bind 实现

Function.prototype.bind = function(obj, ...args) {
  var fn = this;
  return function() {
    return fn.apply(obj, args)    
  }
}

// Does not work with `new funcA.bind(thisArg, args)`
if (!Function.prototype.bind) (function(){
  // Is this an error? We are invoking <call.bind> method before it's defined.
  var slice = Array.prototype.slice.call.bind(Array.prototype.slice);
  Function.prototype.bind = function() {
    var thatFunc = this, thatArg = arguments[0];
    var args = slice(arguments, 1);
    if (typeof thatFunc !== 'function') {
      // closest thing possible to the ECMAScript 5
      // internal IsCallable function
      throw new TypeError('Function.prototype.bind - ' +
             'what is trying to be bound is not callable');
    }
    return function(){
      var funcArgs = args.concat(slice(arguments))
      return thatFunc.apply(thatArg, funcArgs);
    };
  };
})();