[PR proposal] Custom function.length

[ehmicky]

Some users might want to copy all function properties except length. For example:

const newFunc = func.bind(null, true)
// newFunc.length should be one fewer than func.length
mimicFn(newFunc)

A length option could be added to solve this:

const newFunc = func.bind(null, true)
// funcLength is func.length
// If the return value of length() is negative, newFunc.length will be 0
mimicFn(newFunc, { length: funcLength => funcLength - 1 })

This could be useful for any function wrapper that modifies function arity through binding, currying, etc.

I can submit a PR if you think it is a good idea.

[sindresorhus]

Sure, PR welcome :)

Make sure to mention the use-cases for it when you document it.

[ehmicky]

Done in #18

[sindresorhus]

Hmm, I've been thinking about this. First, it seems like an obscure use-case, as no one should really depend on Function#length, that's an anti-pattern. Second, wouldn't it be simpler to just have a {length: false} (to not copy length) option?

[ehmicky]

Not copying length is a much better idea indeed. Actually I don't think there needs to be an option, Function#length should just never be copied, as I don't see a reason why anyone would want this.

It is an obscure use-case indeed, but we could make this lightweight by only doing this: when iterating over properties, skip length. What do you think?

[ehmicky]

Done in #23