Mapped Types 9 exercises

Remapping Object Keys in a Mapped Type

Here's the solution to this challenge:

type AttributeGetters = {
[K in keyof Attributes as `get${Capitalize<K>}`]: () => Attributes[K]

Let's break it down.

The as keyword

This is the first appearance of the as keyword in this workshop.

You might have seen it used like `const

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0:00 Let's examine this solution here. The only new thing that we've got is this as here. The as is basically acting like a key remapper. If we remove this, we end up with the type that we are grabbing from here and the solution that we had before.

0:17 After this, we get to remap the keys as something new. We can change this to wow, if we like. We're going to get an interesting situation, because we're remapping multiple things to the wow. We end up with a function that can return string or number, which is interesting in itself.

0:34 You might have seen this as before as const num = 1 as number for instance or as unknown or things like this. You've seen as kind of this attached a runtime variables, but it can also be attached here. TypeScript has this funny habit of reusing different syntax in different places. Here, as is doing one thing. Here, as is doing a different thing. Good on you TypeScript.

1:03 We've got this as and what it's doing then is we're basically saying, OK, we get to have our cake and eat it too here, because we get access to the original key. This is going to be first name, last name, or age, which means we can still grab attributes K here, which is really critical, but we also get to remap it to something else.

1:25 What's going on here is we're creating a template literal. We've got our K inside here, and if we had this like this, then we would have get lower case firstName, get lower case lastName, get age, but we're wrapping this in capitalize, meaning that we get a nice camel case read out here, get firstName, get lastName, get age.

1:46 This syntax then is available pretty much whenever...You don't need to use keyof for this. You can use firstName, or lastName, or age here, and that will still work, but it gives you this ability to remap keys inside an object, which is of fascinating.