TypeScript Challenges
simple
Pick
typescript
type MyPick<T, K extends keyof T> = {
[P in K]: T[P]
}
- keyof：The keyof operator takes an object type and produces a string or numeric literal union of its keys.
- P：custom key.
ReadOnly
typescript
type MyReadonly<T> = {
readonly [P in keyof T]: T[P]
}
Tuple to Object
typescript
type TupleToObject<T extends readonly any[]> = {
[P in T[number]]: P
}
First of Array
typescript
// type First<T extends any[]> = T extends [] ? never : T[0]
// type First<T extends any[]> = T extends [infer First, ...infer _] ? First : never
type First<T extends any[]> = T['length'] extends 0 ? never : T[0]
Length of Tuple
typescript
type Length<T extends readonly any[]> = T['length']
Exclude
typescript
type MyExclude<T, U> = T extends U ? never : T
- extends: When the type on the left of the extends is assignable to the one on the right, then you’ll get the type in the first branch (the “true” branch); otherwise you’ll get the type in the latter branch (the “false” branch).
Includes
typescript
Awaited
typescript
type MyAwaited<T> = T extends Promise<infer V> ? MyAwaited<V> : T
- infer：Conditional types provide us with a way to infer from types we compare against in the true branch using the infer keyword.
If
typescript
type If<C extends true | false, T, F> = C extends true ? T : F
Concat
typescript
type Concat<T extends any[], U extends any[]> = [...T, ...U]
Includes
typescript