JavaScript中的'new'关键字是什么？

Question

JavaScript中的'new'关键字是什么？

newJavaScript中的关键字在第一次遇到时会非常混乱，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？

• 它解决了什么问题？

• 什么时候适当，什么时候不适合？

Answer 1

让初学者更好地理解它在浏览器控制台中尝试以下代码。function Foo() {     return this; }var a = Foo();       //returns window objectvar b = new Foo();   //returns empty object of fooa instanceof Window;  // truea instanceof Foo;     // falseb instanceof Window;  // falseb instanceof Foo;     // true现在你可以阅读社区wiki的答案 :)

Answer 2

它做了5件事：它创建了一个新对象。该对象的类型只是对象。它将此新对象的内部，不可访问，[[prototype]]（即__ proto__）属性设置为构造函数的外部可访问原型对象（每个函数对象自动具有原型属性）。它使this变量指向新创建的对象。它执行构造函数，只要this提到就使用新创建的对象。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，脚本将检查该对象的[[prototype]]对象。这就是你如何在JavaScript中获得类似于传统类继承的东西。关于这一点最困难的部分是第2点。每个对象（包括函数）都有一个名为[[prototype]]的内部属性。它只能在对象创建时设置，可以是new，使用Object.create，也可以是基于文字（函数默认为Function.prototype，数字为Number.prototype等）。它只能用Object.getPrototypeOf（someObject）读取。有没有其他的方式来设置或读取此值。除了[[prototype]]属性之外，函数还有一个名为prototype的属性，您可以访问和修改这些属性，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 withobj1 = 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'这就像类继承一样，因为现在，您使用的任何对象new ObjMaker()也似乎都继承了'b'属性。如果你想要类似子类的东西，那么你这样做：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.prototypeobj2.c;// returns 'third', from SubObjMaker.prototypeobj2.b;// returns 'second', from ObjMaker.prototypeobj2.a;// returns 'first', from SubObjMaker.prototype, because SubObjMaker.prototype // was created with the ObjMaker function, which assigned a for us在最终找到这个页面之前，我读了很多关于这个主题的垃圾，这里用漂亮的图表很好地解释了这个问题。