How to extract the result from reduce()

How do I reliably extract the final value from a reduce operation

eg
let numItems = -1
_([1, 2, 3]).reduce(0, (acc, value) => acc + 1).each(x => numItems = x)

This works for this simple example but for a more complex situation where the stream is a file that is split into lines its tricky.

At the nodejs command line it works but running in a test harness numItems is -1. I'm thinking that the stream hasn't finished reading the lines by the time I check the value.

How do I wait for it to finish before checking

The issue names Highland's reduce operation, a file stream split into lines, and a test harness but no repository files or tests. Start by reproducing the command-line and test-harness cases, then trace when reduce completes relative to the assertion. Done would require a confirmed explanation and, if a project change is needed, a clearly specified regression test.

Written by the indexing model from the issue text.