partitions.js

const partitions = arr => arr.reduce((map, obj) => Object.keys(obj).reduce((map, key) =>{
  if (map.has(key)) {
    map.set(key, map.get(key) + 1)
  } else {
    map.set(key, 1)
  }
  return map
}, map), new Map())

export default partitions

/*
partitions([
  {"key1": "value1", "key2": "value2"},
  {"key1": "value1", "key3": "value3"}
])

{"key1" => 2, "key2" => 1, "key3" => 1}
*/