JavaScript - group objects by multiple properties in array then sum up their values

You can use reduce() method to create temporary object with unique groups as its properties. Then inside each group sum up the property value of all objects.

// ONLINE-RUNNER:browser;

const groupAndSum = (array, property1, property2, propertyToSum) => {
    const tmp = {};
    array.reduce((result, object) => {
        const group = object[property1] + '-' + object[property2];
        if (!tmp[group]) {
            tmp[group] = Object.assign({}, object);
            result.push(tmp[group]);
        } else {
            tmp[group][propertyToSum] += object[propertyToSum];
        }
        return result;
    }, []);
    return tmp;
};


// Usage example:

const array = [
    { country: 'USA',    type: 'A', quantity: 1 },
    { country: 'USA',    type: 'A', quantity: 2 },
    { country: 'Canada', type: 'B', quantity: 1 },
    { country: 'Canada', type: 'B', quantity: 2 },
    { country: 'USA',    type: 'A', quantity: 2 },
];

const result = groupAndSum(array, 'type', 'country', 'quantity');

console.log(JSON.stringify(result, null, 4));

See also

1. JavaScript - group object by property then sum up property values

References

1. Array.prototype.reduce() - JavaScript | MDN
2. Object.assign() - JavaScript | MDN