Call Class Constructor Without new Keyword in ES6

Given the class definition:

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

It is not possible to directly call the class constructor without the new keyword. This is because classes in ES6 inherently have a constructor function that is invoked when the class is called.

Invoking a class without new results in an error:

Cannot call a class as a function

This error message clearly indicates that the class constructor can only be called with the new operator, which is required to create a new instance of the class.

To overcome this limitation, consider the following approaches:

• Use a regular function instead:

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

• Always call the class with new:

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

• Wrap the class in a function and call with new:

var FooWrapper = function(...args) { return new Foo(...args) };
FooWrapper("world").hello(); // "hello world"