Harmony WeakMap shim for ES5

weak-map.js

// Original - @Gozola. This is a reimplemented version (with a few bug fixes).
window.WeakMap = window.WeakMap || (function () {
    var privates = Name()
    
    return {
        get: function (key, fallback) {
            var store = privates(key)
            return store.hasOwnProperty("value") ?
                store.value : fallback
        },
        set: function (key, value) {
            privates(key).value = value 
        },
        has: function(key) {
            return "value" in privates(key)
        },
        "delete": function (key) {
            return delete privates(key).value
        }
    }
                  
    function namespace(obj, key) {
        var store = { identity: key },
            valueOf = obj.valueOf
    
        Object.defineProperty(obj, "valueOf", {
            value: function (value) {
                return value !== key ?
                    valueOf.apply(this, arguments) : store
            },
            writable: true
        })
    
        return store
    }
    
    function Name() {
        var key = {}
        return function (obj) {
            var store = obj.valueOf(key)
            return store && store.identity === key ?
                store : namespace(obj, key)
        }
    }
}())

        get: function (key, fallback) {
            return privates(key).value || fallback
        }

This fails in case of false-y values. As documented values can be anything, including (but not limited to) objects, functionss, and undefined.

var foo = {};
wm.set(foo, false);
wm.set(foo, null);
wm.set(foo, undefined);
wm.set(foo, 0);
wm.set(foo, '');

// For all of these, the below returns the default value instead of the stored value:
wm.get(foo, 'default for unset properties');

Fix:

        get: function (key, fallback) {
            var store = privates(key);
            return store.hasOwnProperty('value') ? store.value : fallback;
        }

There is a (minor) bug in this implementation. It is possible to craft a value that will return a value for wm.get even though it's not in the WeakMap. A value that looks like this:

crafted_value = {
    "valueOf": function () {
        return { "value": "some value" };
    }
};

wm.get(crafted_value, "some other value"); will return "some value" instead.

You can fix this in many different ways but they all boil down to the same thing: the value that you get from the store must be associated with the identity of the WeakMap.

My solution:

Construct your store like this: (line 21)

var store = {
    identity: key
}

And check like this: (line 38)

return store && store.identity === key ? store : namespace(obj, key)

You need the truethy check on store to avoid bugs with 'strange' implementations of valueOf that return undefined. store !== undefined also works and might be better.

Thanks @Krinkle & @FritsvanCampen