var jsonData = [{x:12, machine1: 7}, {x:15, machine2:7},{x:12, machine2: 8}];

var jsonData = [{x:12, machine1:7, machine2:8}, {x:15, machine2:7}]

// Loop through all objects in the array
for (var i = 0; i < jsonData.length; i++) {

  // Loop through all of the objects beyond i
  // Don't increment automatically; we will do this later
  for (var j = i+1; j < jsonData.length; ) {

    // Check if our x values are a match
    if (jsonData[i].x == jsonData[j].x) {

      // Loop through all of the keys in our matching object
      for (var key in jsonData[j]) {

        // Ensure the key actually belongs to the object
        // This is to avoid any prototype inheritance problems
        if (jsonData[j].hasOwnProperty(key)) {

          // Copy over the values to the first object
          // Note this will overwrite any values if the key already exists!
          jsonData[i][key] = jsonData[j][key];
        }
      }

      // After copying the matching object, delete it from the array
      // By deleting this object, the "next" object in the array moves back one
      // Therefore it will be what j is prior to being incremented
      // This is why we don't automatically increment
      jsonData.splice(j, 1);
    } else {
      // If there's no match, increment to the next object to check
      j++;
    }
  }
}

12: [ {x=12, machine1=7}, {x=12, machine2=8} ],
15: [ {x=15, machine2=7} ]

var jsonData = [{x:12, machine1: 7}, {x:15, machine2:7},{x:12, machine2: 8}];
var groupedByX = _.groupBy(jsonData, 'x');
var result = [];
_.forEach(groupedByX, function(value, key){
   var obj = {};
   for(var i=0; i<value.length; i++) {
     _.defaults(obj, value[i]);
   }
   result.push(obj);
});

const mergeUnique = (list, $M = new Map(), id) => {
  list.map(e => $M.has(e[id]) ? $M.set(e.id, { ...e, ...$M.get(e.id) }) : $M.set(e.id, e));
  return Array.from($M.values());
};