How could typescript check object attribute type at runtime

Question

I defined a service type and my purpose is to create a function with two params.

first one is service type which is key of type Service
second one is a string array of the attribute keys per the first argument.

something like below

type MyService = {
  service1: { x: string, y: number },
  service2: { m: string, n: number },
};

function useService(ns: keyof MyService, serviceNames: Array<keyof MyService[typeof ns extends keyof MyService ? typeof ns : never]>): void {
  console.log(serviceNames);
}

useService('service1', ['x', 'y']);  // now the second parameter type is always never

I don't know whether ts could check the runtime type? Please advice.

score 0 · Answer 1 · answered Dec 21 '20 at 07:12

0

No Typescript cant check this at runtime sadly...

This question might be useful to you

answered Dec 21 '20 at 07:12

Ulrich Stark

401
3
12

score 0 · Answer 2 · answered Dec 21 '20 at 07:20

0

Typescript code when it gets compiled, the resultant JS code never has types. So it cannot check at runtime.

"use strict";
function useService(ns, serviceNames) {
    console.log(serviceNames);
}
useService('service1', ['x', 'y']);

answered Dec 21 '20 at 07:20

Shivam Pandey

3,756
2
19
26

score 0 · Accepted Answer · answered Dec 21 '20 at 07:53

I think this is the solution you might want to.

use generic to get type from argments.

type MyService = {
  service1: { x: string, y: number },
  service2: { m: string, n: number },
};

function useService<T extends keyof MyService>(ns: T, serviceNames: Array<keyof MyService[T]>): void {
  console.log(serviceNames);
}

useService('service1', ['x', 'y']); // it'll remove the error

How could typescript check object attribute type at runtime

3 Answers3