Question

I wanted to use a hashtable as set of some objects for a graphic libray, so that I can quickly remove these objects by saying something like set.remove(v2).

But it appears I cannot use these objects while I try to iterate the set, since the keys don’t have the right properties, even though a == shows that they are the same objects!

What can I do to iterate the set and use the properties or functions which are part of the keys?

Test case shown below, with rather unexpected results (for me)!

v1={'a':2,'b':3};
set={};
set[v1]=true;

// Access the object within set
var v2; for(var k in set) v2=k;

// At this moment, v2 and v1 should be the same object, but...

console.log(v2==v1);    // returns true
console.log(JSON.stringify(v1)); // returns {"a":2,"b":3}
console.log(JSON.stringify(v2)); // returns "[object Object]"   !!!
console.log(v1.a);  // returns 2
console.log(v2.a);  // causes error 'reference to undefined property v2.a' !!!

Answer 1

You’re trying to set an object as the key of another object, which isn’t possible in current ECMAScript implementations. When you call set[v1]=true;, internally the engine converts v1 to a string, the result being "[object Object]" with no identifiable properties.

ECMAScript 6 allows you to map objects to other values using WeakMap, but the API is still in the early stages and currently only available in Chrome—with a command line switch—and Firefox.