function intersection<T>(x: T[], y: T[]): T[] {
  const frequency = x.reduce(
    (map, value) => map.set(value, (map.get(value) ?? 0) + 1),
    new Map<T, number>()
  );

  return y.filter((value) => {
    const count = frequency.get(value) ?? 0;
    frequency.set(value, count - 1);

    return count > 0;
  });
}

Explanation

The intersection function returns the intersection, i.e. the common elements, of two lists. For example, the intersection of [1, 2, 3] and [2, 3, 4] is [2, 3].

This is a TypeScript function that takes in two lists of a generic type T and returns a list of elements of type T. The intersection function uses the reduce method to create a Map object called frequency. This map stores the frequency of each element in the first list x. The reduce method iterates over the x list, and for each element, it checks whether the element already exists in the map. If it does, it increments the frequency of that element by 1. If it doesn’t, it adds the element to the map with a frequency of 1. The reduce method initializes the map variable to an empty Map object.

Next, the function uses the filter method to filter out the elements in the second list y that are not present in the frequency map. The filter method takes a callback function that is called for each element in y. For each element, the function first checks if it exists in the frequency map. If it does, it decrements the frequency of that element by 1 and returns true to keep the element in the resulting list. If the element does not exist in the frequency map, it returns false to filter out that element from the resulting list.