Static Constructor in Javascript ES6

Question

In ES6, I can create static methods like below. But I need to define a static constructor but no success. I need something that runs only once when the class is loaded. I Is

Answer 1

I need something that runs only once when the class is loaded.

You shouldn't be using classes if you just use them as a bag of methods. Use an object instead. However, it's still possible to run such code. Just put it before or after the class definition.

console.log('before class is created')

class Foo {}

console.log('after class was created');

Answer 2

It does seem neater to have class-setup code inside the class body so the "class expression" is self-contained. ES6 accepts the syntax static constructor() {/* do stuff */} in a class body but never runs it. Perhaps it is for future language expansion? Anyway, here is one way to achieve the desired result. The trick is to initialize a static property with an immediately-executed function expression that does your class setup:

class MyClass {
  static _staticConstructorDummyResult = (function() {
    console.log('static constructor called') // once!
  })()
  constructor () {
    console.log('instance constructor called')
  }
}
let obj = new MyClass(),
    obj2 = new MyClass()

Inside the "static constructor" you can add properties to the class object with MyClass.prop = value, or if you're keen to refer to MyClass as this, change the function expression to an arrow function expression.

You can make _staticConstructorDummyResult non-enumerable using Object.defineProperty(), or if you don't mind requiring Chrome (it won't work in Firefox currently), you can add a # at the front of the name to make it a private property.

Answer 3

If you insist on a static constructor: Define a static method and invoke it after the class definition.

class Foo {
  static staticConstructor() {
    console.log('Foo has been constructed statically!');
  }
}

Foo.staticConstructor()

Of course this is not really necessary. Except mabye to clearly express the notion of a static constructor. However this smells like Java.

Felix proposed a fine solution by putting code before or after the class definition.

For example: Do you want to pre-calculate some static members? Just assign the calculation result after the class definition!

class Foo {}

Foo.preCalculated = calculate();

function calculate() {
  console.log('Do some hard work here');
  return 'PRECALCULATED';
}