Bind polyfill

bind.js

/**
 * Function bind polyfill
 * https://github.com/ariya/phantomjs/issues/10522
 */

if (!Function.prototype.bind) {
  Function.prototype.bind = function (context /* ...args */) {
    var fn = this;
    var args = Array.prototype.slice.call(arguments, 1);

    if (typeof(fn) !== 'function') {
      throw new TypeError('Function.prototype.bind - context must be a valid function');
    }

    return function () {
      return fn.apply(context, args.concat(Array.prototype.slice.call(arguments)));
    };
  };
}

On this line, why do you put parentheses around the fn variable? I've seen that before, but always wondered what the purpose is.

It's more a visual thing than a function thing... I like how it reads.

Why do you concatenate an empty array with args and Array.prototype.slice.call(arguments), instead of doing args.concat(Array.prototype.slice.call(arguments))? Also, I feel like it might be more efficient/faster to do Array.prototype.slice.call(arguments) once, and then get the args from that.

I could do args.concat(Array.prototype.slice.call(arguments)). It's just what I typed... 😄

You mean to do Array.prototype.slice.call(arguments) outside of the anonymous function I return? It's key concat both, the args from the call to bind and the args coming into the anonymous function. That's is because you need to handle code like fn.apply.bind where the call to bind gets its own arguments and the callback also gets its own arguments. Or maybe I misunderstood your suggestion.