ix/src/iterable/first.ts

The Interactive Extensions for JavaScript

import { OptionalFindOptions } from './findoptions.js';

/**
 * Returns the first element of an iterable sequence that matches the predicate if provided, or undefined if no such element exists.
 *
 * @template T The type of the elements in the source sequence.
 * @param {Iterable<T>} source Source async-enumerable sequence.
 * @returns {(S | undefined)} The first element in the iterable sequence, or undefined if no such element exists.
 */
export function first<T>(source: Iterable<T>, options?: OptionalFindOptions<T>): T | undefined {
  const { ['thisArg']: thisArg, ['predicate']: predicate = () => true } = options || {};
  let i = 0;
  for (const item of source) {
    if (predicate!.call(thisArg, item, i++)) {
      return item;
    }
  }

  return undefined;
}