bind.js

Function.prototype.bind = function(thisObj) {
	var that = this, args = Array.prototype.slice.call(arguments, 1);
	return function() {
                var xArgs = Array.prototype.slice.call(arguments);
		return that.apply(thisObj, args.concat(xArgs));
	};
};

function fn() {
	return this;
}

var a = { a : 1 };
var b = { b : 2 };

console.log(fn.call(a)); // { a : 1 }
console.log(fn.call(b)); // { b : 1 }

var boundFn = fn.bind(a);
console.log(fn(a)); // global object