Handle arrays when merging snapshots

CHANGELOG.md

@@ -78,6 +78,7 @@
 - `[jest-haste-map]` Remove legacy condition for duplicate module detection ([#7333](https://github.com/facebook/jest/pull/7333))
 - `[jest-haste-map]` Fix `require` detection with trailing commas and ignore `import typeof` modules ([#7385](https://github.com/facebook/jest/pull/7385))
 - `[jest-cli]` Fix to set prettierPath via config file ([#7412](https://github.com/facebook/jest/pull/7412))
+- `[jest-snapshot]` Handle arrays when merging snapshots ([#7089](https://github.com/facebook/jest/pull/7089))
 
 ### Chore & Maintenance
 

packages/jest-snapshot/src/__tests__/utils.test.js

@@ -195,12 +195,74 @@ test('serialize handles \\r\\n', () => {
 
 describe('DeepMerge', () => {
   it('Correctly merges objects with property matchers', () => {
-    const target = {data: {bar: 'bar', foo: 'foo'}};
-    const matcher = expect.any(String);
-    const propertyMatchers = {data: {foo: matcher}};
+    /* eslint-disable sort-keys */
+    // to keep keys in numerical order rather than alphabetical
+    const target = {
+      data: {
+        one: 'one',
+        two: 'two',
+        three: [
+          {
+            four: 'four',
+            five: 'five',
+          },
+          // Include an array element not present in the propertyMatchers
+          {
+            six: 'six',
+            seven: 'seven',
+          },
+        ],
+      },
+    };
+    // Don't use `expect.any(string)` since that will cause a false positive
+    // if deepMerge incorrectly keeps two as 'two' from the target
+    const matcher = '--matcher--';
+    const propertyMatchers = {
+      data: {
+        two: matcher,
+        three: [
+          {
+            four: matcher,
+          },
+        ],
+      },
+    };
+
     const mergedOutput = deepMerge(target, propertyMatchers);
+    expect(mergedOutput).toStrictEqual({
+      data: {
+        one: 'one',
+        two: matcher,
+        three: [
+          {
+            four: matcher,
+            five: 'five',
+          },
+          {
+            six: 'six',
+            seven: 'seven',
+          },
+        ],
+      },
+    });
 
-    expect(mergedOutput).toStrictEqual({data: {bar: 'bar', foo: matcher}});
-    expect(target).toStrictEqual({data: {bar: 'bar', foo: 'foo'}});
+    // Ensure original target is not modified
+    expect(target).toStrictEqual({
+      data: {
+        one: 'one',
+        two: 'two',
+        three: [
+          {
+            four: 'four',
+            five: 'five',
+          },
+          {
+            six: 'six',
+            seven: 'seven',
+          },
+        ],
+      },
+    });
+    /* eslint-enable sort-keys */
   });
 });

packages/jest-snapshot/src/utils.js

@@ -178,13 +178,32 @@ export const saveSnapshotFile = (
   );
 };
 
+const deepMergeArray = (target, source) => {
+  // Clone target
+  const mergedOutput = target.slice();
+
+  source.forEach((element, index) => {
+    if (typeof mergedOutput[index] === 'undefined') {
+      mergedOutput[index] = element;
+    } else if (typeof element === 'undefined') {
+      mergedOutput.push(target[index]);
+    } else {
+      mergedOutput[index] = deepMerge(target[index], element);
+    }
+  });
+
+  return mergedOutput;
+};
+
 export const deepMerge = (target: any, source: any) => {
   const mergedOutput = Object.assign({}, target);
   if (isObject(target) && isObject(source)) {
     Object.keys(source).forEach(key => {
       if (isObject(source[key]) && !source[key].$$typeof) {
         if (!(key in target)) Object.assign(mergedOutput, {[key]: source[key]});
         else mergedOutput[key] = deepMerge(target[key], source[key]);
+      } else if (Array.isArray(source[key])) {
+        mergedOutput[key] = deepMergeArray(target[key], source[key]);
       } else {
         Object.assign(mergedOutput, {[key]: source[key]});
       }