无论如何，JavaScript中的`new`是什么意思？ [英] What does `new` in JavaScript do, anyway?

问题描述

我对Javascrpt的构造函数如何工作感到非常困惑;尽管使用该语言已有好几年了（大多数情况下它就像是LISP的半命令版本），但我想更多地了解对象应该如何工作。

I am very confused about how constructors work in Javascrpt; despite using the language for several years (mostly as if it were like a semi-imperative version of LISP) I would like to know more about how objects are supposed to work in it.

鉴于此代码：

function Foo(x) {
    return {
        bar: function() { return x; }
    };
}

调用 myFoo = Foo有什么区别？ 5）和 myFoo = new Foo（5）？或者，换句话说，Javascript 中的构造函数究竟是什么？

What is the difference between calling myFoo = Foo(5) and myFoo = new Foo(5)? Or, in other words, what exactly does a constructor in Javascript do?

推荐答案

调用 myFoo = Foo（5）和 myFoo = new Foo（5）之间的区别是什么？ ？

What is the difference between calling myFoo = Foo(5) and myFoo = new Foo(5)?

该代码没有区别，因为它返回一个对象，并且规范说：

There's no difference for that code, because it returns an object, and the spec says:

• 让结果成为调用F的 [[Call]] 内部属性的结果，提供 obj 作为这个值并提供传递到 [[Construct]] 的参数列表作为args。


• 如果类型（结果）是对象然后返回结果。

• Let result be the result of calling the [[Call]] internal property of F, providing obj as the this value and providing the argument list passed into [[Construct]] as args.
• If Type(result) is Object then return result.

由于该函数返回的是Object的结果，因此使用其结果。如果它没有返回一个对象，或者它检查了这个，你会注意到一个区别，例如，如果你把它重写为：

Since that function returns a result that is an Object, its result is used. You would notice a difference if it did not return an object, or if it checked this, for example if you rewrote it as:

function Foo(x) {
  if (!(this instanceof Foo)) { return new Foo(x); }
  this.bar = function() { return x; };
}
// Now instanceof works.
alert((new Foo) instanceof Foo);

new 在JavaScript中，无论如何？

What does new in JavaScript do, anyway?

new 运算符导致函数用调用此绑定到新创建的 Object ，其原型是该函数的 prototype property。

The new operator causes the function to be called with this bound to a newly created Object whose prototype is that function's prototype property.

对于用户定义的函数，

new f(a, b, c)

相当于

// Create a new instance using f's prototype.
var newInstance = Object.create(f.prototype), result;

// Call the function
result = f.call(newInstance, a, b, c),

// If the result is a non-null object, use it, otherwise use the new instance.
result && typeof result === 'object' ? result : newInstance

注意，语言规范实际上定义了两个操作的函数， [[Call]] 和 [[Construct]] ，所以在某些极端情况下 new 表现得很奇怪。

Note, that the language specification actually defines functions with two operations, [[Call]] and [[Construct]], so there are some corner cases where new behaves oddly.

例如，绑定和内置函数：

For example, bound and built-in functions:

var g = f.call.bind(f);

应定义一个函数，在调用时，只需调用 f ，所以 g 应该在所有方面与 f 相同，但是

should define a function that when called, just calls f, so g should be the same as f in all respects, but

new g()

生成

TypeError: function call() { [native code] } is not a constructor

因为内置函数 Function.prototype.call 支持 [[Call] ] 但不是 [[Construct]] 。

because the builtin function Function.prototype.call supports [[Call]] but not [[Construct]].

Function.prototype.bind 在 new 和普通电话周围也有不同的表现。 此值在调用时始终是绑定的thisValue，但是当您使用 new 时，它是一个新构造的实例。

Function.prototype.bind also behaves differently around new and regular calls. The this value is always the bound thisValue when called, but is a newly constructed instance when you use new.

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