Why does TypeScript think my number|undefined Record<> property is undefined within a falsy check?

I don’t get why this code raises TS2532: Object is possibly 'undefined'. Why obj[prop] check is not enough and what is the proper way of writing this logic?

function test<T extends string>(obj: Record<T, number | undefined>, prop: T) {
  if (obj[prop]) obj[prop]--;
}

>Solution :

Typescript can’t handle the generics that well which is described in ms/TS#33014. Example:

function f<T extends 'a' | 'b'>(x: T) {
  if (x === 'a') {
    x; //  T extends "a" | "b"
  }
}

The easiest solution would be to store obj[prop] in a separate variable and if that value exists then assign a new value to obj[prop]:

function test<T extends string>(obj: Record<T, number | undefined>, prop: T) {
  let value = obj[prop];

  if (value) {
    obj[prop] = value - 1;
  }
}

playground