I am trying to transform some es5 code to es6 and I stumbled upon the following code and I am wondering if I could replace the util.inherits with the extends keyword for classes. I am a bit confused if they do the same thing.

ES5

var EventEmitter = require('events').EventEmitter;
var util = require('util');

function TheEmitter() {
    EventEmitter.call(this);
}
util.inherits(TheEmitter, EventEmitter);

ES6

const EventEmitter = require('events').EventEmitter;

class TheEmitter extends EventEmitter {
   ...
}

I am trying to transform some es5 code to es6 and I stumbled upon the following code and I am wondering if I could replace the util.inherits with the extends keyword for classes. I am a bit confused if they do the same thing.

ES5

var EventEmitter = require('events').EventEmitter;
var util = require('util');

function TheEmitter() {
    EventEmitter.call(this);
}
util.inherits(TheEmitter, EventEmitter);

ES6

const EventEmitter = require('events').EventEmitter;

class TheEmitter extends EventEmitter {
   ...
}

• javascript
• node.js
• inheritance
• ecmascript-6
• es6-class

• 1 Looks like you already did? – Bergi Commented May 28, 2017 at 11:21

1 Answer 1

Since the class and extends keywords are only syntactic sugar on top of prototypal inheritance, the answer simply is: Yes, you can replace util.inherits by extends and keep the same behavior.

Of course, there are minor things to watch out for, e.g. you need to make sure to call the super constructor in your derived class's constructor, whereas with util.inherits you had to call the constructor function and apply it to this. But effectively, these things are only other syntactic constructs, semantically, they are equivalent.

Then, of course, there are some practical issues, where both options differ from each other. E.g., when defining Foo using the class keyword, you can not call Foo without using the new keyword. Without the class keyword, this is perfectly possible (although not meaningful, so you shouldn't have done this anyway).

So, to cut a long story short: Apart from some strange effects that only happen if you used rare and strange constructs (like calling a constructor function without new), the transition should be seamless.