Summing two objects without altering either
Since you're using ES2015 destructuring assignments you can use Set and spread:

function mashObj(a, b) {
    var sum = {};
    new Set([...Object.keys(a), ...Object.keys(b)])
        .forEach(k =&gt; sum[k] = Math.round(a[k]||0) + Math.round(b[k]||0));
    return sum;
}