Feature request: v8.hasFastProperties

[Nevro]

What is the problem this feature will solve?

Obtaining an accurate threshold by which I could decide whether I want loop over object with for.in (FastProperties) or with for.keys (DictionaryProperties).

Looping over an object with fast properties is much faster (up to 5x) with for.in than with for.keys. Makes a big difference when you looping over an big object (bellow 1021 length) populated with Object.fromEntries method and even with small regular object (bellow 20 length).

What is the feature you are proposing to solve the problem?

Add hasFastProperties method to node:v8 module. This would avoid the need to expose the entire native syntax.

What alternatives have you considered?

try {
    const { setFlagsFromString } = await import('node:v8');
    setFlagsFromString('--allow-natives-syntax');
    var V8NativeSyntax = eval('%Equal(0, 0)');
} catch {
    var V8NativeSyntax = false;
}

function hasFastProperties(obj) {
    return V8NativeSyntax 
        && typeof obj === 'object'
        && eval('%HasFastProperties(obj)');
}

[bnoordhuis]

Not really feasible. There's no V8 C++ API¹ that provides that information so Node.js cannot improve on what you're already doing.

¹ And, channeling the V8 team, I'm pretty sure they won't add one.