Update if exists or add new element to array of objects - elegant way in javascript + lodash

In your first approach, no need for Lodash thanks to findIndex():

function upsert(array, item) { // (1)
  const i = array.findIndex(_item => _item.id === item.id);
  if (i > -1) array[i] = item; // (2)
  else array.push(item);
}

Example:

const array = [
  {id: 0, name: 'Apple', description: 'fruit'},
  {id: 1, name: 'Banana', description: 'fruit'},
  {id: 2, name: 'Tomato', description: 'vegetable'}
];

upsert(array, {id: 2, name: 'Tomato', description: 'fruit'})
console.log(array);
/* =>
[
  {id: 0, name: 'Apple', description: 'fruit'},
  {id: 1, name: 'Banana', description: 'fruit'},
  {id: 2, name: 'Tomato', description: 'fruit'}
]
*/

upsert(array, {id: 3, name: 'Cucumber', description: 'vegetable'})
console.log(array);
/* =>
[
  {id: 0, name: 'Apple', description: 'fruit'},
  {id: 1, name: 'Banana', description: 'fruit'},
  {id: 2, name: 'Tomato', description: 'fruit'},
  {id: 3, name: 'Cucumber', description: 'vegetable'}
]
*/

(1) other possible names: addOrReplace(), addOrUpdate(), appendOrUpdate(), insertOrUpdate()...

(2) can also be done with array.splice(i, 1, item)

Note that this approach is "mutable" (vs "immutable"): it means instead of returning a new array (without touching the original array), it modifies directly the original array.

You can use an object instead of an array:

var hash = {
  '1': {uid: 1, name: "bla", description: "cucu"},
  '2': {uid: 2, name: "smth else", description: "cucarecu"}
};

The keys are the uids. Now your function addOrReplace is simple like this:

function addOrReplace(hash, object) {
    hash[object.uid] = object;
}

---

UPDATE

It's also possible to use an object as an index in addition to the array.
This way you've got fast lookups and also a working array:

var arr = [],
    arrIndex = {};

addOrReplace({uid: 1, name: "bla", description: "cucu"});
addOrReplace({uid: 2, name: "smth else", description: "cucarecu"});
addOrReplace({uid: 1, name: "bli", description: "cici"});

function addOrReplace(object) {
    var index = arrIndex[object.uid];
    if(index === undefined) {
        index = arr.length;
        arrIndex[object.uid] = index;
    }
    arr[index] = object;
}

Take a look at the jsfiddle-demo (an object-oriented solution you'll find here)

Maybe

_.mixin({
    mergeById: function mergeById(arr, obj, idProp) {
        var index = _.findIndex(arr, function (elem) {
            // double check, since undefined === undefined
            return typeof elem[idProp] !== "undefined" && elem[idProp] === obj[idProp];
        });

        if (index > -1) {
            arr[index] = obj; 
        } else {
            arr.push(obj);
        }

        return arr;
    }
});

and

var elem = {uid: 3, name: 'new element name name', description: "cocoroco"};

_.mergeById(arr, elem, "uid");