michael@0: // Array.of calls a "length" setter if one is present.
michael@0: 
michael@0: var hits = 0;
michael@0: var lastObj = null, lastVal = undefined;
michael@0: function setter(v) {
michael@0:     hits++;
michael@0:     lastObj = this;
michael@0:     lastVal = v;
michael@0: }
michael@0: 
michael@0: // when the setter is on the new object
michael@0: function Pack() {
michael@0:     Object.defineProperty(this, "length", {set: setter});
michael@0: }
michael@0: Pack.of = Array.of;
michael@0: var pack = Pack.of("wolves", "cards", "cigarettes", "lies");
michael@0: assertEq(lastObj, pack);
michael@0: assertEq(lastVal, 4);
michael@0: 
michael@0: // when the setter is on the new object's prototype
michael@0: function Bevy() {}
michael@0: Object.defineProperty(Bevy.prototype, "length", {set: setter});
michael@0: Bevy.of = Array.of;
michael@0: var bevy = Bevy.of("quail");
michael@0: assertEq(lastObj, bevy);
michael@0: assertEq(lastVal, 1);