pack.js

'use strict';

function pack(arr){
  // prev_item is the first item
  var prev_item = arr[0];
  // count of the first item is 1 at this stage
  var count_items = [1];
  // first item is very new to unique_items so it is unique
  var unique_items = [prev_item];

  // start from the second item.
  for (var i = 1; i < arr.length; i++) {

    // push new items only
    if (unique_items.indexOf(arr[i]) == -1) {
      unique_items.push(arr[i]);
    }

    // make count_items and unique_items equal in length
    if (count_items.length < unique_items.length){
      count_items.push(1);
    }

    // if the current item == the prev item increase the count by 1
    if (prev_item == arr[i]) {
      var target_item = unique_items.indexOf(prev_item);
      count_items[target_item] = count_items[target_item] + 1;
    }

    prev_item = arr[i];
  }

  if (count_items.length == unique_items.length) {
    var res = [];
    for (var i = 0; i < unique_items.length; i++) {
      res.push(unique_items[i] + ":" + count_items[i]);
    }
    return res
  } else {
    throw "Error: number of items must equals number of their counts.";
  }
}


console.log(pack([5,5,5,7,7,3,4,7]));
// [ '5:3', '7:2', '3:1', '4:1' ]
console.log(pack([5,5,5,7,7,3,4,7,8]));
// [ '5:3', '7:2', '3:1', '4:1', '8:1' ]
// and here is our the trick, which I don't know if it a bug or expected behaviour!!
console.log(pack([7,5,5,5,7,7,3,4,7,8]));
// [ '7:2', '5:3', '3:1', '4:1', '8:1' ]