[![npm][npm]][npm-url]
[![node][node]][node-url]
[![tests][tests]][tests-url]
[![coverage][cover]][cover-url]
[![discussion][discussion]][discussion-url]
[![size][size]][size-url]

webpack-dev-middleware

An express-style development middleware for use with webpack
bundles and allows for serving of the files emitted from webpack.
This should be used for development only.

Some of the benefits of using this middleware include:

- No files are written to disk, rather it handles files in memory
- If files changed in watch mode, the middleware delays requests until compiling
has completed.
- Supports hot module reload (HMR).

Getting Started

First thing's first, install the module:

``console
npm install webpack-dev-middleware --save-dev
`

> [!WARNING]
>
> _We do not recommend installing this module globally._

Usage

`js
const express = require("express");
const webpack = require("webpack");
const middleware = require("webpack-dev-middleware");

const compiler = webpack({
// webpack options
});

const app = express();

app.use(
middleware(compiler, {
// webpack-dev-middleware options
}),
);

app.listen(3000, () => console.log("Example app listening on port 3000!"));
`

See below for an example of use with fastify.

Options

| Name | Type | Default | Description |
| :---------------------------------------------: | :-------------------------------: | :-------------------------------------------: | :------------------------------------------------------------------------------------------------------------------- |
| methods | Array | [ 'GET', 'HEAD' ] | Allows to pass the list of HTTP request methods accepted by the middleware |
| headers | Array\|Object\|Function | undefined | Allows to pass custom HTTP headers on each request. |
| index | boolean\|string | index.html | If false (but not undefined), the server will not respond to requests to the root URL. |
| mimeTypes | Object | undefined | Allows to register custom mime types or extension mappings. |
| mimeTypeDefault | string | undefined | Allows to register a default mime type when we can't determine the content type. |
| etag | boolean\| "weak"\| "strong" | undefined | Enable or disable etag generation. |
| lastModified | boolean | undefined | Enable or disable Last-Modified header. Uses the file system's last modified value. |
| cacheControl | boolean\|number\|string\|Object | undefined | Enable or disable setting Cache-Control response header. |
| cacheImmutable | boolean\ | undefined | Enable or disable setting Cache-Control: public, max-age=31536000, immutable response header for immutable assets. |
| publicPath | string | undefined | The public path that the middleware is bound to. |
| stats | boolean\|string\|Object | stats (from a configuration) | Stats options object or preset name. |
| serverSideRender | boolean | undefined | Instructs the module to enable or disable the server-side rendering mode. |
| writeToDisk | boolean\|Function | false | Instructs the module to write files to the configured location on disk as specified in your webpack configuration. |
| outputFileSystem | Object | memfs | Set the default file system which will be used by webpack as primary destination of generated files. |
| modifyResponseData | Function | undefined | Allows to set up a callback to change the response data. |

The middleware accepts an options Object. The following is a property reference for the Object.

$3

Type: Array
Default: [ 'GET', 'HEAD' ]

This property allows a user to pass the list of HTTP request methods accepted by the middleware\\.

$3

Type: Array|Object|Function
Default: undefined

This property allows a user to pass custom HTTP headers on each request.
eg. { "X-Custom-Header": "yes" }

or

`js
webpackDevMiddleware(compiler, {
headers: () => ({
"Last-Modified": new Date(),
}),
});
`

or

`js
webpackDevMiddleware(compiler, {
headers: (req, res, context) => {
res.setHeader("Last-Modified", new Date());
},
});
`

or

`js
webpackDevMiddleware(compiler, {
headers: [
{
key: "X-custom-header",
value: "foo",
},
{
key: "Y-custom-header",
value: "bar",
},
],
});
`

or

`js
webpackDevMiddleware(compiler, {
headers: () => [
{
key: "X-custom-header",
value: "foo",
},
{
key: "Y-custom-header",
value: "bar",
},
],
});
`

$3

Type: Boolean|String
Default: index.html

If false (but not undefined), the server will not respond to requests to the root URL.

$3

Type: Object
Default: undefined

This property allows a user to register custom mime types or extension mappings.
eg. mimeTypes: { phtml: 'text/html' }.

Please see the documentation for mime-types for more information.

$3

Type: String
Default: undefined

This property allows a user to register a default mime type when we can't determine the content type.

$3

Type: "weak" | "strong"
Default: undefined

Enable or disable etag generation. Boolean value use

$3

Type: Boolean
Default: undefined

Enable or disable Last-Modified header. Uses the file system's last modified value.

