Javascript Sum array of object values

Question

I would think this is a surprisingly common and simple question but I cant seem to find what Im looking for

If I had

var array = [{"a":4,"b":5, "d":6}, {"a":4, "c":5}, {"c":4}]

how do I sum the objects to get

{"a":8,"b":5,"c":9, "d":6}

using underscore, lodash, or something fairly quick and simple one liner?

Travis J · Accepted Answer · 2015-08-20 23:32:19Z

4

You should be able to use reduce for this

var compact = array.reduce(function(prev,cur){
 for(var key in cur){
  if(!cur.hasOwnProperty(key))continue;
  if(prev.hasOwnProperty(key)){
   prev[key] += cur[key];
  }else{
   prev[key] = cur[key];
  }
 }
 return prev;
},{});

answered Aug 20, 2015 at 23:32

Travis J

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2 Comments

Leon Over a year ago

Thank you for the answer Travis. Is there a way you can see of doing this with underscore or lodash?

mu is too short Over a year ago

@Leon: Underscore has _.reduce if you don't want to use standard JavaScript.

Adam Boduch · Accepted Answer · 2015-08-21 12:51:54Z

2

You could combine spread(), merge(), and add() to produce:

_.spread(_.merge)([{}].concat(array, _.add));
// → { a: 8, b: 5, d: 6, c: 13 }

answered Aug 21, 2015 at 12:51

Adam Boduch

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2 Comments

Leon Over a year ago

No clue how it works but this is a brilliant solution using lodash.

Jimmy Thrasibule Over a year ago

Any way someone can explain how this works. I don't get how concat can help having the values added.

manji · Accepted Answer · 2015-08-20 23:33:44Z

2

You can try this solution:

var arr = [{"a":4,"b":5, "d":6}, {"a":4, "c":5}, {"c":4}];
var result = {};
arr.forEach(function(obj) {
  for(var prop in obj) {
    if(obj.hasOwnProperty(prop)) {
        result[prop] = (result[prop] || 0) + obj[prop];
    }
  }
});

$('#result').text(JSON.stringify(result,null,2));

<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.11.0/jquery.min.js"></script>
<div id="result"></div>

answered Aug 20, 2015 at 23:33

manji

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Comments

Felix Kling · Accepted Answer · 2015-08-20 23:35:02Z

2

_.merge with a custom callback would do:

var array = [{"a": 4, "b": 5, "d": 6}, {"a": 4, "c": 5}, {"c": 4}];

var result = _.merge.apply(null, array.concat([function(a, b) {
  if (typeof a === 'number') {
    return a + b;
  }
}]));
console.log(result);

<script src="https://cdnjs.cloudflare.com/ajax/libs/lodash.js/3.10.1/lodash.min.js"></script>

answered Aug 20, 2015 at 23:35

Felix Kling

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2 Comments

Adam Boduch Over a year ago

This mutates the first element of array, FYI.

Felix Kling Over a year ago

@AdamBoduch: true. Should be solvable by injecting an empty object at the beginning.

Ryan Shepherd · Accepted Answer · 2018-08-23 17:24:38Z

0

As of lodash 4.0.0 this is simple using mergeWith:

_.mergeWith({}, ...array, _.add)

answered Aug 23, 2018 at 17:24

Ryan Shepherd

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