关于bind & new 的理解

[wangliang1124]

先看MDN上bind的polyfill

`

if (!Function.prototype.bind) {
  Function.prototype.bind = function(oThis) {
    if (typeof this !== 'function') {
      throw new TypeError('Function.prototype.bind - what is trying to be bound is not callable');
    }

    var aArgs   = Array.prototype.slice.call(arguments, 1),
        fToBind = this,
        fNOP    = function() {},
        fBound  = function() {
          return fToBind.apply(this instanceof fNOP
                 ? this
                 : oThis,
                 aArgs.concat(Array.prototype.slice.call(arguments)));
        };

    if (this.prototype) {
      // Function.prototype doesn't have a prototype property
      fNOP.prototype = this.prototype; 
    }
    fBound.prototype = new fNOP();

    return fBound;
  };
}

`
接着，这是《你不知道的JavaScript》中的一段代码，这段代码怎么执行的？

`

function foo(something) {
  this.a = something;
}
var obj1 = {};
var bar = foo.bind( obj1 ); 
bar( 2 );
console.log( obj1.a ); // 2
var baz = new bar(3);
console.log( obj1.a ); // 2
console.log( baz.a ); // 3

`

首先看这一句var bar = foo.bind( obj1 ); , bind被foo调用（bind的执行环境是foo），因此polyfill中fToBind = this,的this就是foo，模拟过程如下：

`

var obj1 = {'__proto__': foo.prototype}
fNOP.apply(obj1, arguments);
fbound.prototype = obj1;
bar = fBound;

`

然后看这一句var baz = new bar(3);，模拟如下：

`

// bar.prototype === obj1, obj1 === { __proto__: foo.prototype }, foo.prototype === { constructor: foo, __proto__} 
var obj2 = { __proto__: bar.prototype }  //  obj2 = { __proto__: {__proto__: { constructor: foo, __proto__} } }
bar.apply(obj2, arguments)
baz = obj2

`