ES6 Everyday: Extending Built-in Classes

May 14, 2015

Along with inheritance of our own classes, ES6 also offers us the ability to extend built-in classes.

One of the best use cases I’ve seen of this is Axel Rauschmayer’s example of extending the Error class:

class ParsingError extends Error
{
   // ...
}

A practice that is common in other languages, like Java and C#.

Of course, you can go far beyond this; this example from StackOverflow implements a stack by extending Array:

class Stack extends Array
{
   constructor(length)
   {
      super(length);
   }

   top()
   {
      return this[this.length - 1];
   }
}

According to the ES6 Compatibility Table, we should eventually have the option to extend all of the following built-in classes:

Array
Boolean
Error
Function
Map
Number
Promise
RegExp
Set
String

However, the ES6 Compatibility Table also shows us that this is basically unsupported across the board (a board that includes transpilers, browsers, and server runtimes), with a few small exceptions.

So, unfortunately, it’s a little too early to take this one out for a spin.