asyncerator/src/operator/filter.ts

Provide supporting types for AsyncIterable/AsyncIterableIterators, promisified stream.pipeline implementation, and Array-like utility operators, sources and sinks.

// operator/filter.ts

/*
 * Copyright (c) 2021-2024 Check Digit, LLC
 *
 * This code is licensed under the MIT license (see LICENSE.txt for details).
 */

import type { Asyncerator } from '../asyncerator';

import type { Operator } from './index';

export default function <Input, Output extends Input>(
  predicate: (value: Input, index: number) => value is Output,
): Operator<Input, Output>;
export default function <Input>(filterFunction: (value: Input, index: number) => boolean): Operator<Input, Input>;

/**
 * Similar to `Array.filter`, only emit values from input for which filterFunction returns true.
 * @param filterFunction
 */
export default function <Input>(filterFunction: (value: Input, index: number) => boolean): Operator<Input, Input> {
  return async function* (iterator: Asyncerator<Input>) {
    let currentIndex = 0;
    for await (const item of iterator) {
      if (filterFunction(item, currentIndex++)) {
        yield item;
      }
    }
  };
}