Javascript: Sum objects in a collection

Question

I am looking for a way to consolidate a collection of objects in javascript. For example I have a collection:

inventory = [ 
    {count: 1, type: "Apple"},
    {count: 2, type: "Orange"},
    {count: 1, type: "Berry"},
    {count: 2, type: "Orange"},
    {count: 3, type: "Berry"}
]

What I want to end up with is:

   inventory = [
        {count: 1, type: "Apple"},
        {count: 4, type: "Orange"},
        {count: 4, type: "Berry"}
     ]

Is there an elegant way to do this that doesn't involve getting a list of types, searching through my collection for those types, summing the values, and making a new array with the sums?

where are you getting this information? via a database? perhaps it would be better to aggregate natively rather than in JS. — zamnuts
– zamnuts, Commented Nov 13, 2013 at 5:55
I am getting this data through running regex on a list of strings, so unfortunately unable to aggregate before I get to the JS — muggetninja
– muggetninja, Commented Nov 13, 2013 at 5:59

Tim Heap · Accepted Answer · 2013-11-13 05:58:01Z

3

It is not overly pretty, but this will do it. It creates both a dictionary of item types/counts, and a list of the final sums. inventoryDict is used to easily find the existing count, while summedInventory holds the final list of summed items.

var inventory = [ /* ... */ ];
var summedInventory = [];
var inventoryDict = {};

for (var i = 0; i < inventory.length; i++) {
    var item = inventory[i];
    if (!(item.type in inventoryDict)) {
        inventoryDict[item.type] = {type: item.type, count: 0};
        summedInventory.push(inventoryDict[item.type]);
    }
    inventoryDict[item.type].count += item.count;
}

This is is assuming that you do not want to mutate the inventory items in place - the loop can be slightly simplified if you do not mind mutating the items.

To avoid the intermediate variable and do it in a more functional manner, you could use Array.reduce:

var newInventory = inventory.reduce(function(acc, item) {
    var summedInventory = acc[0], inventoryDict = acc[1];

    if (!(item.type in inventoryDict)) {
        inventoryDict[item.type] = {type: item.type, count: 0};
        summedInventory.push(inventoryDict[item.type]);
    }

    inventoryDict[item.type].count += item.count;
    return acc;
}, [[], {}])[0];

edited Nov 13, 2013 at 5:58

answered Nov 13, 2013 at 5:45

Tim Heap

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4 Comments

Zach Saucier Over a year ago

Did you test this? I thought the format would be item['type'] and item['count']

zamnuts Over a year ago

+1 for reduce solution which is a bit more intuitive IMO, albeit not as compatible.

Tim Heap Over a year ago

@Zeaklous JavaScript allows you to use either obj.name or obj['name']. They are equivalent. When the property you are looking up is a valid identifier and known at the time of writing, the preferred option is usually obj.name. The other variant is useful for keys only known at run time, or for keys that are not valid identifiers: keyName = "hello"; obj[keyName] and obj['name with spaces'];

Zach Saucier Over a year ago

Right, right. Had to brush up on my jQuery formatting. All is good!

Saeed Neamati · Accepted Answer · 2013-11-13 06:00:42Z

1

My solution is this:

inventory = [ 
    {count: 1, type: "Apple"},
    {count: 2, type: "Orange"},
    {count: 1, type: "Berry"},
    {count: 2, type: "Orange"},
    {count: 3, type: "Berry"}
];

result = {};
inventory.map(function(item) {
    console.log(item);
    var count = result[item.type] || 0;
    result[item.type] = item.count + count;
});

inventory = [];

for (property in result) {
    inventory.push({count: result[property], type: property});
}

console.log(inventory);

See this jsfiddle.

answered Nov 13, 2013 at 6:00

Saeed Neamati

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Comments

Jeff Sisson · Accepted Answer · 2013-11-13 06:12:41Z

1

Here's a relatively slim way of doing it using Javascript's Array.reduce:

var reduced = inventory
    .reduce(function(sum,current) {
        var found = false
        sum.forEach(function(row,i) {
            if (row.type === current.type) {
                sum[i].count += current.count
                found = true;
            }
        })
        if (found === false) sum.push(current)
        return sum
    }, [])

console.log(reduced)

answered Nov 13, 2013 at 6:12

Jeff Sisson

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