I am running a lambda function predicate over a list of values to find the first value matching the predicate. However, in addition to finding the right value, I want to store the result of an internal calculation in the lambda function for later.

Simplified example:

let helper: number | undefined = undefined;
const array = [1, 2, 3];

const result = array.find((v) => {
  helper = v % 2;
  return helper === 0;
});

console.log({ helper, result });
if (helper !== undefined) {
  console.log(helper.toExponential());
}

Here, result should be the first value matching the predicate and helper should store the internal calculation, which I want to do something with later on. In order to consider the case where no value matches the predicate, I defined the type of helper to be number or undefined and check afterwards, if helper is a number before I do anything with it.

At runtime everything works as expected and the output is:

{ "helper": 0, "result": 2 }
"0e+0"

However, TypeScript complains:

Property 'toExponential' does not exist on type 'never'.

That is, TypeScript does not see the reassignment of helper within the lambda function. Because it does not see it, it deduces that the type of helper can be simplified to undefined.

How can I make TypeScript aware that I am reassigning a variable in a lambda function?