I have come across an issue where Meteor.wrapAsync is not handling undefined properties as it should.

The documentation states that:

Meteor.wrapAsync(func, [context])
Wrap a function that takes a callback function as its final parameter.

But in the case of

const myWrapFunction = Meteor.wrapAsync((...args) => {
  console.log('arguments received in wrapped function:', args)

  const callback = args[args.length - 1] // Last argument
  if (typeof callback !== 'function') {
    throw new Meteor.Error(500, `Callback function is not a function!`)
  }
  callback() // to return
})
myWrapFunction(1, undefined) // does not throw, by pure luck
myWrapFunction(1, undefined, undefined) // throws "Callback function is not a function!"

The arguments provided into myWrapFunction is

myWrapFunction(1, undefined) // gives args = [1, Function] 
myWrapFunction(1, undefined, undefined) // gives args = [1, Function, undefined]

altough I expected this to happen:

myWrapFunction(1, undefined) // should give args = [1, undefined, Function] 
myWrapFunction(1, undefined, undefined) // should give args = [1, undefined, undefined, Function]

This creates a significant problem, since the callback might not be the last parameter - which is in direct conflict with the docs.
This means that myWrapFunction(1, undefined) and myWrapFunction(1) might produce different results, even though they are in general thought of as identical.

This has been addressed previously in #6816 , but I think it should be brought up again for review.

I see two possible solutions:

1. Fix the bug (but that might be breaking and have significant impact)
2. Change the documentation to instead say "Wrap a function that takes a callback function as its last (non-undefined) parameter".

Tested in Meteor 1.10.2
Repo for reproducing the issue: https://github.com/nytamin/meteor-wrapAsync-bug