Is it possible to specify string format with TypeScript?

Question

I just wonder - is it a way to check if the string matches some pattern via TypeScript.

I don't want to do the checks in the runtime.

//What I have now
public id: string; //can be whatever string 

//What I want to see
public id: IdType; //Some type?
id = '0000-0000-0000-0001'//valid
id = 'whatever'//invalid

So is it a way to define string format?

I'm not sure if you can do it. Pattern is not a type. You can though add validator to check the value — Rajesh Dixit
– Rajesh Dixit, Commented Nov 8, 2017 at 10:20
@Rajesh But it will be a runtime check, right? I just interested to check it during compilation — S Panfilov
– S Panfilov, Commented Nov 8, 2017 at 10:23
@SergeiPanfilov Yes. That will be a runtime check. Think about this, how will TS know what the value is just by type? TS can check for type but the value is processed during runtime only. So unless value is actually set, you cannot check it — Rajesh Dixit
– Rajesh Dixit, Commented Nov 8, 2017 at 10:31
Not possible yet. There's only suggestion - exactly what you're looking for — Aleksey L.
– Aleksey L., Commented Nov 8, 2017 at 11:58

Aleksey L. · Accepted Answer · 2020-11-18 04:27:29Z

37

This should now be possible as of TS 4.1 beta

There's now a feature called Template Literal Types, which looks something like this (excuse the basic example):

type idType = `${number}-${number}-${number}-${number}`
const id: idType = '0000-0000-0000-0001';

https://devblogs.microsoft.com/typescript/announcing-typescript-4-1-beta/#template-literal-types

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edited Nov 18, 2020 at 4:27

Aleksey L.

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answered Oct 25, 2020 at 23:09

Daniel Del Core

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4 Comments

Aleksey L. Over a year ago

This unfortunately won't work because template literal types allow only concrete literal types typescriptlang.org/play?ts=4.1.0-beta#code/…

Daniel Del Core Over a year ago

Seems to be working in the nightly builds. Might just be on its way :)

Artif3x Over a year ago

This is now working in 4.1.5: typescriptlang.org/play?ts=4.1.5#code/…

Timar Ivo Batis Over a year ago

How do you make this flexible? Maybe there's more '${number}' groups or fewer? working with paths here.

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