这张图再次表明,每个对象都有一个原型。构造函数
function Foo也有自己的__proto__,也就是function .prototype,
而它又通过__proto__属性再次引用
Object.prototype。因此,重复,Foo。原型只是一个显式
Foo的属性,引用b和c对象的原型。
var b = new Foo(20);
var c = new Foo(30);
__proto__和prototype之间有什么区别?
这一数据来自dmitrysoshnikov.com网站。
注:上述2010年的文章现在有第二版(2017年)。
我认为你需要知道__proto__, [[prototype]]和prototype之间的区别。
公认的答案是有帮助的,但它可能暗示(不完全)__proto__只与在构造函数上使用new创建的对象相关,这是不正确的。
更准确地说:__proto__存在于每个对象上。
But what is __proto__ at all?
Well, it is an object referencing another object which is also a property of all objects, called [[prototype]].
It's worth mentioning that [[prototype]] is something that JavaScript handles internally and is inaccessible to the developer.
Why would we need a reference object to the property [[prototype]] (of all objects)?
Because JavaScript doesn't want to allow getting / setting the [[prototype]] directly, so it allows it through a middle layer which is __proto__. So you can think of __proto__ as a getter/setter of the [[prototype]] property.
What is prototype then?
It is something specific to functions(Initially defined in Function, i.e, Function.prototype and then prototypically inherited by newly created functions, and then again those functions give it to their children, forming a chain of prototypical inheritance).
JavaScript uses a parent function's prototype to set its child functions' [[prototype]] when that parent function is run with new (remember we said all objects have [[prototype]]? well, functions are objects too, so they have [[prototype]] as well). So when the [[prototype]] of a function(child) is set to the prototype of another function(parent), you will have this in the end:
let child = new Parent();
child.__proto__ === Parent.prototype // --> true.
(Remember child.[[prototype]] is inaccessible, so we checked it using __proto__.)
注意1:只要属性不在子对象中,它的__proto__将被“隐式”搜索。例如,if child。Myprop返回一个值,你不能说" Myprop "是子对象的属性,还是父对象原型的属性。这也意味着你永远不需要做这样的事情:child.__proto__.__proto__。我的东西你自己拿去吧,孩子。Myprop会自动为你做这件事。
注意2:即使父对象的原型中有项目,子对象自己的原型最初也是一个空对象。如果您想进一步扩展继承链(将child[ren]添加到child),则可以向其中添加项或手动从其中删除项。或者可以隐式地操纵它,例如使用类语法。)
注意3:如果你需要自己设置/获取[[prototype]],使用__proto__有点过时,现代JavaScript建议使用Object。setPrototypeOf和Object。getPrototypeOf代替。
正如这句话所说
__proto__是用于查找链的实际对象
原型是用来构建的对象
当你用new创建一个对象时:
(新的Foo)。__proto__ === Foo.prototype;
(新的Foo)。原型=== undefined;
我们可以进一步注意到,使用函数构造函数创建的对象的__proto__属性指向相应构造函数的prototype属性所指向的内存位置。
如果我们改变构造函数的prototype的内存位置,派生对象的__proto__仍将继续指向原始地址空间。因此,要使公共属性沿着继承链向下可用,总是将属性附加到构造函数函数原型,而不是重新初始化它(这会改变它的内存地址)。
考虑下面的例子:
function Human(){
this.speed = 25;
}
var himansh = new Human();
Human.prototype.showSpeed = function(){
return this.speed;
}
himansh.__proto__ === Human.prototype; //true
himansh.showSpeed(); //25
//now re-initialzing the Human.prototype aka changing its memory location
Human.prototype = {lhs: 2, rhs:3}
//himansh.__proto__ will still continue to point towards the same original memory location.
himansh.__proto__ === Human.prototype; //false
himansh.showSpeed(); //25