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+import { Observable } from '../Observable';
+import { EmptyError } from '../util/EmptyError';
+import { OperatorFunction, TruthyTypesOf } from '../types';
+import { filter } from './filter';
+import { take } from './take';
+import { defaultIfEmpty } from './defaultIfEmpty';
+import { throwIfEmpty } from './throwIfEmpty';
+import { identity } from '../util/identity';
+
+export function first<T, D = T>(predicate?: null, defaultValue?: D): OperatorFunction<T, T | D>;
+export function first<T>(predicate: BooleanConstructor): OperatorFunction<T, TruthyTypesOf<T>>;
+export function first<T, D>(predicate: BooleanConstructor, defaultValue: D): OperatorFunction<T, TruthyTypesOf<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, S extends T, D>(
+  predicate: (value: T, index: number, source: Observable<T>) => value is S,
+  defaultValue: D
+): OperatorFunction<T, S | D>;
+export function first<T, D = T>(
+  predicate: (value: T, index: number, source: Observable<T>) => boolean,
+  defaultValue?: D
+): OperatorFunction<T, T | D>;
+
+/**
+ * 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>
+ *
+ * ![](first.png)
+ *
+ * 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. Emits an error
+ * notification 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, first } from 'rxjs';
+ *
+ * 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, first } from 'rxjs';
+ *
+ * const div = document.createElement('div');
+ * div.style.cssText = 'width: 200px; height: 200px; background: #09c;';
+ * document.body.appendChild(div);
+ *
+ * const clicks = fromEvent(document, 'click');
+ * const result = clicks.pipe(first(ev => (<HTMLElement>ev.target).tagName === 'DIV'));
+ * result.subscribe(x => console.log(x));
+ * ```
+ *
+ * @see {@link filter}
+ * @see {@link find}
+ * @see {@link take}
+ * @see {@link last}
+ *
+ * @throws {EmptyError} Delivers an `EmptyError` to the Observer's `error`
+ * callback if the Observable completes before any `next` notification was sent.
+ * This is how `first()` is different from `take(1)` which completes instead.
+ *
+ * @param predicate An optional function called with each item to test for condition
+ * matching.
+ * @param defaultValue The default value emitted in case no valid value was found on
+ * the source.
+ * @return A function that returns an Observable that emits the first item that
+ * matches the condition.
+ */
+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(defaultValue!) : throwIfEmpty(() => new EmptyError())
+    );
+}