Pivot an array of objects by key

README.md

Accumulate values by key from an array of objects (key/value mappings):

records = [
  { a: 10, b: 20, c: 30 }, 
  { a: 11, b: 21 }, 
  { a: 12, b: 22, c: 32, d: 42 }
]

pivoted = pivot(records)

Expected result:

pivoted = { 
  a: [ 10, 11, 12 ], 
  b: [ 20, 21, 22 ], 
  c: [ 30, 32 ], 
  d: [ 42 ] 
}

pivot.coffee

# reducing iterator 
iter = (pivoted, rec) ->
  for k, v of rec
    pivoted[k] = [] if not pivoted[k]
    pivoted[k].push(v)
  pivoted

module.exports = pivot = (data) -> data.reduce(iter, {})

pivot.js

module.exports = pivot;

function pivot(data) { 
  var iter = function(pivoted, record) {
    for (var key in record) {
      if (!pivoted[key]) pivoted[key] = []
      pivoted[key].push(record[key])
    }
    return pivoted;
  };
  return data.reduce(iter, {}) 
};

test.coffee

pivot = require 'pivot'
{deepEqual} = require 'assert'

# data to be pivoted should be an array of records
records = [
  { a: 10, b: 20, c: 30 }, 
  { a: 11, b: 21 }, 
  { a: 12, b: 22, c: 32, d: 42 }
]
expected = 
  a: [ 10, 11, 12 ]
  b: [ 20, 21, 22 ]
  c: [ 30, 32 ]
  d: [ 42 ]

deepEqual pivot(records), expected

Worked for me nicely :)