Substring of a string type TypeScript

Question

Is it possible to make a value passed to a function be a partial string (i.e., a substring) in TypeScript

Something along the lines of this?

function transform( str: Substring<'Hello world'> ) {
    // ...
}

And when I then call the function I can pass a substring of that string

transform( 'world' );
// or
transform( 'Hello' );
// or
transform( 'ello' ); // Is valid because it exists in hello
// or
transform( 'orl' ); // Is valid because it exists in world

// Is not valid, altough individual letters exists 
// they are not in the right order
transform( 'hlowrld' )

What do you mean by "partial" when it comes to a string? Is it a substring? like "Hello world".includes("world")? Because one might mean that "Hlo wld" is a substring of "Hello world", and the answer here very much depends on what you're actually asking. — jcalz
– jcalz, Commented Aug 2, 2022 at 13:05
@jcalz Like you describe in your example, "Hello world".includes("world"). So not just individual letter, they must be in order. I thought that this was pretty obvious in the question tho. — Red
– Red, Commented Aug 2, 2022 at 13:08
Would a prefix/suffix type be sufficient here? Or does it necessarily have to be an includes (i.e. any position in the string) type? — spender
– spender, Commented Aug 2, 2022 at 13:10
"I thought that this was pretty obvious in the question tho" Obviousness is in the eye of the beholder. Would you mind editing the question to specify that you are looking for substring and not subsequence or any other possible interpretation of the term "partial"? — jcalz
– jcalz, Commented Aug 2, 2022 at 13:14
I edited the question, I hope this clarifies what I am trying to archieve. My native languages isn't english, so I sometimes have trouble explaining these things, sorry for that. — Red
– Red, Commented Aug 2, 2022 at 13:16

Tobias S. · Accepted Answer · 2022-08-26 17:00:35Z

15

You could certainly make the function generic and check if T is a substring of Hello world via template literal types.

function transform<
  T extends string
>(str: "Hello World" extends `${string}${T}${string}` ? T : never) {}

transform('World')
transform('Hello')
transform('ello')
transform('orl')

transform('hlowrld') 
//         ^^^^^^^ Argument of type 'string' is not assignable to parameter of type 'never'

This can also be converted into a reusable utility type.

type Substring<
  S extends string, 
  T extends string
> = S extends `${string}${T}${string}` ? T : never

function transform<
  T extends string
>(str: Substring<"Hello World", T>) {}

Playground

edited Aug 26, 2022 at 17:00

answered Aug 2, 2022 at 13:16

Tobias S.

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4 Comments

captain-yossarian from Ukraine Over a year ago

Can I use your answer in my TS blog?

Tobias S. Over a year ago

@captain-yossarianfromUkraine sure :)

charlag Over a year ago

related utility can be this, when you don't want to bind the input string early: type EndsWith<S extends string> = `${string}${S}`

Shinigami Over a year ago

For a project of my own, I needed following type: type RemovePrefix<TPrefix extends string, TString extends string> = TString extends `${TPrefix}${infer T}` ? T : never. The TPrefix is the part that should be removed from the string, and then inferred T is the result. So I can get const x: RemovePrefix<'MyPrefix/', 'MyPrefix/A' | 'MyPrefix/B'> = 'A' // or 'B'

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