Typescript 声明定义字符串或任何字符串的类型

Question

I failed to find information on this elsewhere. I want to create a type that has both defined strings or allows any type of string to be used, but giving auto suggestion for the defined entries.

Right now if I add the // | string // | string at the end, it sort of takes over, and doesn't give any auto suggestion.

                                      // if I add this it stops suggestions
type DefinedLetters = 'A' | 'B' | 'C' // | string

const Letter: DefinedLetters  = '' // Should suggest A, B or C, but also allow D, E, F, etc.

Answer 1

I found this cool LiteralUnion type researching this for you, I think it works like a charm!

type LiteralUnion<T extends U, U = string> = T | (U & { });
type DefinedLetters = LiteralUnion<'A' | 'B' | 'C'>

const Letter: DefinedLetters  = "Cool!"

Answer 2

Why not create an enum and then in your usage, you can just specify your type as

let letter2: DefinedLetters | string;

Example

enum DefinedLetters {
    A = 'A',
    B = 'B',
    C = 'C'
}

let letter1: DefinedLetters;
letter1 = DefinedLetters.A; // <- ok
letter1 = 'test'; // <-- ts error

let letter2: DefinedLetters | string;
letter2 = DefinedLetters.A; // <- ok
letter2 = 'test'; // <- ok

Not quite a type approach, but you get the same thing in this example.