Typescript “error TS2532: Object is possibly 'undefined'” even after undefined check

Question

I\'m trying to use the --strict option on tsc but I ran into the following \"weird\" case that I don\'t seem to understand.

If I write:

Answer 1

Because the second access to input.query happens inside another function the arrow function passed in to forEach. Flow analysis does not cross function boundaries, so since you are in another function you need to test again.

An alternative to the double test would be to use a local variable, since the type of the variable is locked in on assignment and it will not include the undefined type :

function testStrict(input: { query?: { [prop: string]: string } }) {
    if (input.query) {
        const query = input.query;
        Object.keys(input.query).forEach(key => {
            query[key];
        })
    }
    return input;
}