`NaN && 1` gets incorrectly typed as `0 | 1`

[antonilol]

🔎 Search Terms

"NaN"

🕗 Version & Regression Information

• This is the behavior in every version I tried, and I reviewed the FAQ for entries about _________

i got the same output for every version in the typescript playground, except 3.3.3 where as const is not supported. without as const this gets typed as just number

⏯ Playground Link

https://www.typescriptlang.org/play/?ts=5.9.0-dev.20250702#code/MYewdgzgLgBGCGYYF4YDl5pgMmzAjDPBDKJFANxA

💻 Code

const nan = NaN && 1 as const;

🙁 Actual behavior

It gets typed as 0 | 1. (See .d.ts output in playground)

🙂 Expected behavior

It should get number or a more specific type.

Additional information about the issue

NaN is falsy in javascript, !!NaN === false

[Andarist]

The problem is here:

            case SyntaxKind.AmpersandAmpersandToken:
            case SyntaxKind.AmpersandAmpersandEqualsToken: {
                const resultType = hasTypeFacts(leftType, TypeFacts.Truthy) ?
                    getUnionType([extractDefinitelyFalsyTypes(strictNullChecks ? leftType : getBaseTypeOfLiteralType(rightType)), rightType]) :
                    leftType;

where getDefinitelyFalsyPartOfType looks like this:

    function getDefinitelyFalsyPartOfType(type: Type): Type {
        return type.flags & TypeFlags.String ? emptyStringType :
            type.flags & TypeFlags.Number ? zeroType :
            type.flags & TypeFlags.BigInt ? zeroBigIntType :
            type === regularFalseType ||
                type === falseType ||
                type.flags & (TypeFlags.Void | TypeFlags.Undefined | TypeFlags.Null | TypeFlags.AnyOrUnknown) ||
                type.flags & TypeFlags.StringLiteral && (type as StringLiteralType).value === "" ||
                type.flags & TypeFlags.NumberLiteral && (type as NumberLiteralType).value === 0 ||
                type.flags & TypeFlags.BigIntLiteral && isZeroBigInt(type as BigIntLiteralType) ? type :
            neverType;
    }

If NaN would be to considered then zeroType shouldn't have to be returned from here.