globalThis is coming to JavaScript

#javascript #webdev #node #learning

One of the first things we learn with JavaScript is the keyword this. What is tricky to understand at first is that its definition is always different. It depends on the scope we're accessing it in.

Well, in all projects there is a "global" this. But it's called something different depending on what context you're in. On the web you may be familiar with it as the window object. In other contexts it's self and sometimes it's this!

As it turns out, there is a function that's been refined over time to always access it.

var getGlobal = function () {
    if (typeof self !== 'undefined') { return self; }
    if (typeof window !== 'undefined') { return window; }
    if (typeof global !== 'undefined') { return global; }
    throw new Error('unable to locate global object');
};

Not exactly pretty. And a pain to include in every project.

But no more! Now at stage 4, globalThis is the latest addition to ECMAScript.

Top comments (10)

Ben Halpern • Nov 21 '19

Nobody will ever be happy with the name globalThis 😂

Seems really useful though!

Nick • Nov 21 '19

Yeaaa... usefulness is an 11/10 but name scheme is about a -1/10

Rasmus Wennerström • Nov 22 '19

I don't see why they couldn't just name it global. There's the "let's not conflict"-argument, and "let's fight around with existing types".

But in all honesty... there's not much you can "assume" about the global object anyways, you will still have to check if (global.feature) for most features if you're building a cross-platform lib, and I'm not sure how globalThis would help with that.

Laurie • Nov 22 '19

"Attempts were made to ship under the name global, but it turns out that this does, in fact, break some existing websites." per the spec github.com/tc39/proposal-global

Rasmus Wennerström • Nov 22 '19

Thank you for sharing! ✌️

Seanmclem • Nov 21 '19

What might be a use-case where I would want a globalThis instead of just manually accessing window, or this?

Laurie • Nov 21 '19 • Edited

It's a standard. So it will work consistently regardless of what environment the code is run in.

Rasmus Wennerström • Nov 22 '19

"As consistent as it can get".

Different global objects will expose very different APIs, so now instead of having to check if (typeof global !== 'undefined') global.nodeFeature(), you can now check:

if (globalThis.nodeFeature) { 
  globalThis.nodeFeature();
} else if (globalThis.webFeature) { 
  globalThis.webFeature();
}

I think it's useful, but it's just kind of deferring the problem, as you will still have to write platform-specific abstractions.

Seanmclem • Nov 22 '19

Yeah I guess that was my concern. If the different 'this' you're targeting can have radically different APIs, I would wonder why target them with a single globalThis instead of individually. I figured I was missing something..