rxjs/src/internal/operators/takeLast.ts

Reactive Extensions for modern JavaScript

import { EMPTY } from '../observable/empty';
import { MonoTypeOperatorFunction } from '../types';
import { operate } from '../util/lift';
import { createOperatorSubscriber } from './OperatorSubscriber';

/**
 * Waits for the source to complete, then emits the last N values from the source,
 * as specified by the `count` argument.
 *
 * ![](takeLast.png)
 *
 * `takeLast` results in an observable that will hold values up to `count` values in memory,
 * until the source completes. It then pushes all values in memory to the consumer, in the
 * order they were received from the source, then notifies the consumer that it is
 * complete.
 *
 * If for some reason the source completes before the `count` supplied to `takeLast` is reached,
 * all values received until that point are emitted, and then completion is notified.
 *
 * **Warning**: Using `takeLast` with an observable that never completes will result
 * in an observable that never emits a value.
 *
 * ## Example
 *
 * Take the last 3 values of an Observable with many values
 *
 * ```ts
 * import { range, takeLast } from 'rxjs';
 *
 * const many = range(1, 100);
 * const lastThree = many.pipe(takeLast(3));
 * lastThree.subscribe(x => console.log(x));
 * ```
 *
 * @see {@link take}
 * @see {@link takeUntil}
 * @see {@link takeWhile}
 * @see {@link skip}
 *
 * @param count The maximum number of values to emit from the end of
 * the sequence of values emitted by the source Observable.
 * @return A function that returns an Observable that emits at most the last
 * `count` values emitted by the source Observable.
 */
export function takeLast<T>(count: number): MonoTypeOperatorFunction<T> {
  return count <= 0
    ? () => EMPTY
    : operate((source, subscriber) => {
        // This buffer will hold the values we are going to emit
        // when the source completes. Since we only want to take the
        // last N values, we can't emit until we're sure we're not getting
        // any more values.
        let buffer: T[] = [];
        source.subscribe(
          createOperatorSubscriber(
            subscriber,
            (value) => {
              // Add the most recent value onto the end of our buffer.
              buffer.push(value);
              // If our buffer is now larger than the number of values we
              // want to take, we remove the oldest value from the buffer.
              count < buffer.length && buffer.shift();
            },
            () => {
              // The source completed, we now know what are last values
              // are, emit them in the order they were received.
              for (const value of buffer) {
                subscriber.next(value);
              }
              subscriber.complete();
            },
            // Errors are passed through to the consumer
            undefined,
            () => {
              // During finalization release the values in our buffer.
              buffer = null!;
            }
          )
        );
      });
}