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

You could use a hash table for the grouping by key.

var array = [{ 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}],
    grouped = [];

array.forEach(function (o) {
    if (!this[o.key]) {
        this[o.key] = { key: o.key, val: 0 };
        grouped.push(this[o.key]);
    }
    this[o.key].val += o.val;
}, Object.create(null));

console.log(grouped);

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Another approach is to collect all key/value pairs in a Map and format the final array with Array.from and a callback for the objects.

var array = [{ 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 }],
    grouped = Array.from(
        array.reduce((m, { key, val }) => m.set(key, (m.get(key) || 0) + val), new Map),
        ([key, val]) => ({ key, val })
    );

console.log(grouped);

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