Pipeable Operators

Starting in version 5.5 we have shipped "pipeable operators", which can be accessed in rxjs/operators (notice the pluralized "operators"). These are meant to be a better approach for pulling in just the operators you need than the "patch" operators found in rxjs-compat package.

NOTE: Using rxjs or rxjs/operators without making changes to your build process can result in larger bundles. See Known Issues section below.

Renamed Operators

Due to having operators available independent of an Observable, operator names cannot conflict with JavaScript keyword restrictions. Therefore the names of the pipeable version of some operators have changed. These operators are:

  1. do -> tap
  2. catch -> catchError
  3. switch -> switchAll
  4. finally -> finalize

The let operator is now part of Observable as pipe and cannot be imported.

source$.let(myOperator) -> source$.pipe(myOperator)

See "Build Your Own Operators" below.

The former toPromise() "operator" has been removed because an operator returns an Observable, not a Promise. There is now an Observable.toPromise()instance method.


Problems with the patched operators for dot-chaining are:

  1. Any library that imports a patch operator will augment the Observable.prototype for all consumers of that library, creating blind dependencies. If the library removes their usage, they unknowingly break everyone else. With pipeables, you have to import the operators you need into each file you use them in.

  2. Operators patched directly onto the prototype are not "tree-shakeable" by tools like rollup or webpack. Pipeable operators will be as they are just functions pulled in from modules directly.

  3. Unused operators that are being imported in apps cannot be detected reliably by any sort of build tool or lint rule. That means that you might import scan, but stop using it, and it's still being added to your output bundle. With pipeable operators, if you're not using it, a lint rule can pick it up for you.

  4. Functional composition is awesome. Building your own custom operators becomes much easier, and now they work and look just like all other operators in rxjs. You don't need to extend Observable or override lift anymore.


What is a pipeable operator? Simply put, a function that can be used with the current let operator. It used to be the origin of the name ("lettable"), but that was confusing so we now call them "pipeable" because they're intended to be used with the pipe utility. A pipeable operator is basically any function that returns a function with the signature: <T, R>(source: Observable<T>) => Observable<R>.

There is a pipe method built into Observable now at Observable.prototype.pipe that сan be used to compose the operators in similar manner to what you're used to with dot-chaining (shown below).

There is also a pipe utility function that can be imported from import { pipe } from 'rxjs';. The pipe function can be used to build reusable pipeable operators from other pipeable operators. For example:

import { pipe } from 'rxjs';
import { map } from 'rxjs/operators';

const mapTwice = <T,R>(fn: (value: T, index: number) => R) => pipe(map(fn), map(fn));


You pull in any operator you need from one spot, under 'rxjs/operators' (plural!). It's also recommended to pull in the Observable creation methods you need directly as shown below with range:

import { range } from 'rxjs';
import { map, filter, scan } from 'rxjs/operators';

const source$ = range(0, 10);

  filter(x => x % 2 === 0),
  map(x => x + x),
  scan((acc, x) => acc + x, 0)
.subscribe(x => console.log(x))

// Logs:
// 0
// 4
// 12
// 24
// 40

Build Your Own Operators Easily

You, in fact, could always do this with let... but building your own operator is now as simple as writing a function. Notice, that you can compose your custom operator in with other rxjs operators seamlessly.

import { Observable, interval } from 'rxjs';
import { filter, map, take, toArray } from 'rxjs/operators';

 * an operator that takes every Nth value
const takeEveryNth = (n: number) => <T>(source: Observable<T>) =>
  new Observable<T>(observer => {
    let count = 0;
    return source.subscribe({
      next(x) {
        if (count++ % n === 0) observer.next(x);
      error(err) { observer.error(err); },
      complete() { observer.complete(); }

 * You can also use an existing operator like so
const takeEveryNthSimple = (n: number) => <T>(source: Observable<T>) =>
  source.pipe(filter((value, index) => index % n === 0 ))

 * And since pipeable operators return functions, you can further simplify like so
const takeEveryNthSimplest = (n: number) => filter((value, index) => index % n === 0);

  map(x => x + x),
  map(x => x * x),
.subscribe(x => console.log(x));
// Logs:
// [0, 2304, 9216]

Known Issues

TypeScript < 2.4

In TypeScript 2.3 and lower, typings will need to be added to functions passed to operators, as types cannot be inferred prior to TypeScript 2.4. In TypeScript 2.4, types will infer via composition properly.

