Consider undefined properties equivalent to missing properties.

packages/equality/src/equality.ts

@@ -43,8 +43,8 @@ function check(a: any, b: any): boolean {
     case '[object Object]': {
       if (previouslyCompared(a, b)) return true;
 
-      const aKeys = Object.keys(a);
-      const bKeys = Object.keys(b);
+      const aKeys = definedKeys(a);
+      const bKeys = definedKeys(b);
 
       // If `a` and `b` have a different number of enumerable keys, they
       // must be different.
@@ -151,6 +151,18 @@ function check(a: any, b: any): boolean {
   return false;
 }
 
+function definedKeys<TObject extends object>(obj: TObject) {
+  // Remember that the second argument to Array.prototype.filter will be
+  // used as `this` within the callback function.
+  return Object.keys(obj).filter(isDefinedKey, obj);
+}
+function isDefinedKey<TObject extends object>(
+  this: TObject,
+  key: keyof TObject,
+) {
+  return this[key] !== void 0;
+}
+
 const nativeCodeSuffix = "{ [native code] }";
 
 function endsWith(full: string, suffix: string) {

packages/equality/src/tests.ts

@@ -63,11 +63,12 @@ describe("equality", function () {
       [1, /*hole*/, 3],
     );
 
-    assertNotEqual(
+    assertEqual(
       [1, /*hole*/, 3],
       [1, void 0, 3],
     );
 
+    // Not equal because the arrays are a different length.
     assertNotEqual(
       [1, 2, /*hole*/,],
       [1, 2],
@@ -102,6 +103,23 @@ describe("equality", function () {
     assertNotEqual(a, b);
   });
 
+  it("should consider undefined and missing object properties equivalent", function () {
+    assertEqual({
+      a: 1,
+      b: void 0,
+      c: 3,
+    }, {
+      a: 1,
+      c: 3,
+    });
+
+    assertEqual({
+      a: void 0,
+      b: void 0,
+      c: void 0,
+    }, {});
+  });
+
   it("should work for Error objects", function () {
     assertEqual(new Error("oyez"), new Error("oyez"));
     assertNotEqual(new Error("oyez"), new Error("onoz"));