How and why would I write a class that extends null?

Question

JavaScript\'s class syntax, added in ES6, apparently makes it legal to extend null:

class foo extends null {}

Some Googling reveals that it was

Answer 1

To answer the second part:

I can't make much sense of this hypothetical use case.

That way, your object won't have Object.prototype in its prototype chain.

class Hash extends null {}
var o = {};
var hash = new Hash;
o["foo"] = 5;
hash["foo"] = 5;
// both are used as hash maps (or use `Map`).
hash["toString"]; // undefined
o["toString"]; // function

As we know, undefined in fact is not a function. In this case we can create objects without fearing a call on a field that shouldn't be there.

This is a common pattern through Object.create(null) and is common in a lot of code bases including big ones like NodeJS.

Answer 2

Instantiating such classes is meant to work; Chrome and Firefox just have bugs. Here's Chrome's, here's Firefox's. It works fine in Safari (at least on master).

There used to be a bug in the spec which made them impossible to instantiate, but it's been fixed for a while. (There's still a related one, but that's not what you're seeing.)

The use case is roughly the same as that of Object.create(null). Sometimes you want something which doesn't inherit from Object.prototype.