TS doesn't understand that NaN is a falsy number

[Validark]

TypeScript Version: latest beta

Search Terms: NaN truthiness falsy 0 is not the only falsy number

Code

function f(a: number, b: string) {
    return a && b;
}

const x: string | 0 = f(0 / 0, "Hello"); // should error but doesn't

Expected behavior: function f should return a string or number, since there currently is no NaN type in TS. If there was, it would return 0 | NaN | string

Actual behavior: function f returns string | 0

[RyanCavanaugh]

Duplicate #15135

[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.

[osyrisrblx]

@RyanCavanaugh I don't think this is actually a duplicate of that issue. The issue you linked is more along the lines of "these are terms in JavaScript and should exist in TypeScript".

Rather, this is a truthiness bug! Here's a more simple example:

function bool(): boolean { return false; }
function number(): number { return NaN; }

let a = number();
let b = bool();

const x = a && b; // type = boolean | 0
console.log(x); // prints NaN

I would expect the type of x here to be boolean | number