forIn.js #1

var hasOwn = require('./hasOwn');

    var _hasDontEnumBug,
        _dontEnums;

    function checkDontEnum(){
        _dontEnums = [
                'toString',
                'toLocaleString',
                'valueOf',
                'hasOwnProperty',
                'isPrototypeOf',
                'propertyIsEnumerable',
                'constructor'
            ];

        _hasDontEnumBug = true;

        for (var key in {'toString': null}) {
            _hasDontEnumBug = false;
        }
    }

    /**
     * Similar to Array/forEach but works over object properties and fixes Don't
     * Enum bug on IE.
     * based on: http://whattheheadsaid.com/2010/10/a-safer-object-keys-compatibility-implementation
     */
    function forIn(obj, fn, thisObj){
        var key, i = 0;
        // no need to check if argument is a real object that way we can use
        // it for arrays, functions, date, etc.

        //post-pone check till needed
        if (_hasDontEnumBug == null) checkDontEnum();

        for (key in obj) {
            if (exec(fn, obj, key, thisObj) === false) {
                break;
            }
        }


        if (_hasDontEnumBug) {
            var ctor = obj.constructor,
                isProto = !!ctor && obj === ctor.prototype;

            while (key = _dontEnums[i++]) {
                // For constructor, if it is a prototype object the constructor
                // is always non-enumerable unless defined otherwise (and
                // enumerated above).  For non-prototype objects, it will have
                // to be defined on this object, since it cannot be defined on
                // any prototype objects.
                //
                // For other [[DontEnum]] properties, check if the value is
                // different than Object prototype value.
                if (
                    (key !== 'constructor' ||
                        (!isProto && hasOwn(obj, key))) &&
                    obj[key] !== Object.prototype[key]
                ) {
                    if (exec(fn, obj, key, thisObj) === false) {
                        break;
                    }
                }
            }
        }
    }

    function exec(fn, obj, key, thisObj){
        return fn.call(thisObj, obj[key], key, obj);
    }

    module.exports = forIn;