# ES dev server

A web server for development without bundling, utilizing the browser's standard module loader and efficient browser caching for simple and fast web development.

npx es-dev-server --node-resolve --watch

Quick overview

See all commands

# Getting started

We recommend following this guide for a step by step overview of different workflows with es-dev-server.

# Setup

With our project scaffolding you can set up a pre-configured project:

npm init @open-wc

# Manual

You can also set up the dev server manually:

npm i -D es-dev-server

Add scripts to your package.json, modify the flags as needed:

  "scripts": {
    "start": "es-dev-server --app-index index.html --node-resolve --watch --open"

Run the server:

npm run start

# Node version

es-dev-server requires node v10 or higher

# Command line flags and Configuration

# Server configuration

name type description
port number The port to use, uses a random free port if not set.
hostname string The hostname to use. Default: localhost
open boolean/string Opens the browser on app-index, root dir or a custom path
app-index string The app's index.html file, sets up history API fallback for SPA routing
root-dir string The root directory to serve files from. Default: working directory
base-path string Base path the app is served on. Example: /my-app
config string The file to read configuration from (JS or JSON)
help none See all options

# Development help

name type description
watch boolean Reload the browser when files are edited
http2 boolean Serve files over HTTP2. Sets up HTTPS with self-signed certificates

# Code transformation

name type description
compatibility string Compatibility mode for older browsers. Can be: auto, always, min, max or none Default auto
node-resolve boolean Resolve bare import imports using node resolve
dedupe boolean/array Deduplicates all modules, or modules from specified packages if the value is an array
preserve-symlinks boolean Preserve symlinks when resolving modules. Set to true, if using tools that rely on symlinks, such as npm link. Default false.
module-dirs string/array Directories to resolve modules from. Used by node-resolve
babel boolean Transform served code through babel. Requires .babelrc
file-extensions string/array Extra file extensions to use when transforming code.
babel-exclude number/array Patterns of files to exclude from babel compilation.
babel-modern-exclude number/array Patterns of files to exclude from babel compilation on modern browsers.
babel-module-exclude number/array Patterns of files to exclude from babel compilation for modules only.

Most commands have an alias/shorthand. You can view them by using --help.

# Configuration files

We pick up an es-dev-server.config.js file automatically if it is present in the current working directory. You can specify a custom config path using the config flag.

Configuration options are the same as command line flags, using their camelCased names. Example:

module.exports = {
  port: 8080,
  watch: true,
  nodeResolve: true,
  appIndex: 'demo/index.html',
  moduleDirs: ['node_modules', 'web_modules'],

In addition to the command-line flags, the configuration file accepts these additional options:

name type description
middlewares array Koa middlewares to add to the server, read more below.
responseTransformers array Functions which transform the server's response.
babelConfig object Babel config to run with the server.
polyfillsLoader object Configuration for the polyfills loader, read more below.
debug boolean Whether to turn on debug mode on the server.
onServerStart (config) => Promise Function called before server is started.

# Node resolve

"Bare imports" are imports which don't specify a full path to a file:

import foo from 'bar';

The browser doesn't know where to find this file called bar. The --node-resolve flag resolves this bare import to the actual file path before serving it to the browser:

import foo from './node_modules/bar/bar.js';

Because we use es-module-lexer for blazing fast analysis to find the imports in a file without booting up a full-blown parser like babel, we can do this without noticeable impact on performance.

For the actual resolve logic, we internally use @rollup/plugin-node-resolve so that you can keep the resolve logic in sync between development and production. When using a config file, the nodeResolve can also be an object which accepts the same options as the rollup plugin. options.

Example config

See the rollup docs for all options and what they do.

Some options like dedupe, fileExtensions, preserveSymlinks and moduleDirs are mapped to options for nodeResolve internally. You can overwrite them with your custom config.

module.exports = {
  nodeResolve: {
    jsnext: true,
    browser: true,
    // set default to false because es-dev-server always
    // runs in the browser
    preferBuiltins: true,
    // will overwrite es-dev-server's fileExtensions option
    extensions: ['.mjs', '.js'],
    // will overwrite es-dev-server's dedupe option
    dedupe: ['lit-html'],
    customResolveOptions: {
      // will overwrite es-dev-server's moduleDirs option
      moduleDirectory: ['node_modules'],
      preserveSymlinks: true,

In future, we are hoping that import maps will make this step unnecessary.

