Groups items in an array by a give selector.

group-by.js

/**
 * Groups items in a list by a give selector.
 * @template T Type of the items.
 * @template K Type of the grouping key.
 * @param {Iterable<T>} list The items to group.
 * @param {(item: T) => K} by Function for the selector by which to group.
 */
function groupBy(list, by)
{
	/** @type {Map<K, T[]>} */
	const groups = new Map();

	for (const item of list)
	{
		const key = by(item);
		if (groups.has(key) == false)
			groups.set(key, []);

		const group = groups.get(key);
		group.push(item);
	};

	const groupList = [];
	for (const [key, value] of groups)
		groupList.push({
			key,
			items: value,
		});

	return groupList;
}