jashkenas/underscore

/********************************************************************************************
 * Native code in underscore.js
 * "emulatedSet" was called in "collectNonEnumProps", which aim to solve the ieEnumBug
 ********************************************************************************************/
// Internal helper to create a simple lookup structure.
// `collectNonEnumProps` used to depend on `_.contains`, but this led to
// circular imports. `emulatedSet` is a one-off solution that only works for
// arrays of strings.
function emulatedSet(keys) {
    var hash = {};
    for (var l = keys.length, i = 0; i < l; ++i) hash[keys[i]] = true;
    return {
        contains: function(key) { return hash[key]; },
        push: function(key) {
            hash[key] = true;
            return keys.push(key);
        }
    };
}

var hasEnumBug = !{ toString: null }.propertyIsEnumerable('toString');
var nonEnumerableProps = ['valueOf', 'isPrototypeOf', 'toString',
    'propertyIsEnumerable', 'hasOwnProperty', 'toLocaleString'
];

// Internal helper. Checks `keys` for the presence of keys in IE < 9 that won't
// be iterated by `for key in ...` and thus missed. Extends `keys` in place if
// needed.nonEnumerableProps
function collectNonEnumProps(obj, keys) {
    keys = emulatedSet(keys);
    var nonEnumIdx = nonEnumerableProps.length;
    var constructor = obj.constructor;
    var proto = Object.prototype.toString.call(constructor) == '[Object Function]' &&
        constructor.prototype || Object.prototype;

    // Constructor is a special case.
    var prop = 'constructor';
    if (obj.hasOwnProperty(prop) && !keys.contains(prop)) keys.push(prop);

    while (nonEnumIdx--) {
        prop = nonEnumerableProps[nonEnumIdx];
        if (prop in obj && obj[prop] !== proto[prop] && !keys.contains(prop)) {
            keys.push(prop);
        }
    }
}

/*********************************************************************************
 * my test code
 *********************************************************************************/
var obj = { toString: null, valueOf: 'value' };
// in IE<9, the keys for obj will be []

var keys = [];
// keys will be [], but we expert ['toString','valueOf']
collectNonEnumProps(obj, keys); // keys = []