ES6:不使用 new 关键字调用类构造函数 [英] ES6: call class constructor without new keyword

问题描述

给定一个简单的类

class Foo {构造函数(x){if (!(this instanceof Foo)) 返回新的 Foo(x);this.x = x;}你好() {返回`你好 ${this.x}`;}}

是否可以在没有new关键字的情况下调用类构造函数?

应允许使用

(new Foo("world")).hello();//你好世界"

或

Foo("world").hello();//你好世界"

但后者失败了

不能将类作为函数调用

解决方案

类有一个类体"，它是一个构造函数.
如果您使用内部 constructor() 函数，则该函数也将是相同的类主体，并且将是调用类时调用的函数，因此类始终是构造函数.>

构造函数需要使用 new 运算符来创建新实例，因此调用没有 new 运算符的类会导致错误，因为它是 需要类构造函数来创建新实例.

错误信息也很具体，正确

<块引用>

TypeError: 类构造函数不能在没有 'new' 的情况下被调用

你可以；

• 使用常规函数代替类¹.
• 始终使用 new 调用类.
• 在常规包装函数中调用类，始终使用 new，这样您就可以获得类的好处，但是仍然可以在使用和不使用 new操作符².

<小时>

¹⁾

function Foo(x) {if (!(this instanceof Foo)) 返回新的 Foo(x);this.x = x;this.hello = 函数(){返回 this.x;}}

²⁾

class Foo {构造函数(x){this.x = x;}你好() {返回`你好 ${this.x}`;}}var _old = Foo;Foo = function(...args) { return new _old(...args) };

Given a simple class

class Foo {
  constructor(x) {
    if (!(this instanceof Foo)) return new Foo(x);
    this.x = x;
  }
  hello() {
    return `hello ${this.x}`;
  }
}

Is it possible to call the class constructor without the new keyword?

Usage should allow

(new Foo("world")).hello(); // "hello world"

Or

Foo("world").hello();       // "hello world"

But the latter fails with

Cannot call a class as a function

解决方案

Classes have a "class body" that is a constructor.
If you use an internal constructor() function, that function would be the same class body as well, and would be what is called when the class is called, hence a class is always a constructor.

Constructors requires the use of the new operator to create a new instance, as such invoking a class without the new operator results in an error, as it's required for the class constructor to create a new instance.

The error message is also quite specific, and correct

TypeError: Class constructors cannot be invoked without 'new'

You could;

• either use a regular function instead of a class¹.
• Always call the class with new.
• Call the class inside a wrapping regular function, always using new, that way you get the benefits of classes, but the wrapping function can still be called with and without the new operator².

---

¹⁾

function Foo(x) {
    if (!(this instanceof Foo)) return new Foo(x);
    this.x = x;
    this.hello = function() {
        return this.x;
    }
}

²⁾

class Foo {
    constructor(x) {
        this.x = x;
    }
    hello() {
        return `hello ${this.x}`;
    }
}

var _old = Foo;
Foo = function(...args) { return new _old(...args) };

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