Sum javascript object propertyA values with same object propertyB in array of objects

Question

How would one take a javascript array of objects such as:

my objArr = [
{key:Mon Sep 23 2013 00:00:00 GMT-0400, val:42},
{key:Mon Sep 24 2013 00:00:00 GMT-04         


        

Answer 1

Here is an alternative for you, but similar to that of Explosion Pills, reuses the original array rather than creating a new one or a different object. The sort may not be necessary and will slow things down a little, but it could be removed.

Javascript

function reduceMyObjArr(arr) {
    var temp = {},
        index;

    for (index = arr.length - 1; index >= 0; index -= 1) {
        key = arr[index].key;
        if (temp.hasOwnProperty(key)) {
            arr[temp[key]].val += arr[index].val;
            arr.splice(index, 1);
        } else {
            temp[key] = index;
        }
    }

    arr.sort(function (a, b) {
        if (a.key === b.key) {
            return 0;
        }

        if (a.key < b.key) {
            return -1;
        }

        return 1;
    });

    return arr;
}

var myObjArr = [{
    key: "Mon Sep 23 2013 00: 00: 00 GMT - 0400",
    val: 42
}, {
    key: "Mon Sep 24 2013 00: 00: 00 GMT - 0400",
    val: 78
}, {
    key: "Mon Sep 25 2013 00: 00: 00 GMT - 0400",
    val: 23
}, {
    key: "Mon Sep 23 2013 00: 00: 00 GMT - 0400",
    val: 54
}];

reduceMyObjArr(myObjArr);

console.log(myObjArr);

jsFiddle

And a jsperf that compares this (with and without the sort) against the accepted answer. You can improve the performance test by extending the data set.