How to extend existing constructor function in javascript?

Question

Let\'s suppose that I have the following object function:

function A(options){
 ...
}

Then I want to create a new function (B) that inherit

Answer 1

Just call A constructor with this

function B(aOptions, bOptions) {
  A.call(this, aOptions);

  // do stuff with bOptions here...
}

Now to setup the prototype

B.prototype = Object.create(A.prototype, {
  constructor: {
    value: B
  }
});

Now B will have the prototype methods from A.

Any new methods added to B's prototype will not be available to A's prototype

---

There's a couple other tweaks that can make your life easier too.

function A(options) {

  // make `new` optional
  if (!(this instanceof A)) {
    return new A(options);
  }

  // do stuff with options here...
}

And do the same for B

function B(aOptions, bOptions) {

  // make `new` optional
  if (!(this instanceof B)) {
    return new B(aOptions, bOptions);
  }

  // call parent constructor
  A.call(this, aOptions);

  // do stuff with bOptions here...
}

Now you can call A(options) or new A(options) to get the same result.

Same with B, B(aOptions, bOptions) or new B(aOptions, bOptions) will get the same result.