How is Async going to support concat functions?

[suguru03]

What version of async are you using?
v2.4.1

Which environment did the issue occur in (Node version/browser version)
Node v8.0.0

What did you do? Please include a minimal reproducable case illustrating issue.

async.concat([1, 2, 3], (val, next) => {
  next(null, val % 2 ? val : 0);
}, (err, result) => {
  console.log(result); // [1, 3]
});

What did you expect to happen?
If the function has the same functionality as Array.prototype.concat, it should be [1, 0, 3].

What was the actual result?
[1, 3]

Also I realized,

[].concat(0); // [0]
[].concat(); // []
[].concat(undefined); // [undefined]

I would like to know how Async is going to support the functions. Thanks :)

[aearly]

Looks like we're just doing a truthy check here, when we should actually do y == null:

https://github.com/caolan/async/blob/master/lib/internal/concat.js#L5

[suguru03]

Sure, I'll follow the PR changes, thanks :)