Merging multiple duplicate objects into one from JavaScript array

Question

My JSON file looks like the following, somewhere around 1000-2000 objects.

[{
    "date": "2015-01-25T22:13:18Z",
    "some_object": {
        "first_group": 20,
        "second_group": 90,
        "third_group": 39,
        "fourth_group": 40
    }
}, {
    "date": "2015-01-25T12:20:32Z",
    "some_object": {
        "first_group": 10,
        "second_group": 80,
        "third_group": 21,
        "fourth_group": 60
    }
}, {
    "date": "2015-02-26T10:53:03Z",
    "some_object": {
        "first_group": 12,
        "second_group": 23,
        "third_group": 13,
        "fourth_group": 30
    }
}]

After copying it in an array I need to perform the following manipulation on it:

First. Remove duplicate objects. 2 objects are considered the same if they have the same date (without taking the time into consideration). So in my JSON, the first two objects are considered the same. Now the tricky part is that when a duplicate is found, we shouldn't just randomly remove one of them, but merge (not sure if merge is the right word) the fields from some_object, so it becomes one object in the array. Therefore, with the JSON above, the first two objects would become one:

{
 "date": "2015-02-26T00:00:00Z",
 "some_object": {
    "first_group": 30, //20+10
    "second_group": 170, //90+80
    "third_group": 60, //39+21
    "fourth_group": 100 //40+60
 }
}

Even trickier is that there could be some 3-10 objects with the same date, but different time in the array. Therefore those should be merged into 1 object according to the rule above.

Second. Sort this array of objects ascending (from oldest to newest of the date field).

So what's so hard? Where did you get stuck?

I found out how to sort the array ascending (based on date) by using this and some of this.
But I have no idea how to do the first point of removing the duplicates and merging, in a time-efficient manner. Maybe something inside:

var array = [];//reading it from the JSON file
var object_date_sort_asc = function (obj1, obj2) {
    if (obj1.date > obj2.date) return 1;
    if (obj1.date < obj2.date) return -1;

    //some magic here
    return 0;
};
array.sort(object_date_sort_asc);

Any ideas?

Answer 1

Use an object whose properties are the dates, to keep track of dates that have already been seen, and the values are the objects. When you encounter a date that's been seen, just merge the elements.

var seen = {};
for (var i = 0; i < objects.length; i++) {
    var cur = objects[i];
    if (cur.date in seen) {
        var seen_cur = seen[cur.date];
        seen_cur.some_object.first_group += cur.some_object..first_group;
        seen_cur.some_object..second_group += cur.some_object..second_group;
        ...
    } else {
        seen[cur.date] = cur;
    }
}

Once this is done, you can convert the seen object to an array and sort it.

var arr = [];
for (var k in seen) {
    arr.push(seen[k]);
}

Answer 2

To remove duplicate objects, you can loop through your array using .map(). In each iteration, you push the dates, parsed using some simple regex (which removes the time), into an array—if and only if it is not present in the array to begin with:

• If it is not in the array, push into array (of unique dates) and return the object
• If it is in the array, do nothing

The logic above can be described as the following, assuming your array is assigned to the data variable:

// Remove duplicates
var dates = [];
var data_noDupes = $.map(data, function(item){ 
  var item_date = item.date.replace(/([\d\-]+)T.*/gi, '$1');
  if (dates.indexOf(item_date) === -1) {
    dates.push(item_date);
    return item;
  } 
});

This should remove all recurring instances of the same date.

With regards to the second part: to sort, you simply sort the returned array by the date, again parsed using some simply regex that removes the time:

// Sort data_noDupes
function sortByDate(a, b){
  var a_item_date = a.date.replace(/([\d\-]+)T.*/gi, '$1'),
      b_item_date = b.date.replace(/([\d\-]+)T.*/gi, '$1');
  return ((a_item_date < b_item_date) ? -1 : ((a_item_date > b_item_date) ? 1 : 0));
}

If you want to be extra safe, you should use momentjs to parse your date objects instead. I have simply modified how the dates are parsed in the functional example below, but with exactly the same logic as described above:

$(function() {
  var data = [{
    "date": "2015-02-26T10:53:03Z",
    "some_object": {
      "first_group": 12,
      "second_group": 23,
      "third_group": 13,
      "fourth_group": 30
    }
  }, {
    "date": "2015-01-25T12:20:32Z",
    "some_object": {
      "first_group": 10,
      "second_group": 80,
      "third_group": 21,
      "fourth_group": 60
    }
  }, {
    "date": "2015-01-25T22:13:18Z",
    "some_object": {
      "first_group": 20,
      "second_group": 90,
      "third_group": 39,
      "fourth_group": 40
    }
  }];

  // Remove duplicates
  var dates = [];
  var data_noDupes = $.map(data, function(item) {
    // Get date and format date
    var item_date = moment(new Date(item.date)).format('YYYY-MM-DD');
    
    // If it is not present in array of unique dates:
    // 1. Push into array
    // 2. Return object to new array
    if (dates.indexOf(item_date) === -1) {
      dates.push(item_date);
      return item;
    }
  });

  // Sort data_noDupes
  function sortByDate(a, b) {
    var a_item_date = moment(new Date(a.date));
    return ((a_item_date.isBefore(b.date)) ? -1 : ((a_item_date.isAfter(b.date)) ? 1 : 0));
  }

  data_noDupes.sort(sortByDate);
  console.log(data_noDupes);

  $('#input').val(JSON.stringify(data));
  $('#output').val(JSON.stringify(data_noDupes));
});

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