How to narrow and secure types with const assertions

I'm continuing my journey of learning TypeScript, and it's a wild ride. Sometimes I feel like I have things under control, but other times, I'm looking at a type definition at work, and all I can do is scratch my head in confusion. It'll be a long journey!

Today I discovered a somewhat basic but very useful feature that's in TS since 3.4 (released in 2019). That's a century in web development, but hey, I'm still starting out!

Let's look at a trivial example.

const resource = {id: '123'};

if (resource.id === '234') {
  console.log('yay!');
}

If you look at it, you'll probably see that there's no way this code logs out yay! because the resource.id isn't changed to 234. But TypeScript doesn't know that, and if you inspect the resulting type, you'll find that resource.id is just a string. And that does make sense because it doesn't matter if you declare an object with the const keyword; you can still change the properties as you like.

But what if you want to treat the object as truly immutable? Turns out, you can narrow types and explicitly tell TypeScript that resource won't change with two words — as const.

const resource = {id: '123'} as const;

if (resource.id === '234') {
  console.log('yay');
}

And voilà — look at this extra level of type safety! 👏

Now that TypeScript knows that resource is immutable, it narrows the property type from string to 123, and it complains if you're trying to alter the constant object. That's pretty handy for all sorts of operations!

The TypeScript docs are a quick read if you want to learn more.