Reduce an array based on the provided keys, optionally summing specified properties.

array-combine-sum.js

/**
 * Combine elements of an array based on the provided keys, optionally summing specified properties
 * @param data
 * @param keys
 * @param sum
 * @returns array
 */
var arrayCombineSum = function(data,keys,sum) {

    /**
     * Generate a value from the provides key(s)
     * @param object
     * @returns {string}
     */
    var createKey = function(object, keys) {
        var output = [];
        keys.forEach(function(key) {
            if (object[key] === undefined) {
                return;
            }
            output.push(object[key]);
        })
        return output.join('');
    }

    // Reduce the array based on user specified key(s)
    return data.reduce(function(output, object) {

        // Generate a primary key for this object based on the user provided keys
        var pk = createKey(object, keys);

        // Add the element to the output data if it doesn't already exist
        if (!(pk in output)) {
            output.data.push(output[pk] = object);
        }
        else {

            // Sum requested values
            sum.forEach(function(key) {
                output[pk][key] += object[key];
            });
        }
        return output;
    }, {data:[]}).data;
};

example.js

data = [];
sites = ["Site A", "Site B"];
locations = ["Location A", "Location B", "Location C"];

for (var month = 7; month <= 8; month++) {
    for (var day = 1; day <= 31; day++) {
        sites.forEach(function(site) {
            locations.forEach(function(location) {
                data.push({
                    site: site,
                    location: location,
                    day: day,
                    month: month,
                    date: day + '/' + month + '/2013',
                    expected: 25 + Math.ceil(Math.random() * 10),
                    actual: 15 + Math.ceil(Math.random() * 10)
                });
            })
        });
    }
}

// Monthly, by site
console.log(arrayCombineSum(data, ['site', 'month'], ['expected', 'actual']));

// Monthly, by site & location
console.log(arrayCombineSum(data, ['site', 'location', 'month'], ['expected', 'actual']));