$ = ;// $.map(fn)
When returning a promise, it is mapped asynchronously (in parallel):
var Q = ;// ...
The stream 'end' event delayed until all promisses are finished.
When you return
undefined from your promise or regular function, the file is
filtered from the stream.
For regular callback style mapping, just use
var map = ;// ...
through2 works well:
var Through = obj;// ...