var hasOwn = Object.prototype.hasOwnProperty;
var toString = Object.prototype.toString;

var isFunction = function (fn) {
	var isFunc = (typeof fn === 'function' && !(fn instanceof RegExp)) || toString.call(fn) === '[object Function]';
	if (!isFunc && typeof window !== 'undefined') {
		isFunc = fn === window.setTimeout || fn === window.alert || fn === window.confirm || fn === window.prompt;
	}
	return isFunc;
};

module.exports = function forEach(obj, fn) {
	if (!isFunction(fn)) {
		throw new TypeError('iterator must be a function');
	}
	var i, k,
		isString = typeof obj === 'string',
		l = obj.length,
		context = arguments.length > 2 ? arguments[2] : null;
	if (l === +l) {
		for (i = 0; i < l; i++) {
			if (context === null) {
				fn(isString ? obj.charAt(i) : obj[i], i, obj);
			} else {
				fn.call(context, isString ? obj.charAt(i) : obj[i], i, obj);
			}
		}
	} else {
		for (k in obj) {
			if (hasOwn.call(obj, k)) {
				if (context === null) {
					fn(obj[k], k, obj);
				} else {
					fn.call(context, obj[k], k, obj);
				}
			}
		}
	}
};