nullish types not narrowed in if block

[errorx666]

🔎 Search Terms

null nullish undefined narrow if

🕗 Version & Regression Information

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

⏯ Playground Link

https://www.typescriptlang.org/play/?ts=6.0.3#code/JYOwLgpgTgZghgYwgAgCJzHZBvAsAKGSOSggGcBXAGzAGUIwB+ALhwOI+WigHsoyWJCHAAmPEFQCeyMmCigA5gG0AusgA+yENSoBudsQC+GrTv35DBAnDKSQCZDAr2wwccgUN0mABQBKVgAFXgBbYDIIAB5vOHVtKioAPhwhMAooEFME3WRLfGtbe0dnBFd3CAAPOBCAByoIfzZCYgRxWWQRDCwAXmQ4AHc4YDAPLy7-cw5gGGQfH07MRgA6UkoaeiYl7j4BJfqQBTAAC2RGRmQABj9kZKumzmIAekeOrpXyajoGLahefmQEHAQAByEYAIxQ8SoyCO0AgABo+mRZgs4MtVp8NsttvxlvtDiczpdrrcDA9WiAyDx6j8-j5Xph3msvmBaTslgArHigenAgA6IOQ1z8kyMBDyBEq1TqDRFBCAA

💻 Code

interface Data {
    resultSet?: {
        errors?: readonly string[] | null;
    } | null;
}

async function getData(): Promise<Data|null> { return null; }

async function example() {
    const data = await getData();
    if ((data?.resultSet?.errors?.length ?? 0) > 0) {
        // data.resultSet.errors can't be null here, as (data?.resultSet?.errors?.length ?? 0) > 0
        console.error( data.resultSet.errors.join( '\n' ) );
    }
}

example();

🙁 Actual behavior

Errors in code

'data' is possibly 'null'.
'data.resultSet' is possibly 'null' or 'undefined'.
'data.resultSet.errors' is possibly 'null' or 'undefined'.

🙂 Expected behavior

No errors

Additional information about the issue

The if condition should eliminate nullish values from the body.

[snarbles2]

It's able to narrow if you write it like so:

    if (data?.resultSet?.errors?.length) {
        console.error( data.resultSet.errors.join( '\n' ) );
    }

I think the comparison operator makes the expression too complex for the CFA.

[errorx666]

Updated my code as suggested by snarbles2. I prefer the original code because it doesn't rely on coercing a number to a boolean.
Leaving open in hopes of getting slightly smarter CFA.

[errorx666]

Related?
It doesn't like data?.resultSet?.errors?.length > 0 because the left-hand-side might be null or undefined, but that's fine because nullish values can't be greater than 0 so the result is the same.