[SOLVED] Typescript - Incorrectly inferring 'never'

Typescript - Incorrectly inferring 'never'

This is a basic use-case: Initializing a variable with null then changing the value in some nested loop/function:

let a: number | null = null;
[1].forEach(() => {
  a = 1;
});

if (a != null)
  a.toFixed(); // Error: Property 'toFixed' does not exist on type 'never'.

However typescript infers a's type to be never. I would assume that without the if it would assume it to be null | number in which case I could get an error stating the property does not exist on null, but why is it assuming it to be never based on just the initial assignment value.

Am I doing something wrong?

Solution

If you are absolutely sure that a has a value there, than you can put the ! after the variable

let a: number | null = null;
[1].forEach(() => {
  a = 1;
});

if (a !== null)
  a!.toFixed(); //

I would not use null thought but undefined, so no need to use !

let a: number | undefined;
[1].forEach(() => {
  a = 1;
});

if (a) { // <-- if undefined or 0
  a.toFixed(); // No problem here
}

Also as recommendation use !== not !=