Grouping consecutive elements together using Javascript

Question

I have an array of elements like so:

messages[i], where messages[i] may only exist for certain values of i. For instance messages[0] and messages[2] may exist but not messages[1].

Now I would like to group together elements with continuous indices, for example if the indices for which messages existed were:

2, 3, 4, 5, 8, 9, 12, 13, 14, 15, 16, 17, 20

I would like to group them like so:

2, 3, 4, 5

8, 9

12, 13, 14, 15, 16, 17

20

What would be an effective way to do so using Javascript?

EDIT:

for (i = 0; i < messages.length; i++) {
   if (messages[i].from_user_id == current_user_id) {
   // group the continuous messages together
      } else {
  //group these continuous messages together
   }
}

Answer 1

You can use a counter variable which has to be incremented and the difference between the index and the consecutive elements are the same, group them in a temporary array. If the difference is varies for two consecutive array elements, the temporary element has to be moved to the result and the temporary array has to be assigned a new array object.

var array = [2, 3, 4, 5, 8, 9, 12, 13, 14, 15, 16, 17, 20];

var result = [], temp = [], difference;
for (var i = 0; i < array.length; i += 1) {
    if (difference !== (array[i] - i)) {
        if (difference !== undefined) {
            result.push(temp);
            temp = [];
        }
        difference = array[i] - i;
    }
    temp.push(array[i]);
}

if (temp.length) {
    result.push(temp);
}

console.log(result);
# [ [ 2, 3, 4, 5 ], [ 8, 9 ], [ 12, 13, 14, 15, 16, 17 ], [ 20 ] ]