Bug 1054426 - Make Object.assign skip null/undefined source arguments if any are passed, to match not-yet-drafted semantics agreed upon in TC39 meeting minutes. Also fixes Facebook bustage. r=mjrosenb over IRC a=nigelb,facebookbustage

js/src/builtin/Object.js

@@ -14,8 +14,14 @@ function ObjectStaticAssign(target, firstSource) {
     // Step 4.
     var i = 1;
     do {
-        // Step 5.a-b.
+        // Step 5.a-b, plus an unspecified flourish to skip null/undefined, so
+        // any site depending on agreed-upon (but not-yet-drafted) semantics
+        // from TC39 meeting minutes will work. (Yes, implausibly, such a site
+        // exists. See bug 1054426.)
         var nextSource = arguments[i];
+        if (nextSource === null || nextSource === undefined)
+            continue;
+
         var from = ToObject(nextSource);
 
         // Step 5.c-d.
@@ -58,8 +64,7 @@ function ObjectStaticAssign(target, firstSource) {
         // Step 5.k.
         if (pendingException !== MISSING)
             throw pendingException;
-        i++;
-    } while (i < arguments.length);
+    } while (++i < arguments.length);
 
     // Step 6.
     return to;

js/src/tests/ecma_6/Object/assign.js

@@ -38,12 +38,27 @@ function basicSymbols() {
 }
 basicSymbols();
 
-// Calls ToObject() for target and source
+// Calls ToObject() for target, skips null/undefined sources, uses
+// ToObject(source) otherwise.
 function testToObject() {
     assertThrowsInstanceOf(() => Object.assign(null, null), TypeError);
     assertThrowsInstanceOf(() => Object.assign(), TypeError);
     assertThrowsInstanceOf(() => Object.assign(null, {}), TypeError);
-    assertThrowsInstanceOf(() => Object.assign({}, null), TypeError);
+    assertEq(Object.assign({}, null) instanceof Object, true);
+    assertEq(Object.assign({}, undefined) instanceof Object, true);
+
+    // Technically an embedding could have this as extension acting differently
+    // from ours, so a feature-test is inadequate.  We can move this subtest
+    // into extensions/ if that ever matters.
+    if (typeof objectEmulatingUndefined === "function") {
+        var falsyObject = objectEmulatingUndefined();
+        falsyObject.foo = 7;
+
+        var obj = Object.assign({}, falsyObject);
+        assertEq(obj instanceof Object, true);
+        assertEq(obj.foo, 7);
+    }
+
     assertEq(Object.assign(true, {}) instanceof Boolean, true);
     assertEq(Object.assign(1, {}) instanceof Number, true);
     assertEq(Object.assign("string", {}) instanceof String, true);