/**
* Groups array items into a Map, given a function to produce grouping key.
*/
export function groupBy(list, keyFn) {
const result = new Map();
for (const item of list) {
const key = keyFn(item);
const group = result.get(key);
if (group === undefined) {
result.set(key, [item]);
} else {
group.push(item);
}
return result;