meld a list of dictionaries into a smaller list of dictionaries

meld.js

glue = (x,y) => x === undefined ? y : Array.isArray(x) ? x.concat(y) : [x,y]

function meld(xs) {
  let idx = {}, res = [], it
  for (x of xs) {
    if (x.id in idx) it = res[idx[x.id]]
    else { it = {id: x.id}; idx[x.id] = res.length; res.push(it) }
    for (k of Object.keys(x)) if (k!=='id') it[k] = glue(it[k], x[k]) }
  return res }

JSON.stringify(meld([
  {a: 'abcd', id: '1'},
  {b: 'bcd', id: '2'},
  {c: 'cded', id: '3'},
  {d: 'abc', id: '4'},
  {a: 'abc', id: '1'},
  {c: 'cde', id: '3'},
  {a: 'abcde', id: '1'}])) 

// "[{"id":"1","a":["abcd","abc","abcde"]},{"id":"2","b":"bcd"},{"id":"3","c":["cded","cde"]},{"id":"4","d":"abc"}]"