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
}

Answer 1

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 } ]

Answer 2

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]}});