--- /dev/null
+import { Operator } from '../Operator';
+import { Subscriber } from '../Subscriber';
+import { Observable } from '../Observable';
+import { OperatorFunction, MonoTypeOperatorFunction, TeardownLogic } from '../types';
+
+/* tslint:disable:max-line-length */
+export function filter<T, S extends T>(predicate: (value: T, index: number) => value is S,
+ thisArg?: any): OperatorFunction<T, S>;
+export function filter<T>(predicate: (value: T, index: number) => boolean,
+ thisArg?: any): MonoTypeOperatorFunction<T>;
+/* tslint:enable:max-line-length */
+
+/**
+ * Filter items emitted by the source Observable by only emitting those that
+ * satisfy a specified predicate.
+ *
+ * <span class="informal">Like
+ * [Array.prototype.filter()](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/filter),
+ * it only emits a value from the source if it passes a criterion function.</span>
+ *
+ * ![](filter.png)
+ *
+ * Similar to the well-known `Array.prototype.filter` method, this operator
+ * takes values from the source Observable, passes them through a `predicate`
+ * function and only emits those values that yielded `true`.
+ *
+ * ## Example
+ * Emit only click events whose target was a DIV element
+ * ```ts
+ * import { fromEvent } from 'rxjs';
+ * import { filter } from 'rxjs/operators';
+ *
+ * const clicks = fromEvent(document, 'click');
+ * const clicksOnDivs = clicks.pipe(filter(ev => ev.target.tagName === 'DIV'));
+ * clicksOnDivs.subscribe(x => console.log(x));
+ * ```
+ *
+ * @see {@link distinct}
+ * @see {@link distinctUntilChanged}
+ * @see {@link distinctUntilKeyChanged}
+ * @see {@link ignoreElements}
+ * @see {@link partition}
+ * @see {@link skip}
+ *
+ * @param {function(value: T, index: number): boolean} predicate A function that
+ * evaluates each value emitted by the source Observable. If it returns `true`,
+ * the value is emitted, if `false` the value is not passed to the output
+ * Observable. The `index` parameter is the number `i` for the i-th source
+ * emission that has happened since the subscription, starting from the number
+ * `0`.
+ * @param {any} [thisArg] An optional argument to determine the value of `this`
+ * in the `predicate` function.
+ * @return {Observable} An Observable of values from the source that were
+ * allowed by the `predicate` function.
+ * @method filter
+ * @owner Observable
+ */
+export function filter<T>(predicate: (value: T, index: number) => boolean,
+ thisArg?: any): MonoTypeOperatorFunction<T> {
+ return function filterOperatorFunction(source: Observable<T>): Observable<T> {
+ return source.lift(new FilterOperator(predicate, thisArg));
+ };
+}
+
+class FilterOperator<T> implements Operator<T, T> {
+ constructor(private predicate: (value: T, index: number) => boolean,
+ private thisArg?: any) {
+ }
+
+ call(subscriber: Subscriber<T>, source: any): TeardownLogic {
+ return source.subscribe(new FilterSubscriber(subscriber, this.predicate, this.thisArg));
+ }
+}
+
+/**
+ * We need this JSDoc comment for affecting ESDoc.
+ * @ignore
+ * @extends {Ignored}
+ */
+class FilterSubscriber<T> extends Subscriber<T> {
+
+ count: number = 0;
+
+ constructor(destination: Subscriber<T>,
+ private predicate: (value: T, index: number) => boolean,
+ private thisArg: any) {
+ super(destination);
+ }
+
+ // the try catch block below is left specifically for
+ // optimization and perf reasons. a tryCatcher is not necessary here.
+ protected _next(value: T) {
+ let result: any;
+ try {
+ result = this.predicate.call(this.thisArg, value, this.count++);
+ } catch (err) {
+ this.destination.error(err);
+ return;
+ }
+ if (result) {
+ this.destination.next(value);
+ }
+ }
+}