fix: don't mutate the sample in expect.objectContaining() (jestjs#10711)

Author: ninevra

CHANGELOG.md

@@ -6,6 +6,8 @@
 
 ### Fixes
 
+- `[expect]` Stop modifying the sample in `expect.objectContaining()` ([#10711](https://github.com/facebook/jest/pull/10711))
+
 ### Chore & Maintenance
 
 - `[jest-cli]` chore: standardize files and folder names ([#10698](https://github.com/facebook/jest/pull/1098))

packages/expect/src/__tests__/asymmetricMatchers.test.ts

@@ -211,6 +211,14 @@ test('ObjectContaining throws for non-objects', () => {
   jestExpect(() => objectContaining(1337).asymmetricMatch()).toThrow();
 });
 
+test('ObjectContaining does not mutate the sample', () => {
+  const sample = {foo: {bar: {}}};
+  const sample_json = JSON.stringify(sample);
+  expect({foo: {bar: {}}}).toEqual(expect.objectContaining(sample));
+
+  expect(JSON.stringify(sample)).toEqual(sample_json);
+});
+
 test('ObjectNotContaining matches', () => {
   [
     objectNotContaining({}).asymmetricMatch('jest'),

packages/expect/src/asymmetricMatchers.ts

@@ -178,18 +178,15 @@ class ObjectContaining extends AsymmetricMatcher<Record<string, unknown>> {
       return true;
     } else {
       for (const property in this.sample) {
-        if (
+        const expected =
           typeof this.sample[property] === 'object' &&
           !(this.sample[property] instanceof AsymmetricMatcher)
-        ) {
-          this.sample[property] = objectContaining(
-            this.sample[property] as Record<string, unknown>,
-          );
-        }
+            ? objectContaining(this.sample[property] as Record<string, unknown>)
+            : this.sample[property];
 
         if (
           !hasProperty(other, property) ||
-          !equals(this.sample[property], other[property])
+          !equals(expected, other[property])
         ) {
           return false;
         }