Overload function based on string content in TypeScript

Question

Say I have a function which accepts an input string parameter. If input begins with a "!" character, the function should return a boolean type. If input begins with a "?" character, the function should return a string[] type.

The question is, is it possible to document overloads for this kind of behavior in a TypeScript definition file?

No; that's a matter of value, not of type. If you know the full set of values you could use string literal types, but I don't think you can do it for a prefix. — jonrsharpe
– jonrsharpe, Commented Aug 31, 2020 at 21:45

Alex Wayne · Accepted Answer · 2020-09-22 19:25:01Z

1

This is not possible.

A string in typescript can either be a constant literal string, for example:

const a = "A"

Or it may be of type string which means it may have any content, for example:

const str = prompt("What is your name?")`

There is no way to define a string type that matches any sort of pattern.

I don't know what you are trying to do, but there's probably a better way to design your API here.

If you do know all possibilities, you can treat them like constant strings.

type Bang = "!A" | "!B" | "!C"
type Question = "?A" | "?B" | "?C"

function doStuff(str: Bang): boolean
function doStuff(str: Question): string[]

function doStuff(str: Bang | Question): boolean | string[] {
  if (str.startsWith("!")) {
    return true
  } else {
    return ['strings', 'here']
  }
}

doStuff('!A') // boolean
doStuff('?A') // string[]

Playground

Update for Typescript 4.1:

Typescript 4.1 (just released in beta on Sept 18, 2020) includes support for Template Literal Types. This lets you have the contents of string literals be strongly typed. You can now pull parts of string literals out and use them on their own.

With this feature, your problem could be solved like so:

// Typescript 4.1
function doStuff<
  Prefix extends '!',
  Suffix extends string
>(str: `${Prefix}${Suffix}`): boolean

function doStuff<
  Prefix extends '?',
  Suffix extends string
>(str: `${Prefix}${Suffix}`): string[]

function doStuff<
  Prefix extends '!' | '?',
  Suffix extends string
>(str: `${Prefix}${Suffix}`): boolean | string[] {
  if (str.startsWith("!")) {
    return true
  } else {
    return ['strings', 'here']
  }
}

doStuff('!A') // boolean
doStuff('?A') // string[]
doStuff('bad format') // type error

4.1 Playground

edited Sep 22, 2020 at 19:25

answered Aug 31, 2020 at 21:58

Alex Wayne

188k52 gold badges328 silver badges363 bronze badges

Sign up to request clarification or add additional context in comments.

4 Comments

Alex Wayne Over a year ago

I've added an example for using constant strings if you know all possible values the strings could be ahead of time. Maybe that will help save some of those hours :)

Alex Wayne Over a year ago

I don't what your dataset looks like but, this would be easy if you could type that data structure like { id: string, type: 'entity' | 'item' } instead of relying on a specific format of identifier. But that probably doesn't help you :)

spacefluff432 Over a year ago

I was able to update my implementation and no longer need the prefix thing, but I really appreciate the fact that you just gave a straight answer, you're a rare breed on this website. Keep it up man.

Alex Wayne Over a year ago

@HarrisonSmith Typescript 4.1 was just released in beta and it has a feature that gives you what you initially asked for. Just in case this is still a problem for you, see my update.

Collectives™ on Stack Overflow

Overload function based on string content in TypeScript

1 Answer 1

4 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

4 Comments

Your Answer

Sign up or log in

Post as a guest

Related