Falsy instanceof check doesn't affect righthand or expression

[LeviticusMB]

🔎 Search Terms

instanceof or unknown

🕗 Version & Regression Information

• Didn't think of it until 4.4 when caught values became unknown, but the actual problem has probably been present for a long time.

See #38309, for instance.

⏯ Playground Link

https://www.typescriptlang.org/play?ts=5.2.0-beta#code/MYGwhgzhAEDqCmAjAogJ1Qe1deAPALvAHYAmMAqgEoCSam2A3gLABQ070E+Y+ArhAH4AXNCK8AtonioA3KwC+rVvlQBPaMzYcA9NugBZMAGt40MNFTwAjr3hcF0YD2AALaAAppqAJQbWHaABLADMPLyCiLjAiYHgMUIQUdCxoAF506GCwEAhTAB88nHQAOii+GABCVOgAFgAGAEZfTQCA4AxIjBB4Yq8sT3RvOS12RRYxoA

💻 Code

class WebError extends URIError {
    status?: number;
}

try {
    // Make a request
} catch (err) {
    if (err instanceof WebError === false || err.status != 401) {
        console.error(err);
    }
}

🙁 Actual behavior

The compiler thinks err is unknown and does not understand that err.status must be a valid property.

🙂 Expected behavior

No compiler error.

Additional information about the issue

No response

[nmain]

The recognized syntactic form for this one is !(a instanceof b)

https://www.typescriptlang.org/play?ts=5.2.0-beta#code/MYGwhgzhAEDqCmAjAogJ1Qe1deAPALvAHYAmMAqgEoCSam2A3gLABQ070E+Y+ArhAH4AXNCK8AtonioA3KwC+rVvlQBPaMzYcA9NugBZMAGt40MNFTwAjr3hcF0YD2AALaAAppqAJQbWHaABLADMPAEJPdCCiLjAiYHgMUIQUdCxfAB8MnHQAOli+GDCAXmgAFgAGAEZfTQCA4AwYjBB4XK8sSJ85LXZFFn6gA

[Andarist]

=== false variant should start working once this PR lands: #53714

[LeviticusMB]

Oh I see, it was just because of the === false part ... I always write it like that because those extra parentheses annoy me.

Thanks for the tip.