ES6: call class constructor without new keyword

Question

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

Answer 1

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) };