improved failure message readability of failing ObjectContaining tests

CHANGELOG.md

@@ -40,6 +40,7 @@
 
 - `[babel-plugin-jest-hoist]` Use `denylist` instead of the deprecated `blacklist` for Babel 8 support ([#14109](https://github.com/jestjs/jest/pull/14109))
 - `[expect]` Check error instance type for `toThrow/toThrowError` ([#14576](https://github.com/jestjs/jest/pull/14576))
+- `[expect]` Improve diff for failing `expect.objectContaining` ([#15038](https://github.com/jestjs/jest/pull/15038))
 - `[jest-changed-files]` Print underlying errors when VCS commands fail ([#15052](https://github.com/jestjs/jest/pull/15052))
 - `[jest-changed-files]` Abort `sl root` call if output resembles a steam locomotive ([#15053](https://github.com/jestjs/jest/pull/15053))
 - `[jest-circus]` [**BREAKING**] Prevent false test failures caused by promise rejections handled asynchronously ([#14315](https://github.com/jestjs/jest/pull/14315))

packages/expect/src/__tests__/__snapshots__/matchers.test.js.snap

@@ -2176,13 +2176,13 @@ exports[`.toEqual() {pass: false} expect({"a": 1, "b": 2}).toEqual(ObjectContain
 <d>expect(</><r>received</><d>).</>toEqual<d>(</><g>expected</><d>) // deep equality</>
 
 <g>- Expected  - 2</>
-<r>+ Received  + 3</>
+<r>+ Received  + 2</>
 
 <g>- ObjectContaining {</>
 <g>-   "a": 2,</>
 <r>+ Object {</>
 <r>+   "a": 1,</>
-<r>+   "b": 2,</>
+<d>    "b": 2,</>
 <d>  }</>
 `;
 

packages/expect/src/asymmetricMatchers.ts

@@ -243,11 +243,19 @@ class ObjectContaining extends AsymmetricMatcher<
     const matcherContext = this.getMatcherContext();
     const objectKeys = getObjectKeys(this.sample);
 
+    const otherKeys = other ? getObjectKeys(other) : [];
+
     for (const key of objectKeys) {
       if (
         !hasProperty(other, key) ||
         !equals(this.sample[key], other[key], matcherContext.customTesters)
       ) {
+        // Result has already been determined, mutation only affects diff output
+        for (const key of otherKeys) {
+          if (!hasProperty(this.sample, key)) {
+            this.sample[key] = other[key];
+          }
+        }
         result = false;
         break;
       }