Return a value other than the class in ES6

Question

Recently I've been testing out classes with ES6, I've noticed that when creating a class you cannot specify the value given by the constructor.

Previously in ES5 this was possible.

In both cases I would instantiate the class with new MyClass The reason I want to do this is so I can return a subset of the current class with only functions on it.

ES5 - returns My class was init with: Blah

var MyClass = function() {
  this.initVar = 'Blah'

  return 'My Class was init with: ' + this.initVar
}

ES6 - returns {}

class Bob {
  constructor() {
   return 'hello' 
  }
}

Answer 1

According to the Class article from the TC39 web site, the ES6 class syntax has an implicit constructor function that is called if no such function is provided in the class definition.

This can be overridden by providing your own constructor and returning whatever object you want, e.g.:

class Bob {
  constructor(name) {
    return {hi: name};  // returns an object other than *this*.
  }
}

In action:

var bob = new Bob('bob');
console.log(bob.hi); // 'bob'

To extend the class, you can do:

class Bill extends Bob {
}

However extends also has an implicit constructor, so it will return a new instance of Bill that inherits from Bob.prototype. Since hi was defined in the constructor and not inherited, you get:

var bill = new Bill('bill');
console.log(bill.hi);  // undefined

To initialise Bill the same as Bob, provide a constructor that calls super. This call also changes the this object of Bill to the value returned by super, e.g.

class Bill extends Bob {
  constructor(name) {
    super(name);
  }
}

Now:

var bill = new Bill('bill');
console.log(bill.hi); // bill

Also worth noting that the body of a classDeclaration is always strict mode code.

As a runnable snippet:

class Bob {
  constructor(name) {
    return {hi: name};  // returns an object other than *this*.
  }
}

var bob = new Bob('bob');
console.log(bob.hi); // 'bob'

class Bill extends Bob {
  constructor(name) {
    super(name);
  }
}

var bill = new Bill('bill');
console.log(bill.hi); // bill