A more readable and annotated version of the Javascript groupBy from Ceasar Bautista (https://stackoverflow.com/a/34890276/1376063)

groupBy.js

const groupBy = (data, keys) => { // `data` is an array of objects, `keys` is the array of keys (or property accessor) to group by
  // reduce runs this anonymous function on each element of `data` (the `item` parameter,
  // returning the `storage` parameter at the end
  return data.reduce((storage, item) => {
    // 
    const groupValues = keys.reduce((values, key) => {
      values[key] = item[key];
      return values;
    }, {});
    
    // get the first instance of the key by which we're grouping
    const group = Object.values(groupValues).join(' ');

    // set `storage` for this instance of group to the outer scope (if not empty) or initialize it
    storage[group] = storage[group] || [];

    // add this item to its group within `storage`
    if (keys.every((key) => item[key] === groupValues[key])) {
      storage[group].push(item);
    }

    // return the updated storage to the reduce function, which will then loop through the next 
    return storage; 
  }, {}); // {} is the initial value of the storage
};