bind.js

Function.prototype.bind = function (thisArg) {
  var fn = aFunction(this),
    slice = [].slice,
    partArgs = slice.call(arguments, 1),
    boundF

  boundF = function () {
    var args = partArgs.concat(slice.call(arguments))
    var arr = []
    for (var i = 0; i < args.length; i++) {
      arr.push('a[' + i + ']')
    }
    return this instanceof boundF
      ? Function('C,a', 'return new C(' + arr + ')')(fn, args)
      : fn.apply(thisArg, args)
  }
  return boundF
}