Merge objects with same key in javascript

Question

I have been trying to figure this out for a long time, if i have an array of objects like so:

var my_array = [
    Object {Project: A, Hours: 2},
    Object {Project: B, Hours: 3},
    Object {Project: C, Hours: 5},
    Object {Project: A, Hours: 6},
    Object {Project: C, Hours: 9}
]

I want to merge all the objects with the same key together into one object, such that their hours get added up:

Expected output:

my_array = [
    Object {Project: A, Hours: 8}
    Object {Project: B, Hours: 3}
    Object {Project: C, Hours: 14}
]

How can I approach this issue? It has taken me a long time to get my data formatted in this way, this is the last step!

My attempt, I get that I am looping through array, not sure how to deal with the merging of the objects:

for (var i =0; i<my_array.length; i++) {
   my_array[i].Project   // access the object project key
   my_array[i].Hours     // need to increment hours
}

thefourtheye · Accepted Answer · 2016-03-22 06:23:10Z

You create another object where you can actually group the projects and accumulate the corresponding hours, like this

var groups = my_array.reduce(function(resultObject, currentObject) {

    // if this is the first time the project appears in the array, use zero as the
    // default hours
    resultObject[currentObject.Project] = resultObject[currentObject.Project] || 0;

    // add the current hours corresponding to the project
    resultObject[currentObject.Project] += currentObject.Hours;

    return resultObject;
}, {});

At this point, your groups will look like this

console.log(groups);
// { A: 8, B: 3, C: 14 }

Now, you just have to expand this object, like this

var result = Object.keys(groups).map(function(currentGroup) {
    return {Project: currentGroup, Hours: groups[currentGroup]};
});

Now, the result will be

[ { Project: 'A', Hours: 8 },
  { Project: 'B', Hours: 3 },
  { Project: 'C', Hours: 14 } ]

gurvinder372 · Accepted Answer · 2016-03-22 06:23:01Z

In your attempt, you are missing out on creating a new array

var newArray = [];
var uniqueprojects = {};
for (var i =0; i<my_array.length; i++) {

   if ( !uniqueproject[my_array[i].Project] )
   {
     uniqueproject[my_array[i].Project] = 0;
   }
   uniqueproject[my_array[i].Project] += my_array[i].Hours;
   //my_array[i].Project   // access the object project key
   //my_array[i].Hours     // need to increment hours
}

Now create the final output array out of uniqueproject map

newArray = Object.keys(uniqueproject).map(function(key){return {Project:key, Hours:uniqueproject[key]}});

FYI I just tested your code on jsPerf and compared it to the other proposed solution and yours is much faster. See my post below.

Miqi180 · Accepted Answer · 2016-03-22 18:18:50Z

2

Both thefourtheye's and gurvinder372's proposed solutions work, so I set up a benchmark test on jsPerf to test which is faster. You can see it here.

It appears that gurvinder372's code is by far the fastest.

P.S. please ignore the Uncaught TypeError as it is a jsPerf issue currently being fixed that has nothing to do with the results of the test. For more info, see this and this.

edited Mar 22, 2016 at 18:18

answered Mar 22, 2016 at 9:43

Miqi180

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