Bug 42801 – [ES5] index setters on Array.prototype not run when index set on array instance

• Status: RESOLVED
• Resolution: DUPLICATE
• Component: JavaScriptCore

[Brendan Eich]

From a SpiderMonkey shell:

js> Array.prototype.__defineSetter__("foo", function() { print("Foo!"); });
js> Array.prototype.__defineSetter__(0, function() { print("Zero!"); });
js> var a = Array();
js> a.foo = 0;
Foo!
js> a[0] = 0;
Zero!

In a JSC shell:

> Array.prototype.__defineSetter__("foo", function() { print("Foo!"); });
undefined
> Array.prototype.__defineSetter__(0, function() { print("Zero!"); });
undefined
> var a = Array();
undefined
> a.foo = 0;
Foo!
0
> a[0] = 0;
0

V8 matches JSC. The __defineSetter__ stuff predates ES5, but ES5 codifies what Mozilla promulgated for __defineSetter__:

8.12.5 [[Put]] ( P, V, Throw )
 . . .
1. If the result of calling the [[CanPut]] internal method of O with argument P is false, then
   a. If Throw is true, then throw a TypeError exception.
   b. Else return.
2. Let ownDesc be the result of calling the [[GetOwnProperty]] internal method of O with argument P.
3. If IsDataDescriptor(ownDesc) is true, then
   a. Let valueDesc be the Property Descriptor {[[Value]]: V}.
   b. Call the [[DefineOwnProperty]] internal method of O passing P, valueDesc, and Throw as arguments.
   c. Return.
4. Let desc be the result of calling the [[GetProperty]] internal method of O with argument P. This may be either an own or inherited accessor property descriptor or an inherited data property descriptor.5. If IsAccessorDescriptor(desc) is true, then
   a. Let setter be desc.[[Set]] which cannot be undefined.
   b. Call the [[Call]] internal method of setter providing O as the this value and providing V as the sole argument.
5. If IsAccessorDescriptor(desc) is true, then
   a. Let setter be desc.[[Set]] which cannot be undefined.
   b. Call the [[Call]] internal method of setter providing O as the this value and providing V as the sole argument.
6. Else, create a named data property named P on object O as follows
   a. Let newDesc be the Property Descriptor {[[Value]]: V, [[Writable]]: true, [[Enumerable]]: true, [[Configurable]]: true}.
   b. Call the [[DefineOwnProperty]] internal method of O passing P, newDesc, and Throw as arguments.
7. Return.

/be

[Gavin Barraclough]

*** This bug has been marked as a duplicate of bug 96596 ***