TS 2.3 and under

range(0, 10).pipe(
  map((n: number) => n + '!'),
  map((s: string) => 'Hello, ' + s),
).subscribe(x => console.log(x))

TS 2.4 and up

range(0, 10).pipe(
  map(n => n + '!'),
  map(s => 'Hello, ' + s),
).subscribe(x => console.log(x))

Build and Treeshaking

When importing from a manifest (or re-export) file, an application bundle can sometimes grow. Pipeable operators can now be imported from rxjs/operators, but doing so without changing your build process will often result in a larger application bundle. This is because by default rxjs/operators will resolve to the CommonJS output of rxjs.

In order to use the new pipeable operators and not gain bundle size, you will need to change your Webpack configuration. This will only work with Webpack 3+ as it relies on the new ModuleConcatenationPlugin from Webpack 3.


Published along with rxjs 5.5 is builds of rxjs in ECMAScript Module format (imports and exports) with both ES5 and ES2015 language level. You can find these distributions in node_modules/rxjs/_esm5 and node_modules/rxjs/_esm2015 ("esm" stands for ECMAScript Modules and the number "5" or "2015" is for the ES language level). In your application source code, you should import from rxjs/operators, but in your Webpack configuration file you will need to re-map imports to the ESM5 (or ESM2015) version.

If you require('rxjs/_esm5/path-mapping'), you will receive a function that returns an object of key-value pairs mapping each input to it's file location on disk. Utilize this mapping as follows:


Simple configuration:

const rxPaths = require('rxjs/_esm5/path-mapping');
const webpack = require('webpack');
const path = require('path');

module.exports = {
  entry: 'index.js',
  output: 'bundle.js',
  resolve: {
    // Use the "alias" key to resolve to an ESM distribution
    alias: rxPaths()
  plugins: [
    new webpack.optimize.ModuleConcatenationPlugin()

More complete configuration (closer to a real-world scenario):

const webpack = require('webpack');
const path = require('path');
const HtmlWebpackPlugin = require('html-webpack-plugin');
const DashboardPlugin = require('webpack-dashboard/plugin');
const nodeEnv = process.env.NODE_ENV || 'development';
const isProd = nodeEnv === 'production';
const rxPaths = require('rxjs/_esm5/path-mapping');

var config = {
    devtool: isProd ? 'hidden-source-map' : 'cheap-eval-source-map',
    context: path.resolve('./src'),
    entry: {
        app: './index.ts',
        vendor: './vendor.ts'
    output: {
        path: path.resolve('./dist'),
        filename: '[name].bundle.js',
        sourceMapFilename: '[name].map',
        devtoolModuleFilenameTemplate: function (info) {
            return "file:///" + info.absoluteResourcePath;
    module: {
        rules: [
            { enforce: 'pre', test: /\.ts$|\.tsx$/, exclude: ["node_modules"], loader: 'ts-loader' },
            { test: /\.html$/, loader: "html" },
            { test: /\.css$/, loaders: ['style', 'css'] }
    resolve: {
        extensions: [".ts", ".js"],
        modules: [path.resolve('./src'), 'node_modules'],
        alias: rxPaths()
    plugins: [
        new webpack.DefinePlugin({
            'process.env': { // eslint-disable-line quote-props
                NODE_ENV: JSON.stringify(nodeEnv)
        new webpack.HashedModuleIdsPlugin(),
        new webpack.optimize.ModuleConcatenationPlugin(),
        new HtmlWebpackPlugin({
            title: 'Typescript Webpack Starter',
            template: '!!ejs-loader!src/index.html'
        new webpack.optimize.CommonsChunkPlugin({
            name: 'vendor',
            minChunks: Infinity,
            filename: 'vendor.bundle.js'
        new webpack.optimize.UglifyJsPlugin({
            mangle: false,
            compress: { warnings: false, pure_getters: true, passes: 3, screw_ie8: true, sequences: false },
            output: { comments: false, beautify: true },
            sourceMap: false
        new DashboardPlugin(),
        new webpack.LoaderOptionsPlugin({
            options: {
                tslint: {
                    emitErrors: true,
                    failOnHint: true

module.exports = config;

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Code licensed under an Apache-2.0 License. Documentation licensed under CC BY 4.0.