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

groupBy.js

var groupBy = function(data, key) { // `data` is an array of objects, `key` is the key (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(function(storage, item) {
    // get the first instance of the key by which we're grouping
    var group = item[key];
    
    // 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`
    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
};