Personally, I prefer Object.getOwnPropertyDescriptor as a property existence
test because it doesn't have the reflection the meta-circularity concern. I
would write Dave's original function as:
function getDefiningObject(obj, key) {
var obj={}.valueOf.call(obj); // ToObject in case primitive value passed
while (obj) {
if (Object.getOwnPropertyDescriptor(obj,key)) return obj;
obj = Object.getPrototypeOf(obj);
}
throw new Error("key " + key + " not found");
}
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