fixed asymmetrical equality of cyclic objects

CHANGELOG.md

@@ -91,6 +91,7 @@
 - `[jest-runtime]` Exclude setup/teardown files from coverage report ([#7790](https://github.com/facebook/jest/pull/7790)
 - `[babel-jest]` Throw an error if `babel-jest` tries to transform a file ignored by Babel ([#7797](https://github.com/facebook/jest/pull/7797))
 - `[babel-plugin-jest-hoist]` Ignore TS type references when looking for out-of-scope references ([#7799](https://github.com/facebook/jest/pull/7799)
+- `[expect]` fixed asymmetrical equality of cyclic objects ([#7730](https://github.com/facebook/jest/pull/7730))
 
 ### Chore & Maintenance
 

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

@@ -606,6 +606,59 @@ describe('.toEqual()', () => {
     });
     expect(actual).toEqual({x: 3});
   });
+
+  describe('cyclic object equality', () => {
+    test('properties with the same circularity are equal', () => {
+      const a = {};
+      a.x = a;
+      const b = {};
+      b.x = b;
+      expect(a).toEqual(b);
+      expect(b).toEqual(a);
+
+      const c = {};
+      c.x = a;
+      const d = {};
+      d.x = b;
+      expect(c).toEqual(d);
+      expect(d).toEqual(c);
+    });
+
+    test('properties with different circularity are not equal', () => {
+      const a = {};
+      a.x = {y: a};
+      const b = {};
+      const bx = {};
+      b.x = bx;
+      bx.y = bx;
+      expect(a).not.toEqual(b);
+      expect(b).not.toEqual(a);
+
+      const c = {};
+      c.x = a;
+      const d = {};
+      d.x = b;
+      expect(c).not.toEqual(d);
+      expect(d).not.toEqual(c);
+    });
+
+    test('are not equal if circularity is not on the same property', () => {
+      const a = {};
+      const b = {};
+      a.a = a;
+      b.a = {};
+      b.a.a = a;
+      expect(a).not.toEqual(b);
+      expect(b).not.toEqual(a);
+
+      const c = {};
+      c.x = {x: c};
+      const d = {};
+      d.x = d;
+      expect(c).not.toEqual(d);
+      expect(d).not.toEqual(c);
+    });
+  });
 });
 
 describe('.toBeInstanceOf()', () => {

packages/expect/src/jasmineUtils.ts

@@ -64,9 +64,9 @@ function asymmetricMatch(a: any, b: any) {
 function eq(
   a: any,
   b: any,
-  aStack: any,
-  bStack: any,
-  customTesters: any,
+  aStack: Array<unknown>,
+  bStack: Array<unknown>,
+  customTesters: Array<Tester>,
   hasKey: any,
 ): boolean {
   var result = true;
@@ -149,14 +149,17 @@ function eq(
     return false;
   }
 
-  // Assume equality for cyclic structures. The algorithm for detecting cyclic
-  // structures is adapted from ES 5.1 section 15.12.3, abstract operation `JO`.
+  // Used to detect circular references.
   var length = aStack.length;
   while (length--) {
     // Linear search. Performance is inversely proportional to the number of
     // unique nested structures.
-    if (aStack[length] == a) {
-      return bStack[length] == b;
+    // circular references at same depth are equal
+    // circular reference is not equal to non-circular one
+    if (aStack[length] === a) {
+      return bStack[length] === b;
+    } else if (bStack[length] === b) {
+      return false;
     }
   }
   // Add the first object to the stack of traversed objects.