ix/src/asynciterable/iif.ts

The Interactive Extensions for JavaScript

import { AsyncIterableX } from './asynciterablex.js';
import { defer } from './defer.js';
import { empty } from './empty.js';

/**
 * If the specified condition evaluates true, select the thenSource sequence.
 * Otherwise, select the elseSource sequence.
 *
 * @template TSource The type of the elements in the result sequence.
 * @param {((signal?: AbortSignal) => boolean | Promise<boolean>)} condition Condition evaluated to decide which sequence to return.
 * @param {AsyncIterable<TSource>} thenSource Sequence returned in case evaluates true.
 * @param {AsyncIterable<TSource>} [elseSource=empty()] Sequence returned in case condition evaluates false.
 * @returns {AsyncIterableX<TSource>} thenSource if condition evaluates true; elseSource otherwise.
 */
export function iif<TSource>(
  condition: (signal?: AbortSignal) => boolean | Promise<boolean>,
  thenSource: AsyncIterable<TSource>,
  elseSource: AsyncIterable<TSource> = empty()
): AsyncIterableX<TSource> {
  return defer<TSource>(async (signal) => ((await condition(signal)) ? thenSource : elseSource));
}