ix/src/iterable/iif.ts

The Interactive Extensions for JavaScript

import { IterableX } from './iterablex.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 {(() => boolean)} condition Condition evaluated to decide which sequence to return.
 * @param {Iterable<TSource>} thenSource Sequence returned in case evaluates true.
 * @param {Iterable<TSource>} [elseSource=empty()] Sequence returned in case condition evaluates false.
 * @returns {IterableX<TSource>} thenSource if condition evaluates true; elseSource otherwise.
 */
export function iif<TSource>(
  fn: () => boolean,
  thenSource: Iterable<TSource>,
  elseSource: Iterable<TSource> = empty()
): IterableX<TSource> {
  return defer<TSource>(() => (fn() ? thenSource : elseSource));
}