rxjs/src/internal/operators/takeWhile.ts

Reactive Extensions for modern JavaScript

import { OperatorFunction, MonoTypeOperatorFunction, TruthyTypesOf } from '../types';
import { operate } from '../util/lift';
import { createOperatorSubscriber } from './OperatorSubscriber';

export function takeWhile<T>(predicate: BooleanConstructor, inclusive: true): MonoTypeOperatorFunction<T>;
export function takeWhile<T>(predicate: BooleanConstructor, inclusive: false): OperatorFunction<T, TruthyTypesOf<T>>;
export function takeWhile<T>(predicate: BooleanConstructor): OperatorFunction<T, TruthyTypesOf<T>>;
export function takeWhile<T, S extends T>(predicate: (value: T, index: number) => value is S): OperatorFunction<T, S>;
export function takeWhile<T, S extends T>(predicate: (value: T, index: number) => value is S, inclusive: false): OperatorFunction<T, S>;
export function takeWhile<T>(predicate: (value: T, index: number) => boolean, inclusive?: boolean): MonoTypeOperatorFunction<T>;

/**
 * Emits values emitted by the source Observable so long as each value satisfies
 * the given `predicate`, and then completes as soon as this `predicate` is not
 * satisfied.
 *
 * <span class="informal">Takes values from the source only while they pass the
 * condition given. When the first value does not satisfy, it completes.</span>
 *
 * ![](takeWhile.png)
 *
 * `takeWhile` subscribes and begins mirroring the source Observable. Each value
 * emitted on the source is given to the `predicate` function which returns a
 * boolean, representing a condition to be satisfied by the source values. The
 * output Observable emits the source values until such time as the `predicate`
 * returns false, at which point `takeWhile` stops mirroring the source
 * Observable and completes the output Observable.
 *
 * ## Example
 *
 * Emit click events only while the clientX property is greater than 200
 *
 * ```ts
 * import { fromEvent, takeWhile } from 'rxjs';
 *
 * const clicks = fromEvent<PointerEvent>(document, 'click');
 * const result = clicks.pipe(takeWhile(ev => ev.clientX > 200));
 * result.subscribe(x => console.log(x));
 * ```
 *
 * @see {@link take}
 * @see {@link takeLast}
 * @see {@link takeUntil}
 * @see {@link skip}
 *
 * @param predicate A function that evaluates a value emitted by the source
 * Observable and returns a boolean. Also takes the (zero-based) index as the
 * second argument.
 * @param inclusive When set to `true` the value that caused `predicate` to
 * return `false` will also be emitted.
 * @return A function that returns an Observable that emits values from the
 * source Observable so long as each value satisfies the condition defined by
 * the `predicate`, then completes.
 */
export function takeWhile<T>(predicate: (value: T, index: number) => boolean, inclusive = false): MonoTypeOperatorFunction<T> {
  return operate((source, subscriber) => {
    let index = 0;
    source.subscribe(
      createOperatorSubscriber(subscriber, (value) => {
        const result = predicate(value, index++);
        (result || inclusive) && subscriber.next(value);
        !result && subscriber.complete();
      })
    );
  });
}