All files / collections / unstable_drop_last_while.ts

// Copyright 2018-2026 the Deno authors. MIT license.
// This module is browser compatible.

/**
 * Returns an array that drops all elements in the given iterable until the
 * last element that does not match the given predicate.
 *
 * @experimental **UNSTABLE**: New API, yet to be vetted.
 *
 * @typeParam T The type of the elements in the input iterable.
 *
 * @param iterable The iterable to drop elements from.
 * @param predicate The function to test each element for a condition. The
 * function receives the element and its index.
 *
 * @returns An array that drops all elements until the last element that does
 * not match the given predicate.
 *
 * @example Basic usage
 * ```ts
 * import { dropLastWhile } from "@std/collections/unstable-drop-last-while";
 * import { assertEquals } from "@std/assert";
 *
 * const numbers = [11, 42, 55, 20, 33, 44];
 *
 * const notFortyFour = dropLastWhile(numbers, (number) => number > 30);
 *
 * assertEquals(notFortyFour, [11, 42, 55, 20]);
 * ```
 *
 * @example Using the index parameter
 * ```ts
 * import { dropLastWhile } from "@std/collections/unstable-drop-last-while";
 * import { assertEquals } from "@std/assert";
 *
 * const array = [20, 30, 20];
 * const result = dropLastWhile(array, (_, index) => index > 1);
 *
 * assertEquals(result, [20, 30]);
 * ```
 */
export function dropLastWhile<T>(
  iterable: Iterable<T>,
  predicate: (el: T, index: number) => boolean,
): T[] {
  const array = Array.isArray(iterable) ? iterable : Array.from(iterable);
  let offset = array.length - 1;
  while (offset >= 0 && predicate(array[offset]!, offset)) {
    offset--;
  }
  return array.slice(0, offset + 1);
}