Calling ES6 Class Constructors Without "new"

In ES6, classes are syntactic sugar for constructor functions. When a class is invoked without the new keyword, it does not create a new instance of the class. Instead, it calls the class's constructor function directly.

Consider the following class:

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

If we try to call this class without the new keyword, we get an error:

Cannot call a class as a function

This is because class constructors are designed to create new instances of the class. Invoking them without the new operator is equivalent to calling a regular function.

To allow calling a class constructor without the new keyword, we can use a combination of a constructor function and an arrow function as follows:

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

const FooWithoutNew = () => new Foo(...arguments);

Now, we can call the class constructor without the new keyword using FooWithoutNew:

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

However, it's important to note that this approach has some drawbacks. First, it requires creating a separate function, which can be inconvenient. Second, it breaks the constructor's behavior of returning a new instance.

In general, it is recommended to always call class constructors with the new keyword for clarity and consistency.