Exclude

Filter out unwanted types from unions with the Exclude utility! 🚫

In this easy-level challenge, you'll implement the built-in Exclude<T, U> utility type without using it directly. Exclude removes from union type T all types that are assignable to U. This is one of TypeScript's most fundamental utility types and understanding how it works will deepen your knowledge of conditional types and distributive behavior.

You'll learn how conditional types distribute over union types, a powerful feature that enables many advanced type manipulations in TypeScript.

For this challenge, you will need to change the following code to make the tests pass (no type check errors).

type Result = MyExclude<'a' | 'b' | 'c', 'a'> // 'b' | 'c'

Detailed Explanation

The Exclude utility leverages TypeScript's distributive conditional types to filter union members.

type MyExclude<T, U> = T extends U ? never : T

How it works:

• When T is a union, the conditional type distributes over each member
• For each member: if it extends U, return never, otherwise return the member
• never in a union disappears, effectively filtering out those types

Example breakdown for MyExclude<'a' | 'b' | 'c', 'a'>:

1. Distributes to: ('a' extends 'a' ? never : 'a') | ('b' extends 'a' ? never : 'b') | ('c' extends 'a' ? never : 'c')
2. Evaluates to: never | 'b' | 'c'
3. Simplifies to: 'b' | 'c'

This pattern is fundamental to many type utilities in TypeScript.