Feature request: IsStringLiteral

[Ayc0]

What do you think of adding a type that can say if a string is a string literal (like 'foo/bar') and not a regular string?

export type IsStringLiteral<T> = string extends T ? false : true;

[sindresorhus]

What are some real-world use-cases for it?

[Ayc0]

In one of my projects, I'm using a library like react-router, and I only allow it to use literal strings for the path variable. This allows me to use a custom TS type to parse the path and propose the route parameters (like /foo/:bar/:paz) as props in the Route component.

I already have this type util in my project, but I was wondering if other people in the community wanted to use it too.

If you find it useful, feel free to add it to this awesome project!

[tommy-mitchell]

In tsd, expectDocCommentIncludes expects a generic string literal type parameter, but checks for it through the TypeScript compiler. This type could help users when testing.

---

Related, a better way to write this type would be:

type IsStringLiteral<T> =
  T extends string
    ? string extends T
      ? false
      : true
    : false;

Would adding types to check for other literals be useful? E.g. IsNumericLiteral, etc. Perhaps:

import type {Primitive} from 'type-fest';

type IsLiteral<T extends Primitive> =
  | IsStringLiteral<T>
  | IsNumericLiteral<T>
  | IsBooleanLiteral<T>
  | IsSymbolLiteral<T>
  // ...

I'd be open to adding a PR. Other use cases of IsStringLiteral or other IsXLiteral types would be helpful.

[sindresorhus]

Would adding types to check for other literals be useful?

👍 It would make sense to do these too for completeness.

[sindresorhus]

This is now accepted. Pull request welcome.