# Dedupe

When your dependencies depend on different versions of the same package, package managers like yarn or npm may end up installing multiple versions of the same package with nested node_modules directories. This can create problems when modules expect to be singletons on the page, or when it includes side-effects such as custom element registration which can only be run once.

You can use the dedupe option to ensure a particular package is only loaded once by resolving from the root of the package instead of relative to the originating module.

When dedupe is a boolean, all packages are deduplicated:

es-dev-server --dedupe

When it is an array, only the specified packages are deduplicated. An array can only be set from a config:

module.exports = {
  dedupe: ['foo', 'bar'],

# Folder structure

es-dev-server serves static files using the same structure as your file system. It cannot serve any files outside of the root of the webserver. You need to make sure any files requested, including node modules, are accessible for the webserver.

Outside of that one requirement, however, es-dev-server does not have any opinions on how you should scaffold your project. The following are examples of a variety of different suggested strategies for setting up your project's folder structure.

# index.html in the Root

The simplest setup, making sure that all files are accessible, is to place your index.html at the root of your project

Consider this example directory structure in the web root:


If you run the es-dev-server command from the root of the project, you can access your app at / or /index.html in the browser.

# index.html in a Subfolder

If you move your index.html inside a subfolder:

Use the `--open` parameter for when you'd like to keep you index.html in a subfolder.

You can access your app in the browser at /src/ or /src/index.html. You can tell es-dev-server to explicitly open at this path:

# with app-index flag
es-dev-server --app-index src/index.html --open
# without app-index flag
es-dev-server --open src/

You can also change the root directory of the dev server:

es-dev-server --root-dir src --open

Now your index.html is accessible at / or /index.html. However, the dev server cannot serve any files outside of the root directory. So if your app uses any node modules, they will no longer because accessible.

If you want your index in a subfolder without this being visible in the browser URL, you can set up a file rewrite rule. Read more here

# Monorepos

Use `--app-index` or `--root-dir` when your index.html and web root are in different places, e.g.. in a monorepo setup.

If you are using es-dev-server in a monorepo, your node modules are in two different locations. In the package's folder and the repository root:


You will need to make sure the root node_modules folder is accessible to the dev server.

If your working directory is packages/my-package you can use this command:

# with app-index
es-dev-server --root-dir ../../ --app-index packages/my-package/index.html --open
# without app-index
es-dev-server --root-dir ../../ --open packages/my-package/index.html

If your working directory is the root of the repository you can use this command:

es-dev-server --app-index packages/my-package/index.html --open

This is the same approach as serving an index.html in a subdirectory, so the section above applies here as well.

# Base Element

Use platform features to specify your web root, e.g. in SPAs

You can set up a <base href=""> element to modify how files are resolved relatively to your index.html. This can be very useful when your index.html is not at the root of your project.

If you use SPA routing, using a base element is highly recommended. Read more

# Order of execution

The order of execution for the es-dev-server is:

  1. Custom middlewares
  2. Es-dev-server static file middleware
  3. Response transformers
  4. Es-dev-server code transform middlewares
  5. Es-dev-server response cache (it also caches the code transformations!)
  6. Deferred custom middlewares

Take this into account when deciding between response transformers, custom middlewares and whether or not you defer your custom middleware.

For example, a deferred custom middleware may be necessary if you need to do something with the response body after caching.

async function myMiddleware(ctx, next) {
  ctx.url = ctx.url.replace('foo', 'bar');
  // before es-dev-server
  await next();
  // deferred, after es-dev-server
  ctx.body = ctx.body.replace('foo', 'bar');

# Custom middlewares / proxy

You can install custom middlewares, using the middlewares property.

Read more

The middleware should be a standard koa middleware. Read more about koa here.

