Node: Handling `Function.length` currently with `util.promisify`

[moshest]

• Version: v9.2.0
• Platform: Darwin 17.2.0 (MacOS)
• Subsystem: util

Some code relay on the Function.length to detect how many arguments the function expect to handle.

For example:

const unlink = (file, cb) => fs.unlink(file, cb);
console.log(unlink.length); // 2

// Usage: (path, callback)
unlink('test.log', err => err && console.error(err));

Promisifying a function reduce the last callback arguments so I think that the length of the promisify function should be equal to 1. Currently it's 2:

const unlinkAsync = util.promisify(unlink);
console.log(unlinkAsync.length); // 2

// Usage: (path)
unlinkAsync('test.log').catch(err => console.error(err));

[addaleax]

I’d say patches are welcome, but relying on fn.length is never going to be a 100 % solution due to the way JS functions work.

[TimothyGu]

Case in point 1:

const defaultCallback = console.log.bind(console);
function myAsyncFunc(param1, param2, callback = defaultCallback) {
  // ...
}

myAsyncFunc.length === 2;

Case in point 2:

function myAsyncFunc(param1, param2, callback) {
  // ...
}

/*
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*/
Object.defineProperty(myAsyncFunc, 'length', { value: 1 });

myAsyncFunc.length === 1;

[BridgeAR]

Should this stay open? I think there is little to do on our side. Any function could use the arguments object instead of predefined arguments as well. So relying on the length is really bad.

[bnoordhuis]

Doesn't look like it. I'll close this out.