ES6：没有new关键字的调用类构造函数 [英] ES6: call class constructor without new keyword

问题描述

给定一个简单的类

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

可以调用类构造函数， code> new 关键字？

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

使用应允许

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

或

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

但是后者失败了

Cannot call a class as a function

推荐答案

类具有类体，是构造函数。

如果使用内部< c $ c> constructor（）函数，该函数将是相同的类体，并且将在类被调用时被调用，因此一个类总是一个构造函数。

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.

构造函数需要使用 new 运算符来创建一个新实例，因为这样调用不带 new 运算符会导致错误，因为类构造函数需要 创建新实例。

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.

错误消息也非常具体，并且正确

The error message is also quite specific, and correct

TypeError：无法在没有'new'的情况下调用类构造函数

TypeError: Class constructors cannot be invoked without 'new'

您可以;

• 使用常规函数

• 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） p>

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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