JavaScript 中的 new 运算符是如何工作的? [英] How does the new operator work in JavaScript?

问题描述

可能是 JavaScript 中最不为人知的部分，站在原型链旁边.

Probably the least understood part of JavaScript, standing beside the prototype chain.

所以问题是:如何...

So the question is: how does...

new dataObj(args); 

...实际上创建一个对象，并定义其原型链/构造函数等?

...actually create an object, and define its prototype chain/constructors/etc?

最好是展示一个替代方案，以完全理解这个关键字.

Best is to show an alternative, to fully understand this keyword.

推荐答案

new 操作符使用内部的 [[Construct]] 方法，它基本上做了以下事情:

The new operator uses the internal [[Construct]] method, and it basically does the following:

• 初始化一个新的本地对象
• 设置此对象的内部[[Prototype]]，指向Function prototype 属性.
  • 如果函数的 prototype 属性不是对象(原始值，例如 Number、String、Boolean、Undefined 或 Null)，则使用 Object.prototype相反.
  • Initializes a new native object
  • Sets the internal [[Prototype]] of this object, pointing to the Function prototype property.
    • If the function's prototype property is not an object (a primitive values, such as a Number, String, Boolean, Undefined or Null), Object.prototype is used instead.

    new 操作符的等效实现可以这样表达(假设 ECMAScript 5 Object.create 方法可用):

    An equivalent implementation of what the new operator does, can be expressed like this (assuming that the ECMAScript 5 Object.create method is available):

    function NEW(f) {
      var obj, ret, proto;
    
      // Check if `f.prototype` is an object, not a primitive
      proto = Object(f.prototype) === f.prototype ? f.prototype : Object.prototype;
    
      // Create an object that inherits from `proto`
      obj = Object.create(proto);
    
      // Apply the function setting `obj` as the `this` value
      ret = f.apply(obj, Array.prototype.slice.call(arguments, 1));
    
      if (Object(ret) === ret) { // the result is an object?
        return ret;
      }
      return obj;
    }
    
    // Example usage:
    function Foo (arg) {
      this.prop = arg;
    }
    Foo.prototype.inherited = 'baz';
    
    var obj = NEW(Foo, 'bar');
    obj.prop;          // 'bar'
    obj.inherited;     // 'baz'
    obj instanceof Foo // true
    

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