Everything from

map

gets written to

reduce

into

context.values

like so:

{
  key1: [val1forKey1, val2forKey1, ...],
  key2: [val1forKey2, val2forKey2, ...],
  ...
}

so in your

reduce

, when you do

JSON.parse(context.values)

, you will get the Array of values for the current key