Tell TS that function argument ran and to respects its side effects

Question

I have a retry util that takes a function as an argument. This retry util defines a variable in the scope.

let code: number;

retry(() => {
  code = 123;
});

const msg = `Your code is: ${code}!`;
console.log('msg:', msg);

function retry(cb: () => void) {
  cb();
}

Typescript underlines code with error:

Variable 'code' is used before being assigned.(2454)

Because it doesn't know that cb got ran. Is it possible to type up retry such that TS knows the cb argument ran? So it applies the type affects of that argument?

Togira · Answer 1 · 2022-07-03T16:49:49.720

0

What you can do is tell the compiler that you are sure this variable was assigned to a value, like this:

let code: number;

retry(() => {
  code = 123;
});

const msg = `Your code is: ${code!}!`;
console.log('msg:', msg);

function retry(cb: () => void) {
  cb();
}

See this post for more information on the "!" operator

edited Jul 03 '22 at 16:49

answered Jul 03 '22 at 14:12

Togira

355
3
11

Thanks, I was trying to avoid `!` though and was trying to get TS to read the flow, is that possible? – Noitidart Jul 03 '22 at 14:26
I don't know any way to do that... – Togira Jul 03 '22 at 16:49

Tell TS that function argument ran and to respects its side effects

1 Answers1