first.ts
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import { Observable } from '../Observable';
import { Operator } from '../Operator';
import { Subscriber } from '../Subscriber';
import { EmptyError } from '../util/EmptyError';
import { OperatorFunction } from '../../internal/types';
import { filter } from './filter';
import { take } from './take';
import { defaultIfEmpty } from './defaultIfEmpty';
import { throwIfEmpty } from './throwIfEmpty';
import { identity } from '../util/identity';
/* tslint:disable:max-line-length */
export function first<T, D = T>(
predicate?: null,
defaultValue?: D
): OperatorFunction<T, T | D>;
export function first<T, S extends T>(
predicate: (value: T, index: number, source: Observable<T>) => value is S,
defaultValue?: S
): OperatorFunction<T, S>;
export function first<T, D = T>(
predicate: (value: T, index: number, source: Observable<T>) => boolean,
defaultValue?: D
): OperatorFunction<T, T | D>;
/* tslint:enable:max-line-length */
/**
* Emits only the first value (or the first value that meets some condition)
* emitted by the source Observable.
*
* <span class="informal">Emits only the first value. Or emits only the first
* value that passes some test.</span>
*
* 
*
* If called with no arguments, `first` emits the first value of the source
* Observable, then completes. If called with a `predicate` function, `first`
* emits the first value of the source that matches the specified condition. It
* may also take a deprecated `resultSelector` function to produce the output
* value from the input value, and a `defaultValue` to emit in case the source
* completes before it is able to emit a valid value. Throws an error if
* `defaultValue` was not provided and a matching element is not found.
*
* ## Examples
* Emit only the first click that happens on the DOM
* ```ts
* import { fromEvent } from 'rxjs';
* import { first } from 'rxjs/operators';
*
* const clicks = fromEvent(document, 'click');
* const result = clicks.pipe(first());
* result.subscribe(x => console.log(x));
* ```
*
* Emits the first click that happens on a DIV
* ```ts
* import { fromEvent } from 'rxjs';
* import { first } from 'rxjs/operators';
*
* const clicks = fromEvent(document, 'click');
* const result = clicks.pipe(first(ev => ev.target.tagName === 'DIV'));
* result.subscribe(x => console.log(x));
* ```
*
* @see {@link filter}
* @see {@link find}
* @see {@link take}
*
* @throws {EmptyError} Delivers an EmptyError to the Observer's `error`
* callback if the Observable completes before any `next` notification was sent.
*
* @param {function(value: T, index: number, source: Observable<T>): boolean} [predicate]
* An optional function called with each item to test for condition matching.
* @param {R} [defaultValue] The default value emitted in case no valid value
* was found on the source.
* @return {Observable<T|R>} An Observable of the first item that matches the
* condition.
* @method first
* @owner Observable
*/
export function first<T, D>(
predicate?: ((value: T, index: number, source: Observable<T>) => boolean) | null,
defaultValue?: D
): OperatorFunction<T, T | D> {
const hasDefaultValue = arguments.length >= 2;
return (source: Observable<T>) => source.pipe(
predicate ? filter((v, i) => predicate(v, i, source)) : identity,
take(1),
hasDefaultValue ? defaultIfEmpty<T | D>(defaultValue) : throwIfEmpty(() => new EmptyError()),
);
}