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