Can Typescript perform run-time check at compile-time ?

gistfile1.txt

// Here we will put the solution. 
// But before let's have a talk bellow.

JAVASCRIPT SOLUTION

I do not know other way in JS to DETECT this kind of error. Here is my solution:

const f = a_string => { 
    if (typeof(a_string) === 'string') 
        return 2 
    else
        throw Error('Error! input it is not a string') 
}

const g = a_number => { 
     if (typeof(a_number ) === 'number') 
        return 3 
    else
        throw Error('Error! Input it is not a number') 
}

const a = [1,1,1]
 
const r0 = a.map(g) // OK
const r1 = a.map(f)  // BAD but we'll CATCH that error at run-time with a beaultfull message.

SOLUTION CONSIDERATIONS

Ok, now at cost of some additional lines we have a "run-time error detection" mechanism that can stop our program execution if it's making a previsible mistake.

TYPESCRIPT SOLUTION

ERROR MESSAGE

Here is the error that bellow code produces at static-time in Typescript Playgound:

Argument of type '(_: string) => number' is not assignable to ...
   Type 'number' is not assignable to type 'string'.(2345)

SOLUTION CODE

The code bellow is: informative, shorter at development and at run-time, and just has what is necessary to solve the proposed problem:

const f = (_:string) => 2        // note: sorry the strange syntax, it is just a function (nothing more than that).
const g = (_:number) => 3

const a = [1,1,1]
// const s = ['z','z','z'] , etc...

const r0 = a.map(g) // ok!
const r1 = a.map(f)   // BAD ERROR!! But we know that at exactly moment we typed it. we cannot proceed until you solve that.

JAVSCRIPT TRANSPILATED CODE

Look that the Typescript output code is exactly equals the initial problem. No "run-time error detection code" will be generated or necessary.

"use strict";
const f = (_) => 2; // note: sorry the strange syntax, it is just a function (nothing more than that).
const g = (_) => 3;
const a = [1, 1, 1];
// const s = ['z','z','z'] , etc...
const r0 = a.map(g); // ok!
const r1 = a.map(f); // BAD ERROR!! But we know that at exactly moment we typed it. we cannot proceed until you solve that.

STUDY CASE 01 - CONCLUSION

Original answer:

Does Typescript type check in runtime ?

Response:

yes, in the sense that it avoids runtime checks bring the effect that they generate to static-time.

WHAT MORE ?

Can this technique be generalized ?