Sum array of objects by unique keys
What do you mean optimal?

Your thing transforms the object lots of times, which is liable to not be very efficient, if that's what you meant.

Here's something that I hope is simple to read that does your task.

function sumObjects(obj1, obj2) {
    return Object.keys(obj2).reduce(
        (acc, currKey) =&gt; ({
        ...acc,
        [currKey]: acc[currKey] ? acc[currKey] + obj2[currKey] : obj2[currKey]
        }),
        obj1
    );
}

const result = dimensions.reduce(sumObjects);


The observation is that you just need to see how to do your task for two objects and the rest is just a reduce.

The most efficient thing will likely be just a for loop, like 

const res = {};
for (const obj of dimensions) {
    for (const key in obj) {
        res[key] = res[key] ? res[key] + obj[key] : obj[key];
    }
}