You can use custom middlewares to set up a proxy, for example:

const proxy = require('koa-proxies');

module.exports = {
  port: 9000,
  middlewares: [
    proxy('/api', {
      target: 'http://localhost:9001',

# Rewriting request urls

You can rewrite certain file requests using a simple custom middleware. This can be useful for example to serve your index.html from a different file location or to alias a module.

Read more

Serve /index.html from /src/index.html:

module.exports = {
  middlewares: [
    function rewriteIndex(context, next) {
      if (context.url === '/' || context.url === '/index.html') {
        context.url = '/src/index.html';

      return next();

# Response transformers

With the responseTransformers property, you can transform the server's response before it is sent to the browser. This is useful for injecting code into your index.html, performing transformations on files or to serve virtual files programmatically.

Read more

A response transformer is a function which receives the original response and returns an optionally modified response. This transformation happens before any other built-in transformations such as node resolve, babel or compatibility. You can register multiple transformers, they are called in order.

The functions can be sync or async, see the full signature below:

({ url: string, status: number, contentType: string, body: string }) => Promise<{ body?: string, contentType?: string } | null>

Some examples:

Rewrite the base path of your index.html:

module.exports = {
  responseTransformers: [
    function rewriteBasePath({ url, status, contentType, body }) {
      if (url === '/' || url === '/index.html') {
        const rewritten = body.replace(/<base href=".*">/, '<base href="/foo/">');
        return { body: rewritten };

Serve a virtual file, for example an auto generated index.html:

const indexHTML = generateIndexHTML();

module.exports = {
  responseTransformers: [
    function serveIndex({ url, status, contentType, body }) {
      if (url === '/' || url === '/index.html') {
        return { body: indexHTML, contentType: 'text/html' };

Transform markdown to HTML:

const markdownToHTML = require('markdown-to-html-library');

module.exports = {
  responseTransformers: [
    async function transformMarkdown({ url, status, contentType, body }) {
      if (url === '/readme.md') {
        const html = await markdownToHTML(body);
        return {
          body: html,
          contentType: 'text/html',

Polyfill CSS modules in JS:

module.exports = {
  responseTransformers: [
    async function transformCSS({ url, status, contentType, body }) {
      if (url.endsWith('.css')) {
        const transformedBody = `
          const stylesheet = new CSSStyleSheet();
          export default stylesheet;
        return { body: transformedBody, contentType: 'application/javascript' };

# Typescript support

es-dev-server is based around developing without any build tools but you can make it work with typescript as well.

Read more

The easiest way to use the server with typescript is to compile your typescript to javascript before running the server. Just run tsc in watch mode and include the compiled js files from your index.html.

You can also configure the dev server to consume your typescript files directly. This is done by running the server with a babel plugin to compile your typescript files to javascript.

Note that when compiling typescript with babel it does not do any type checking or special typescript compilation such as decorators, class fields and enums. You can configure babel to cover most of these, but not all. Read more about babel typescript here.

  1. Install the preset:
npm i --save-dev @babel/preset-typescript
  1. Add a babel.config.js or .babelrc to your project:
  "presets": ["@babel/preset-typescript"]
  1. Import a typescript file from your index.html

    <script type="module" src="./my-app.ts"></script>
  1. Run es-dev-server with these flags:
es-dev-server --file-extensions .ts --node-resolve --babel --open

To add support for experimental features that are normally handled by the typescript compiler, you can add extra babel plugins:

  1. Install the plugins:
npm i --save-dev @babel/plugin-proposal-decorators @babel/plugin-proposal-class-properties
  1. Update your babel configuration:
  "presets": ["@babel/preset-typescript"],
  // for libraries that support babel decorators (lit-element) use:
  "plugins": [
    ["@babel/plugin-proposal-decorators", { "decoratorsBeforeExport": true }],
  // for libraries that only support typescript:
  // "plugins": [
  //   ["@babel/plugin-proposal-decorators", { "legacy": true }],
  //   ["@babel/plugin-proposal-class-properties", { "loose": true }]
  // ],

# Compatibility mode

Compatibility mode enables bundle-free development using modern browsers features on older browsers. Automatic compatibility mode is enabled by default.

