Validation of variable is not undefined doesn't work if it's not done directly inside if statement

[eliezra236]

Bug Report

🔎 Search Terms

🕗 Version & Regression Information

• This is the behavior in every version I tried

⏯ Playground Link

Playground link with relevant code

💻 Code

const map = new Map<string, number>();
map.set('a', 1);
const a = map.get('a');

const isAValid = Boolean(a);

if (!isAValid) {
    throw Error('no a');
}

const aCopy: number = a;

if (!a) {
    throw Error('no a')
}

const aCopy2: number = a;

🙁 Actual behavior

In the first example aCopy gives error because it can be undefined according to typescript

🙂 Expected behavior

aCopy should be defined to be a number.

[DanielRosenwasser]

We don't really understand that Boolean is testing for truthiness. If you change your code to use !!, you do get the desired result.

declare const a: number | undefined;

const isAValid = !!a; // <- here

if (!isAValid) {
    throw Error('no a');
}

const aCopy: number = a;

If you really do want to use Boolean like this, I guess you could add a signature with a type predicate like this to the global state:

interface BooleanConstructor {
    <T>(x: T): x is Exclude<T, null | undefined | 0 | "" | false>;
}

[RyanCavanaugh]

#16655

[typescript-bot]

This issue has been marked as a 'Duplicate' and has seen no recent activity. It has been automatically closed for house-keeping purposes.