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- import { OperatorFunction } from '../types';
- import { reduce } from './reduce';
- /**
- * Counts the number of emissions on the source and emits that number when the
- * source completes.
- *
- * <span class="informal">Tells how many values were emitted, when the source
- * completes.</span>
- *
- * 
- *
- * `count` transforms an Observable that emits values into an Observable that
- * emits a single value that represents the number of values emitted by the
- * source Observable. If the source Observable terminates with an error, `count`
- * will pass this error notification along without emitting a value first. If
- * the source Observable does not terminate at all, `count` will neither emit
- * a value nor terminate. This operator takes an optional `predicate` function
- * as argument, in which case the output emission will represent the number of
- * source values that matched `true` with the `predicate`.
- *
- * ## Examples
- *
- * Counts how many seconds have passed before the first click happened
- *
- * ```ts
- * import { interval, fromEvent, takeUntil, count } from 'rxjs';
- *
- * const seconds = interval(1000);
- * const clicks = fromEvent(document, 'click');
- * const secondsBeforeClick = seconds.pipe(takeUntil(clicks));
- * const result = secondsBeforeClick.pipe(count());
- * result.subscribe(x => console.log(x));
- * ```
- *
- * Counts how many odd numbers are there between 1 and 7
- *
- * ```ts
- * import { range, count } from 'rxjs';
- *
- * const numbers = range(1, 7);
- * const result = numbers.pipe(count(i => i % 2 === 1));
- * result.subscribe(x => console.log(x));
- * // Results in:
- * // 4
- * ```
- *
- * @see {@link max}
- * @see {@link min}
- * @see {@link reduce}
- *
- * @param predicate A function that is used to analyze the value and the index and
- * determine whether or not to increment the count. Return `true` to increment the count,
- * and return `false` to keep the count the same.
- * If the predicate is not provided, every value will be counted.
- * @return A function that returns an Observable that emits one number that
- * represents the count of emissions.
- */
- export function count<T>(predicate?: (value: T, index: number) => boolean): OperatorFunction<T, number> {
- return reduce((total, value, i) => (!predicate || predicate(value, i) ? total + 1 : total), 0);
- }
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