Why do immutable.js classes not require "new"?

Question

For example, this is great code. (Style - ES6)

import {List} from 'immutable'; console.log(List()); // List [] 

However, this fails.

 class Foo {} Foo(); // TypeError: Cannot call a class as a function 

In addition, this also fails.

 class Foo extends List {} Foo(); // TypeError: Cannot call a class as a function 

Answer 1

The magic for immutable seems to be happening in its custom transporter addon here .

They basically create their own createClass , which skips the check. This is a snippet from the forwarded (via babel ) version of my code above.

 var Foo = (function (_List) { _inherits(Foo, _List); function Foo() { _classCallCheck(this, Board); _get(Object.getPrototypeOf(Foo.prototype), 'constructor', this).apply(this, arguments); } return Foo; })(_immutable.List); 

Where _classCallCheck as follows.

 function _classCallCheck(instance, Constructor) { if (!(instance instanceof Constructor)) { throw new TypeError('Cannot call a class as a function'); } } 

For immutable it seems that ES6 code is first converted to already include createClass calls.