它做了 5 件事：

1. 它创建一个新对象。这个对象的类型就是object。
2. 它将这个新对象的内部、不可访问的[[prototype]]（即proto）属性设置为构造函数的外部、可访问的原型对象（每个函数对象都自动具有原型属性）。
3. 它使this变量指向新创建的对象。
4. this每当提到时，它都会使用新创建的对象执行构造函数。
5. 它返回新创建的对象，除非构造函数返回非null对象引用。在这种情况下，将返回该对象引用。

注意：构造函数是指new关键字后面的函数，如

new ConstructorFunction(arg1, arg2)

完成此操作后，如果请求新对象的未定义属性，脚本将改为检查对象的[[prototype]]对象的属性。这就是在 JavaScript 中获得类似于传统类继承的方法。

最困难的部分是第 2 点。每个对象（包括函数）都有这个称为[[prototype]]的内部属性。它只能在对象创建时设置，可以使用new、Object.create或基于字面量（函数默认为 Function.prototype，数字为 Number.prototype 等）。它只能用Object.getPrototypeOf(someObject)读取。没有其他方法可以设置或读取此值。

函数，除了隐藏的[[prototype]]属性外，还有一个称为原型的属性，您可以访问和修改它，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 the same
// as {}. 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。

如果你想要一个子类之类的东西，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