Typescript declare type of defined strings or any string

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 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.

Nick · Accepted Answer · 2019-10-25 00:43:41Z

6

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!"

answered Oct 25, 2019 at 0:43

Nick

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2 Comments

Nicholas Tower Over a year ago

Neat! Seems that if you want you can get the same effect like: type DefinedLetters = 'A' | 'B' | 'C' | (string & { });

Nick Over a year ago

@NicholasTower Indeed, that works too, the LiteralUnion just encapsulates that last string bit for re-use

mwilson · Accepted Answer · 2019-10-25 01:03:56Z

1

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.

edited Oct 25, 2019 at 1:03

answered Oct 25, 2019 at 0:54

mwilson

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1 Comment

JCQuintas Over a year ago

Thanks for the effort, but Nick's answer is more in line of what I want right now.

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