Example of WeakMap usage. Run in Firefox Nightly (>6.x)

weakmap.js

var k = {}, v, 
    w = new WeakMap();

w.set( k, "value" );

console.log(
  w.get( k )
);

// ...At some point the program no longer needs this
k = null;

// Calling w.has( k ) using the es-labs implementation will return a useful `undefined`, here:
// http://code.google.com/p/es-lab/source/browse/trunk/src/ses/WeakMap.js?r=491
// The current FF6.x implementation throws an exception "value is not a non-null object"
if ( w.has( k ) ) {
  console.log(
    w.get( k )
  );
}

@be got it. I tried to walk through that logic rather poorly, above. dherman implied that Map was being heavily considered as well. Is there a reason we'd ever want Map over WeakMap? And if not, why not just called WeakMap -> Map - and just have it behave nicely with GC? (and thanks for your patience)

It's a good q, we have considered adding only WeakMap to Harmony, and Map and Set are not yet in harmony:proposals. But WeakMaps cost noticeably more in some applications than Maps, and if you want strong refs and that cost matters, then you'll grow to hate Map=WeakMap. So we're considering Map (and Set, which is just Map with key only, no value).

/be

@BrendanEich So, that's exactly what I expected, except that I only saw one log line - because I didn't notice the tiny "2" on the end of the line (incidentally, I was also pulled over sitting in the back seat of my Jeep when posted that - my phone had buzzed alerting me of the first few comments and couldn't wait until I got home) In a nutshell: rush job b/w a good story.