Using Ruby next in map

While I was refactoring the code base of my client application, I came accross a code block, similar to this:

arr = []
data.each do |d|
  status = d['status']
  next if IGNORED_STATUS.include? status
  arr << d['value']
end

I think, “Ha, I can replace each with map here.”

I go ahead, and make the changes.

arr = data.map do |d|
  status = d['status']
  next if IGNORED_STATUS.include? status
  d['value']
end

Little did I know that, this will break the production system in the future.

After the application went live, after a few hours, my client reported that there were errors in the system disrupting the user from using it normally. After some investigation, I found out that it is caused by nil. I patched the error quickly by using compact.

From something like this:

timeslots = get_timeslots(params) # [1, nil, 2, 3]
timeslots.each { |x| ... }

to:

timeslots = get_timeslots(params).compact # [1, 2, 3]
timeslots.each { |x| ... }

Later on, diving into the source code, I found out that the nil is introduced by my own refactored code. It turns out that, inside a map, using next will return nil instead of skipping it altogether. I only learn this when I came across this blog article. For example:

arr = [1,2,3,4,5,6]
arr.map do |i|
  next if i % 2 == 0
  i # Return the value
end

#=> return [1, nil, 3, nil, 5, nil]
#=> instead of [1, 3, 5]

Well, thanks myself for not writing test ahead before refactoring the code base, I get to learn this the hard way.

Lesson Learned

1. Always write test before refactoring.
2. Using next in a map return nil
3. Array#compact can be used to remove nil elements.