Exclude type under the hood in TypeScript

Sixth challenge is Exclude

Don't mix up Exclude with Omit:

• Exclude is used for union types
• Omit is used for object types so you can remove specified keys within it

Iteration over a union type

We already know how to iterate over:

• Object types (see Pick under the hood)
• Tuple types (see Making object out of tuple)

For union types there are Distributive Conditional Types:

Let's check what's going on step by step:

So it's literally applied to each element of a union type 💫

We can use Distributive Conditional Types with different conditions:

1. Type extends any
2. Type extends unknown
3. Reversed Type extends never (as this is false for every Type)

It works the same way because any and unknown are top types in TypeScript and never is bottom type.

Solution

For Exclude we want to do the same way but we need to remove specified elements from a union:

T extends U is what we need:

🔥🔥🔥 Great job!

Please check out the solution with test cases in Playground