56 lines
2.5 KiB
JavaScript
56 lines
2.5 KiB
JavaScript
/** PURE_IMPORTS_START .._operators_first PURE_IMPORTS_END */
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import { first as higherOrder } from '../operators/first';
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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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* <img src="./img/first.png" width="100%">
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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 `resultSelector` function to produce the output value from
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* the input value, and a `defaultValue` to emit in case the source completes
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* before it is able to emit a valid value. Throws an error if `defaultValue`
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* was not provided and a matching element is not found.
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*
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* @example <caption>Emit only the first click that happens on the DOM</caption>
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* var clicks = Rx.Observable.fromEvent(document, 'click');
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* var result = clicks.first();
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* result.subscribe(x => console.log(x));
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*
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* @example <caption>Emits the first click that happens on a DIV</caption>
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* var clicks = Rx.Observable.fromEvent(document, 'click');
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* var result = clicks.first(ev => ev.target.tagName === 'DIV');
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* result.subscribe(x => console.log(x));
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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 {function(value: T, index: number): R} [resultSelector] A function to
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* produce the value on the output Observable based on the values
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* and the indices of the source Observable. The arguments passed to this
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* function are:
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* - `value`: the value that was emitted on the source.
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* - `index`: the "index" of the value from the source.
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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(predicate, resultSelector, defaultValue) {
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return higherOrder(predicate, resultSelector, defaultValue)(this);
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}
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//# sourceMappingURL=first.js.map
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