Validate type of regex-matched string type in Typescript?

Question

I would like to check whether provided value match defined regex-match string type. Based on that I would return proper value. I need that to properly override my method.

EXAMPLE

type RegexType =`${string}/:${string}`

function myFunction(url: string): string;
function myFunction<P extends RegexType>(
  url: P,
): number;
function myFunction<P extends string>(value: P) {
  // value is typeof RegexType if yes then reutrn numberFunction otherwise stringFunction
}


const numberFunction = (): number => 1;
const stringFunction = (): string => '1';

PLAYGROUND

I know that I can use regex inside of the function, but then I meet another problem, that type is not properly interfered for template string

type RegexType =`${string}/:${string}`

function myFunction<P extends RegexType>(
  url: P,
): number;
function myFunction(url: string): string;
function myFunction<P extends string>(value: P) {
  // value is typeof RegexType if yes then reutrn numberFunction otherwise stringFunction
  return value.match('\/:') ? numberFunction() : stringFunction();
}


const numberFunction = (): number => 1;
const stringFunction = (): string => '1';
const testValue = '123/:123';

// ERROR
const valueNum: number = myFunction(`${testValue}`);
// VALID
const valueNum123: number = myFunction('123/:123');

PLAYGROUND

Sam Lanning · Accepted Answer · 2021-03-12 23:48:15Z

This should do what you want. Unfortunately it requires an any in the return statement, but the return type of the function causes an error as you wanted:

type RegexType =`${string}/:${string}`

function myFunction<P extends string>(value: P): P extends RegexType ? number : string {
  return value.match('\/:') ? numberFunction() : stringFunction() as any;
}

const numberFunction = (): number => 1;
const stringFunction = (): string => '1';
const testValue = '123/:123';

// ERROR
const valueNum: number = myFunction(`${testValue}`);
// VALID
const valueNum123: number = myFunction('123/:123');

Unfortunately, doing this can easily make way for potential type errors by making incorrect assumptions when you're not able to determine the type of a string, for example the following is considered correct in typescript, but will result in the error "valueNum.substr is not a function":

const testValue = '123/:123';
const valueNum: string = myFunction(`${testValue}`);
valueNum.substr(1)

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