
+});

packages/jest-diff/src/diffLines.ts

@@ -7,6 +7,7 @@
 
 import diff from '@jest/diff-sequences';
 import {DIFF_DELETE, DIFF_EQUAL, DIFF_INSERT, Diff} from './cleanupSemantic';
+import {escapeControlCharacters} from './escapeControlCharacters';
 import {
   joinAlignedDiffsExpand,
   joinAlignedDiffsNoExpand,
@@ -101,8 +102,8 @@ export const diffLinesUnified = (
 ): string =>
   printDiffLines(
     diffLinesRaw(
-      isEmptyString(aLines) ? [] : aLines,
-      isEmptyString(bLines) ? [] : bLines,
+      isEmptyString(aLines) ? [] : aLines.map(escapeControlCharacters),
+      isEmptyString(bLines) ? [] : bLines.map(escapeControlCharacters),
     ),
     normalizeDiffOptions(options),
   );

packages/jest-diff/src/escapeControlCharacters.ts

@@ -0,0 +1,26 @@
+/**
+ * Copyright (c) Meta Platforms, Inc. and affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+// Escape control characters to make them visible in diffs
+export const escapeControlCharacters = (str: string): string =>
+  str.replaceAll(
+    /[\u0000-\u0008\u000B\u000C\u000E-\u001F\u007F-\u009F]/g,
+    (match: string) => {
+      switch (match) {
+        case '\b':
+          return '\\b';
+        case '\f':
+          return '\\f';
+        case '\v':
+          return '\\v';
+        default: {
+          const code = match.codePointAt(0);
+          return `\\x${code!.toString(16).padStart(2, '0')}`;
+        }
+      }
+    },
+  );

packages/jest-diff/src/index.ts

@@ -15,6 +15,7 @@ import {
 import {DIFF_DELETE, DIFF_EQUAL, DIFF_INSERT, Diff} from './cleanupSemantic';
 import {NO_DIFF_MESSAGE, SIMILAR_MESSAGE} from './constants';
 import {diffLinesRaw, diffLinesUnified, diffLinesUnified2} from './diffLines';
+import {escapeControlCharacters} from './escapeControlCharacters';
 import {normalizeDiffOptions} from './normalizeDiffOptions';
 import {diffStringsRaw, diffStringsUnified} from './printDiffs';
 import type {DiffOptions} from './types';
@@ -96,7 +97,11 @@ export function diff(a: any, b: any, options?: DiffOptions): string | null {
 
   switch (aType) {
     case 'string':
-      return diffLinesUnified(a.split('\n'), b.split('\n'), options);
+      return diffLinesUnified(
+        escapeControlCharacters(a).split('\n'),
+        escapeControlCharacters(b).split('\n'),
+        options,
+      );
     case 'boolean':
     case 'number':
       return comparePrimitive(a, b, options);