$3

Type: Boolean | Number | String | { maxAge?: number, immutable?: boolean }
Default: undefined

Depending on the setting, the following headers will be generated:

- Boolean - Cache-Control: public, max-age=31536000000
- Number - Cache-Control: public, max-age=YOUR_NUMBER
- String - Cache-Control: YOUR_STRING
- { maxAge?: number, immutable?: boolean } - Cache-Control: public, max-age=YOUR_MAX_AGE_or_31536000000, also , immutable can be added if you set the immutable option to true

Enable or disable setting Cache-Control response header.

$3

Type: Boolean
Default: undefined

Enable or disable setting Cache-Control: public, max-age=31536000, immutable response header for immutable assets (i.e. asset with a hash like image.a4c12bde.jpg).
Immutable assets are assets that have their hash in the file name therefore they can be cached, because if you change their contents the file name will be changed.
Take preference over the cacheControl option if the asset was defined as immutable.

$3

Type: String
Default: output.publicPath (from a configuration)

The public path that the middleware is bound to.

_Best Practice: use the same publicPath defined in your webpack config. For more information about publicPath, please see the webpack documentation._

$3

Type: Boolean|String|Object
Default: stats (from a configuration)

Stats options object or preset name.

$3

Type: Boolean
Default: undefined

Instructs the module to enable or disable the server-side rendering mode.
Please see Server-Side Rendering for more information.

$3

Type: Boolean|Function
Default: false

If true, the option will instruct the module to write files to the configured location on disk as specified in your webpack config file.
_Setting writeToDisk: true won't change the behavior of the webpack-dev-middleware, and bundle files accessed through the browser will still be served from memory._
This option provides the same capabilities as the WriteFilePlugin.

This option also accepts a Function value, which can be used to filter which files are written to disk.
The function follows the same premise as Array#filter in which a return value of false _will not_ write the file, and a return value of true _will_ write the file to disk. eg.

`js
const webpack = require("webpack");

const configuration = {
/ Webpack configuration /
};
const compiler = webpack(configuration);

middleware(compiler, {
writeToDisk: (filePath) => /superman\.css$/.test(filePath),
});
`

$3

Type: Object
Default: memfs

Set the default file system which will be used by webpack as primary destination of generated files.
This option isn't affected by the writeToDisk option.

You have to provide .join() and mkdirp method to the outputFileSystem instance manually for compatibility with webpack@4.

This can be done simply by using path.join:

`js
const path = require("node:path");
const mkdirp = require("mkdirp");
const myOutputFileSystem = require("my-fs");
const webpack = require("webpack");

myOutputFileSystem.join = path.join.bind(path); // no need to bind
myOutputFileSystem.mkdirp = mkdirp.bind(mkdirp); // no need to bind

const compiler = webpack({
/ Webpack configuration /
});

middleware(compiler, { outputFileSystem: myOutputFileSystem });
`

$3

Allows to set up a callback to change the response data.

`js
const webpack = require("webpack");

const configuration = {
/ Webpack configuration /
};
const compiler = webpack(configuration);

middleware(compiler, {
// Note - if you send the Range header you will have ReadStream
// Also data can be string or Buffer
modifyResponseData: (req, res, data, byteLength) =>
// Your logic
// Don't use res.end() or res.send() here
({ data, byteLength }),
});
`

API

webpack-dev-middleware also provides convenience methods that can be use to
interact with the middleware at runtime:

$3

Instructs webpack-dev-middleware instance to stop watching for file changes.

#### Parameters

##### callback

Type: Function
Required: No

A function executed once the middleware has stopped watching.

`js
const express = require("express");
const webpack = require("webpack");

const compiler = webpack({
/ Webpack configuration /
});

const middleware = require("webpack-dev-middleware");

const instance = middleware(compiler);

// eslint-disable-next-line new-cap
const app = new express();

app.use(instance);

setTimeout(() => {
// Says webpack to stop watch changes
instance.close();
}, 1000);
`

$3

Instructs webpack-dev-middleware instance to recompile the bundle, e.g. after a change to the configuration.

#### Parameters

##### callback

Type: Function
Required: No

A function executed once the middleware has invalidated.

`js
const express = require("express");
const webpack = require("webpack");

const compiler = webpack({
/ Webpack configuration /
});

const middleware = require("webpack-dev-middleware");

const instance = middleware(compiler);

// eslint-disable-next-line new-cap
const app = new express();

app.use(instance);

setTimeout(() => {
// After a short delay the configuration is changed and a banner plugin is added to the config
new webpack.BannerPlugin("A new banner").apply(compiler);

// Recompile the bundle with the banner plugin:
instance.invalidate();
}, 1000);
`

$3

Executes a callback function when the compiler bundle is valid, typically after
compilation.

