Return a Map of items to their indices in the array #typescript

findItemIndices.ts

/**
 * Return a Map of items to their indices in the array.
 *
 * The Map will have an entry for each item in A, with the item as the key and
 * an array of indices as the value.
 *
 * There will be as many items in the Map as there are unique items in `A`.
 *
 * @param A an array of items of any type `T`
 *
*/
function findItemIndices<T>(A: T[]) {
  // Create a map to store the indices of each item in A
  const itemIndices = new Map<T, number[]>();

  A.forEach((item, index) => {
    if (!itemIndices.has(item)) {
      itemIndices.set(item, [index]);
    } else {
      itemIndices.get(item).push(index);
    }
  });

  return itemIndices;
}