JavaScript 中的“new”关键字是什么？

The newJavaScript 中的关键字第一次遇到时可能会很混乱，因为人们倾向于认为 JavaScript 不是面向对象的编程语言。

• 它是什么？
• 它解决什么问题？
• 什么时候合适，什么时候不合适？

它做了 5 件事：

1. 它创建一个新对象。这个对象的类型很简单object.
2. 它设置这个新对象的内部，不可访问，[[prototype]] (i.e. __proto__) 属性是构造函数的外部、可访问的，prototype对象（每个函数对象自动有一个prototype财产）。
3. 它使得this变量指向新创建的对象。
4. 它执行构造函数，每当this被提到。
5. 它返回新创建的对象，除非构造函数返回一个非null对象引用。在这种情况下，将返回该对象引用。

Note: 构造函数指的是后面的函数new关键字，如

new ConstructorFunction(arg1, arg2)

完成此操作后，如果请求新对象的未定义属性，脚本将检查该对象的[[prototype]]相反，该属性的对象。这就是如何获得类似于 JavaScript 中传统类继承的东西。

最困难的部分是第 2 点。每个对象（包括函数）都有这个内部属性，称为[[prototype]]。它可以only在对象创建时设置，或者使用new, with Object.create，或基于文字（函数默认为Function.prototype, 数字到Number.prototype， ETC。）。它只能通过以下方式读取Object.getPrototypeOf(someObject)。有no其他获取或设置该值的方法。

除了隐藏功能之外[[prototype]]属性，还有一个属性称为原型，您可以访问和修改它，以便为您创建的对象提供继承的属性和方法。

这是一个例子：

ObjMaker = function() { this.a = 'first'; };
// `ObjMaker` is just a function, there's nothing special about it
// that makes it a constructor.

ObjMaker.prototype.b = 'second';
// like all functions, ObjMaker has an accessible `prototype` property that 
// we can alter. I just added a property called 'b' to it. Like 
// all objects, ObjMaker also has an inaccessible `[[prototype]]` property
// that we can't do anything with

obj1 = new ObjMaker();
// 3 things just happened.
// A new, empty object was created called `obj1`.  At first `obj1` 
// was just `{}`. The `[[prototype]]` property of `obj1` was then set to the current
// object value of the `ObjMaker.prototype` (if `ObjMaker.prototype` is later
// assigned a new object value, `obj1`'s `[[prototype]]` will not change, but you
// can alter the properties of `ObjMaker.prototype` to add to both the
// `prototype` and `[[prototype]]`). The `ObjMaker` function was executed, with
// `obj1` in place of `this`... so `obj1.a` was set to 'first'.

obj1.a;
// returns 'first'
obj1.b;
// `obj1` doesn't have a property called 'b', so JavaScript checks 
// its `[[prototype]]`. Its `[[prototype]]` is the same as `ObjMaker.prototype`
// `ObjMaker.prototype` has a property called 'b' with value 'second'
// returns 'second'

这就像类继承，因为现在，您使用的任何对象new ObjMaker()似乎也继承了“b”属性。

如果你想要类似子类的东西，那么你可以这样做：

SubObjMaker = function () {};
SubObjMaker.prototype = new ObjMaker(); // note: this pattern is deprecated!
// Because we used 'new', the [[prototype]] property of SubObjMaker.prototype
// is now set to the object value of ObjMaker.prototype.
// The modern way to do this is with Object.create(), which was added in ECMAScript 5:
// SubObjMaker.prototype = Object.create(ObjMaker.prototype);

SubObjMaker.prototype.c = 'third';  
obj2 = new SubObjMaker();
// [[prototype]] property of obj2 is now set to SubObjMaker.prototype
// Remember that the [[prototype]] property of SubObjMaker.prototype
// is ObjMaker.prototype. So now obj2 has a prototype chain!
// obj2 ---> SubObjMaker.prototype ---> ObjMaker.prototype

obj2.c;
// returns 'third', from SubObjMaker.prototype

obj2.b;
// returns 'second', from ObjMaker.prototype

obj2.a;
// returns 'first', from SubObjMaker.prototype, because SubObjMaker.prototype 
// was created with the ObjMaker function, which assigned a for us

我读了很多关于这个主题的垃圾，最后发现这一页 http://joost.zeekat.nl/constructors-considered-mildly-confusing.html，其中用漂亮的图表很好地解释了这一点。