Underscore.js groupBy multiple values

Question

Using Underscore.js, I\'m trying to group a list of items multiple times, ie

Group by SIZE then for each SIZE, group by CATEGORY...

http://jsfiddle.net/ricky

Answer 1

Grouping by a composite key tends to work better for me in most situations:

const groups = _.groupByComposite(myList, ['size', 'category']);

Demo using OP's fiddle

Mixin

_.mixin({
  /*
   * @groupByComposite
   *
   * Groups an array of objects by multiple properties. Uses _.groupBy under the covers,
   * to group by a composite key, generated from the list of provided keys.
   *
   * @param {Object[]} collection - the array of objects.
   * @param {string[]} keys - one or more property names to group by.
   * @param {string} [delimiter=-] - a delimiter used in the creation of the composite key.
   *
   * @returns {Object} - the composed aggregate object.
   */
  groupByComposite: (collection, keys, delimiter = '-') =>
    _.groupBy(collection, (item) => {
      const compositeKey = [];
      _.each(keys, key => compositeKey.push(item[key]));
      return compositeKey.join(delimiter);
    }),
});