Three arguments

[dcousens]

universalify does not support calling the callback with more than three arguments

Where does this limit appear?
The limits I found were:

universalify/index.js

Lines 8 to 11 in ab4b5cd

	arguments[arguments.length] = (err, res) => {
	if (err) return reject(err)
	resolve(res)
	}

That indicates you only support err and res. eg (err => res => void).

universalify/index.js

Line 23 in ab4b5cd

else fn.apply(this, arguments).then(r => cb(null, r), cb)

And the same here.

[RyanZim]

You're correct, those are the only limits.

My point was that universalify doesn't work with things like fs.read(), which has a callback with arguments of the form: (err, bytesRead, buffer). For such a case, universalify won't pass the buffer argument as part of the promise resolution value.

There's nothing that expressly prevents you from using universalify with such functions if you don't care about the third argument, it's just a warning that universalify won't handle the third argument.

[dcousens]

Ok. So it is a typo? s/three/two? Or have I miscount?

[RyanZim]

Ah, yes, it's a typo in the docs, and I totally misread it here as well. Should read:

universalify does not support calling the callback with three or more arguments

Good catch; thanks!