TypeScript
Table of Contents
- The
Objectinterface in typescript means "any thing but null or undefined"- Usually, it's better to use the empty interface
{}(e.g.<T extends {})
- Usually, it's better to use the empty interface
- The
objecttype is "any thing that is not number, string, boolean, symbol, null, or undefined" nevercan be assigned toanyorunknown
1. any v.s. unknown
unknown is the parent type of all other types. it's a regular type in the type system.
any means "disable the type check". it's a compiler directive.
Much like any, any value is assignable to unknown; however, unlike any, you cannot access any properties on values with the type unknown, nor can you call/construct them. Furthermore, values of type unknown can only be assigned to unknown or any.
2. Type-level programming
// https://github.com/microsoft/TypeScript/issues/23182#issuecomment-583330532
type NeverAlternative<T, P, N> = [T] extends [never] ? P : N;
type Union<T, U> = NeverAlternative<T, U, T | U>;
class C<T = never> {
frob<U>(obj: Record<keyof U, any>): C<Union<T, keyof U>> {
return new C<Union<T, keyof U>>();
}
get(key: T) {
}
}
// x : C<never>
const x = new C();
// y : C<'a' | 'b'>
const y = x.frob({ 'a': 1, 'b': 2 });
y.get('a');
// z : C<'a' | 'b' | 'bleh'>
const z = y.frob({ bleh: 32 })
2.1. Plural and singluar strings
type Plural<S extends string> = S extends `${infer _}s` ? S : `${S}s`;
type Singular<S extends string> = S extends `${infer U}s` ? U : S;
3. Backlinks
- Related notes (cheatsheet.org)