#### Parameters

##### callback

Type: Function
Required: No

A function executed when the bundle becomes valid.
If the bundle is valid at the time of calling, the callback is executed immediately.

`js
const express = require("express");
const webpack = require("webpack");

const compiler = webpack({
/ Webpack configuration /
});

const middleware = require("webpack-dev-middleware");

const instance = middleware(compiler);

// eslint-disable-next-line new-cap
const app = new express();

app.use(instance);

instance.waitUntilValid(() => {
console.log("Package is in a valid state");
});
`

$3

Get filename from URL.

#### Parameters

##### url

Type: String
Required: Yes

URL for the requested file.

`js
const express = require("express");
const webpack = require("webpack");

const compiler = webpack({
/ Webpack configuration /
});

const middleware = require("webpack-dev-middleware");

const instance = middleware(compiler);

// eslint-disable-next-line new-cap
const app = new express();

app.use(instance);

instance.waitUntilValid(() => {
const filename = instance.getFilenameFromUrl("/bundle.js");

console.log(Filename is ${filename});
});
`

FAQ

$3

Since output.publicPath and output.filename/output.chunkFilename can be dynamic, it's not possible to know which files are webpack bundles (and they public paths) and which are not, so we can't avoid blocking requests.

But there is a solution to avoid it - mount the middleware to a non-root route, for example:

`js
const express = require("express");
const webpack = require("webpack");
const middleware = require("webpack-dev-middleware");

const compiler = webpack({
// webpack options
});

const app = express();

// Mounting the middleware to the non-root route allows avoids this.
// Note - check your public path, if you want to handle /dist/, you need to setup output.publicPath to / value.
app.use(
"/dist/",
middleware(compiler, {
// webpack-dev-middleware options
}),
);

app.listen(3000, () => console.log("Example app listening on port 3000!"));
`

Server-Side Rendering

_Note: this feature is experimental and may be removed or changed completely in the future._

In order to develop an app using server-side rendering, we need access to the
stats, which is
generated with each build.

With server-side rendering enabled, webpack-dev-middleware sets the stats to res.locals.webpack.devMiddleware.stats
and the filesystem to res.locals.webpack.devMiddleware.outputFileSystem before invoking the next middleware,
allowing a developer to render the page body and manage the response to clients.

_Note: Requests for bundle files will still be handled by
webpack-dev-middleware and all requests will be pending until the build
process is finished with server-side rendering enabled._

Example Implementation:

`js
const express = require("express");
const isObject = require("is-object");
const webpack = require("webpack");
const middleware = require("webpack-dev-middleware");

const compiler = webpack({
/ Webpack configuration /
});

// eslint-disable-next-line new-cap
const app = new express();

// This function makes server rendering of asset references consistent with different webpack chunk/entry configurations
function normalizeAssets(assets) {
if (isObject(assets)) {
return Object.values(assets);
}

return Array.isArray(assets) ? assets : [assets];
}

app.use(middleware(compiler, { serverSideRender: true }));

// The following middleware would not be invoked until the latest build is finished.
app.use((req, res) => {
const { devMiddleware } = res.locals.webpack;
const { outputFileSystem } = devMiddleware;
const jsonWebpackStats = devMiddleware.stats.toJson();
const { assetsByChunkName, outputPath } = jsonWebpackStats;

// Then use assetsByChunkName for server-side rendering
// For example, if you have only one main chunk:
res.send(

${normalizeAssets(assetsByChunkName.main)
.filter((path) => path.endsWith(".js"))
.map((path) => )
.join("\n")}


);
});
`

Support

We do our best to keep Issues in the repository focused on bugs, features, and
needed modifications to the code for the module. Because of that, we ask users
with general support, "how-to", or "why isn't this working" questions to try one
of the other support channels that are available.

Your first-stop-shop for support for webpack-dev-server should by the excellent
[documentation][docs-url] for the module. If you see an opportunity for improvement
of those docs, please head over to the [webpack.js.org repo][wjo-url] and open a
pull request.

From there, we encourage users to visit the [webpack discussions][chat-url] and
talk to the fine folks there. If your quest for answers comes up dry in chat,
head over to [StackOverflow][stack-url] and do a quick search or open a new
question. Remember; It's always much easier to answer questions that include your
webpack.config.js and relevant files!

If you're twitter-savvy you can tweet [#webpack][hash-url] with your question
and someone should be able to reach out and lend a hand.

