30-seconds-of-code/snippets/findLastN.md

title, tags, expertise, cover, firstSeen, lastUpdated

title	tags	expertise	cover	firstSeen	lastUpdated
Find last n matches	array	intermediate	blog_images/interior-16.jpg	2021-05-09T13:31:36+03:00	2021-05-09T13:31:36+03:00

Finds the last n elements for which the provided function returns a truthy value.

• Use a for loop to execute the provided matcher for each element of arr.
• Use Array.prototype.unshift() to prepend elements to the results array and return them if its length is equal to n.

const findLastN = (arr, matcher, n = 1) => {
  let res = [];
  for (let i = arr.length - 1; i >= 0; i--) {
    const el = arr[i];
    const match = matcher(el, i, arr);
    if (match) res.unshift(el);
    if (res.length === n) return res;
  }
  return res;
};

findLastN([1, 2, 4, 6], n => n % 2 === 0, 2); // [4, 6]
findLastN([1, 2, 4, 6], n => n % 2 === 0, 5); // [2, 4, 6]