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)
        }
    }
}())

Thanks @Krinkle & @FritsvanCampen