function handle(arr) {
    var res = [],
        obj = {},
        index = 0;
    arr.forEach(val => {
        if (obj.hasOwnProperty(val.UID)) {
            res[obj[val.UID]].value = res[obj[val.UID]].value + ',' + val.value;
        } else {
            obj[val.UID] = index++;
            res.push({
                UID: val.UID,
                value: val.value
            });
        }
    });
    return res;
}
var arr = [{
    UID: "222",
    value: "111"
}, {
    UID: "222",
    value: "103"
}, {
    UID: "123",
    value: "302"
}, {
    UID: "123",
    value: "031"
}]
console.log(handle(arr))

var arr = [];
var json = [
{UID:"222",value:"111"},
{UID:"222",value:"103"},
{UID:"123",value:"302"},
{UID:"123",value:"031"}
];
function checkRepeat(uid) {
 const index = arr.findIndex(function(value, index, arr) {
   return value.UID === uid;
 });
 if (index === -1) return false;
 return true;
}
json.map((item, index) => {
  if (checkRepeat(item.UID)) {
    arr.map((it, i) => {
      if (item.UID === it.UID) arr[i].value += ',' + item.value;
    })
  } else {
    arr.push({UID:item.UID, value: item.value});
  }
});
console.log(arr);