Read more

Compatibility mode can be configured using the --compatibility flag. The possible options are: auto, min, max and none. The default is mode is auto.

auto auto compatibility looks at the current browser to determine the level of compatibility to enable. On the latest 2 versions of the major browsers, it doesn't do any work. This keeps the server as fast as possible in the general case.

On older browsers, the server uses the browser's user agent and @babel/preset-env to do a targeted transformation for that specific browser and version. @babel/preset-env only works with with stage 4 javascript features, they should become an official standard before they can be used.

If the browser does not support es module scripts, dynamic imports or import.meta.url es modules are transformed to system-js.

This works down to at least IE11. Depending on what browser features you are using, it might work with earlier version too but this is not tested.

always always compatibility is the same as auto, except that it doesn't skip compiling on the latest 2 versions of the major browsers. This makes it a bit slower on modern browsers, but allows you to use new features before they are implemented in the browser.

min min compatibility forces the same level of compatibility on all browsers. It makes code compatible with the latest two versions of the major browsers, and does not transform es modules.

max max compatibility forces the same level of compatibility on all browsers. It compiles everything to es5 and system-js.

none none disables compatibility mode entirely.

# Polyfills loader

When compatibility mode is enabled, polyfills are loaded using polyfills-loader.

Read more

You can customize the polyfill loader configuration from your configuration file. Check the docs for the polyfills-loader for all possible options.

module.exports = {
  polyfillsLoader: {
    polyfills: {
      fetch: false,
      custom: [
          name: 'my-feature-polyfill',
          path: require.resolve('my-feature-polyfill'),
          test: "!('myFeature' in window)",

By default, es-dev-server wraps all scripts and are deferred until polyfills are loaded. Loading order of scripts are preserved, but this can create problems if you rely on a script being executed before HTML is parsed. You can configure es-dev-server to exclude certain types of scripts:

module.exports = {
  polyfillsLoader: {
    exclude: {
      jsModules: true,
      inlineJsModules: true,
      jsScripts: true,
      inlineJsScripts: true,

# Using es-dev-server programmatically

You can use different components from es-dev-server as a library and integrate it with other tools:

Read more

# createConfig

When using the server from javascript you are going to need a config object to tell the server what options to turn on and off. It's best to use createConfig for this as this converts the public API to an internal config structure and sets up default values.

By default, all options besides static file serving are turned off, so it's easy to configure based on your requirements.

The config structure is the same as the configuration explained in the configuration files section

import { createConfig } from 'es-dev-server';

const config = createConfig({
  http2: true,
  babel: true,
  open: true,

# createMiddlewares

createMiddlewares creates the dev server's middlewares based on your configuration. You can use this to hook them up to your koa server.

Returns an array of koa middleware functions.

import Koa from 'koa';
import { createConfig, createMiddlewares } from 'es-dev-server';

const config = createConfig({});
const middlewares = createMiddlewares(config);

const app = new Koa();
middlewares.forEach(middleware => {

# createServer

createServer creates an instance of the dev server including all middlewares, but without starting the server. This is useful if you want to be in control of starting the server yourself.

Returns the koa app and a node http or http2 server.

import Koa from 'koa';
import { createConfig, createServer } from 'es-dev-server';

const config = createConfig({ ... });
const { app, server } = createServer(config);

# watch mode

createMiddlewares and createServer requires a chokidar fileWatcher if watch mode is enabled. You need to pass this separately because the watcher needs to be killed explicitly when the server closes.

import Koa from 'koa';
import chokidar from 'chokidar';
import { createConfig, createMiddlewares, createServer } from 'es-dev-server';

const config = createConfig({ ... });
const fileWatcher = chokidar.watch([]);

// if using createMiddlewares
createMiddlewares(config, fileWatcher);
// if using createServer
createServer(config, fileWatcher);

// close filewatcher when no longer necessary

# startServer

startServer creates and starts the server, listening on the configured port. It opens the browser if configured and logs a startup message.

Returns the koa app and a node http or http2 server.

import Koa from 'koa';
import { createConfig, startServer } from 'es-dev-server';

const config = createConfig({ ... });
const { app, server } = startServer(config, fileWatcher);