Sum up several array through several objects

Question

I have one parent's object, which contains several child's objects. On each of them, I have an array, and I want to sum those values.

dataArray = {
    "dribbble":{
        "username":"JeremDsgn",
        "followers":242,
        "diff":{
            "data":[230, 231, 236, 238, 239, 239, 242]
        }
    },
    "twitter":{
        "username":"jeremdsgn",
        "followers":"592",
        "diff":{
            "data":[576, 578, 578, 581, 584, 589, 592]
        }
    },
    "behance":{
        "username":"JeremDsgn",
        "followers":17,
        "diff":{
            "data":[16, 15, 15, 16, 16, 16, 17]
        }
    }
}

So in my case, I would like to create a total array who sum value by value.

total = [822, 824, 829, 835, 839, 844, 851]

I have two loop to go through objects, and through arrays, but didn't works.

for (var site in dataArray) {
  var total = [];

  for (var i = 0; i < dataArray[site].diff.data.length; i++) {    
    total += parseInt(dataArray[site].diff.data[i]);
  }
}

But it didn't affect value to the array for each index values, and it returns NaN.

Answer 1

There's a few problems with your code.

• You are clearing the total array each time you step in to the inner loop. To fix this, declare the array outside of both loops.

• You need to initialize total with 7 indices (all starting at zero).

• You need to increment total at the specified index, i.e. total[i] +=:

JSFiddle

Working Code

var total = [0,0,0,0,0,0,0];

for (var site in dataArray) {
    for (var i = 0; i < dataArray[site].diff.data.length; i++) {
        total[i] += parseInt(dataArray[site].diff.data[i], 10);
    }
}

console.log(total) // [822, 824, 829, 835, 839, 844, 851]

Also: When using parseInt always remember to specify the radix as the second parameter.

Answer 2

In your code, at no point are you adding elements to the array. What you are doing is, this:

total = [];
total += (number);

In javascript, if you add a number to an array, you get the string representation of that array:

[] + 10 === "10"; // Evaluates to true

That is because an array in addition gets converted to a string, hence an empty array gets converted to an empty string and then adds the number to it. Also, you are declaring the main array within the first loop. This will cause it to erase the array every time it iterates over an element. And the same thing is happening in the second case. To solve this, what you need to do is:

var total = [];

for (var site in dataArray) {
    var sub_total = 0;

    for (var i = 0; i < dataArray[site].diff.data.length; i++) {
        sub_total += parseInt(dataArray[site].diff.data[i]);
    }

    total.push(sub_total);
}

Answer 3

You're declaring total twice, and trying to += on an array. Instead you should target the required index of the total array.

Fiddle

var total = [];

for (var site in dataArray) {
  for (var i = 0; i < dataArray[site].diff.data.length; i++) {
    if(typeof total[i] == 'undefined'){
        total[i] = 0;      
    }
    total[i] += parseInt(dataArray[site].diff.data[i]);
  }
}

Outputs

[822, 824, 829, 835, 839, 844, 851]