merge 2 javascript objects by recursively adding their values

Question

I want to be able to merge two objects by adding their values together.

> a
{ "a" : 1, "b" : 3, "d": {"da": 1}}
> b
{ "a" : 1, "c" : 34, "d": {"da": 2} }

I want to obtain :

> { "a" : 2, "b": 3, "c" : 34, "d": {"da": 3} }

I've tried this but it doesn't work :

 function MergeRecursive(obj1, obj2) {
   for (var p in obj2) {
     try {
       // Property in destination object set; update its value.
       if ( obj2[p].constructor==Object ) {
         obj1[p] += MergeRecursive(obj1[p], obj2[p]);

       } else {
             obj1[p] = obj2[p];
       }

     } catch(e) {
       // Property in destination object not set; create it and set its value.
             obj1[p] = obj2[p];
     }
   }
   return obj1;
 }

Any ideas ?

georg · Accepted Answer · 2016-01-29 11:18:16Z

First, let's define an abstract function that applies a func to a combination of two objects, and then use it together with the summation function.

function merge(x, y, fn) {
    var result = {};

    Object.keys(x).forEach(function(k) {
        result[k] = x[k];
    });

    Object.keys(y).forEach(function(k) {
        result[k] = k in x ? fn(x[k], y[k]) : y[k];
    });

    return result;
}

function add(p, q) {
    if(typeof p === 'object' && typeof q === 'object') {
        return merge(p, q, add);
    }
    return p + q;
}

a = { "a" : 1, "b" : 3, "d": {"da": 1}};
b = { "a" : 1, "c" : 34, "d": {"da": 2}};

sum = merge(a, b, add)
document.write('<pre>'+JSON.stringify(sum,0,3));

merge can be also written in a more functional style, like this:

function clone(x) {
    return Object.keys(x).reduce(function(res, k) {
        res[k] = x[k];
        return res;
    }, {});
}

function merge(x, y, fn) {
    return Object.keys(y).reduce(function(res, k) {
        res[k] = k in x ? fn(x[k], y[k]) : y[k];
        return res;
    }, clone(x));
}

If you're fine with the first object being changed, you can skip the clone step and just pass x.

Sam · Accepted Answer · 2016-01-29 11:10:51Z

1

My attempt

function MergeRecursive(obj1, obj2) {
    var k = Object.keys(obj1), i = 0;
    for (var p in obj2) {
        if(typeof obj1[p] == 'object' && typeof obj2[p] == 'object')
            obj2[p] = MergeRecursive(obj1[p],obj2[p]);
        else if(obj1[p] != null)
            obj2[p] += obj1[p];
        else
            obj2[k[i]] = obj1[k[i]];
        i++;
    }
    return obj2;
}

To use as

MergeRecursive({ "a" : 1, "b" : 3, "d": {"da": 1}},{ "a" : 1, "c" : 34, "d": {"da": 2} })

answered Jan 29, 2016 at 11:10

Sam

2,9601 gold badge20 silver badges26 bronze badges

Comments

Glubus · Accepted Answer · 2016-01-29 11:17:31Z

Create a resultArray to avoid complexity with obj1. Then just add each value in obj1 to the resultArray with the value of obj2 accumulated if the corresponding key in obj2 exists.

Then add each key in obj2 that was not yet added by the first iteration to the resultArray, so that you don't lose the data from obj2.

   function MergeRecursive(obj1, obj2) {
       resultArray = [];
       for (var key in obj1) {
           if ( typeof obj1[key] === 'object') {
               resultArray[key] = MergeRecursive(obj1[p], obj2[p]);
           } else {
               resultArray[key] = obj1[key] + obj2[p] ?? 0;
           }
       }
       for (var key in obj2) {
           if (key in resultArray){
               continue;
           }
           resultArray[key] = obj2[key];
       }
       return resultArray;
     }

Collectives™ on Stack Overflow

merge 2 javascript objects by recursively adding their values

3 Answers 3

Comments

Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

Comments

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related