If you have discovered a :bug:, have a feature suggestion, or would like to see
a modification, please feel free to create an issue on Github. _Note: The issue
template isn't optional, so please be sure not to remove it, and please fill it
out completely._

Other servers

Examples of use with other servers will follow here.

$3

`js
const http = require("node:http");
const connect = require("connect");
const webpack = require("webpack");
const devMiddleware = require("webpack-dev-middleware");
const webpackConfig = require("./webpack.config.js");

const compiler = webpack(webpackConfig);
const devMiddlewareOptions = {
/* Your webpack-dev-middleware-options /
};
const app = connect();

app.use(devMiddleware(compiler, devMiddlewareOptions));

http.createServer(app).listen(3000);
`

$3

`js
const http = require("node:http");
const finalhandler = require("finalhandler");
const Router = require("router");
const webpack = require("webpack");
const devMiddleware = require("webpack-dev-middleware");
const webpackConfig = require("./webpack.config.js");

const compiler = webpack(webpackConfig);
const devMiddlewareOptions = {
/* Your webpack-dev-middleware-options /
};

// eslint-disable-next-line new-cap
const router = Router();

router.use(devMiddleware(compiler, devMiddlewareOptions));

const server = http.createServer((req, res) => {
router(req, res, finalhandler(req, res));
});

server.listen(3000);
`

$3

`js
const express = require("express");
const webpack = require("webpack");
const devMiddleware = require("webpack-dev-middleware");
const webpackConfig = require("./webpack.config.js");

const compiler = webpack(webpackConfig);
const devMiddlewareOptions = {
/* Your webpack-dev-middleware-options /
};
const app = express();

app.use(devMiddleware(compiler, devMiddlewareOptions));

app.listen(3000, () => console.log("Example app listening on port 3000!"));
`

$3

`js
const Koa = require("koa");
const webpack = require("webpack");
const middleware = require("webpack-dev-middleware");
const webpackConfig = require("./webpack.simple.config");

const compiler = webpack(webpackConfig);
const devMiddlewareOptions = {
/* Your webpack-dev-middleware-options /
};
const app = new Koa();

app.use(middleware.koaWrapper(compiler, devMiddlewareOptions));

app.listen(3000);
`

$3

`js
const Hapi = require("@hapi/hapi");
const webpack = require("webpack");
const devMiddleware = require("webpack-dev-middleware");
const webpackConfig = require("./webpack.config.js");

const compiler = webpack(webpackConfig);
const devMiddlewareOptions = {};

const server = Hapi.server({ port: 3000, host: "localhost" });

await server.register({
plugin: devMiddleware.hapiPlugin(),
options: {
// The compiler option is required
compiler,
...devMiddlewareOptions,
},
});

await server.start();

console.log("Server running on %s", server.info.uri);

process.on("unhandledRejection", (err) => {
console.log(err);
process.exit(1);
});
`

$3

Fastify interop will require the use of fastify-express instead of middie for providing middleware support. As the authors of fastify-express recommend, this should only be used as a stopgap while full Fastify support is worked on.

`js
const fastify = require("fastify")();
const webpack = require("webpack");
const devMiddleware = require("webpack-dev-middleware");
const webpackConfig = require("./webpack.config.js");

const compiler = webpack(webpackConfig);
const devMiddlewareOptions = {
/* Your webpack-dev-middleware-options /
};

await fastify.register(require("@fastify/express"));
await fastify.use(devMiddleware(compiler, devMiddlewareOptions));
await fastify.listen(3000);
`

$3

`js
import { serve } from "@hono/node-server";
import { Hono } from "hono";
import webpack from "webpack";
import devMiddleware from "webpack-dev-middleware";
import webpackConfig from "./webpack.config.js";

const compiler = webpack(webpackConfig);
const devMiddlewareOptions = {
/* Your webpack-dev-middleware-options /
};

const app = new Hono();

app.use(devMiddleware.honoWrapper(compiler, devMiddlewareOptions));

serve(app);
``

Contributing

Please take a moment to read our contributing guidelines if you haven't yet done so.

CONTRIBUTING

License

MIT

[npm]: https://img.shields.io/npm/v/webpack-dev-middleware.svg
[npm-url]: https://npmjs.com/package/webpack-dev-middleware
[node]: https://img.shields.io/node/v/webpack-dev-middleware.svg
[node-url]: https://nodejs.org
[tests]: https://github.com/webpack/webpack-dev-middleware/workflows/webpack-dev-middleware/badge.svg
[tests-url]: https://github.com/webpack/webpack-dev-middleware/actions
[cover]: https://codecov.io/gh/webpack/webpack-dev-middleware/branch/main/graph/badge.svg
[cover-url]: https://codecov.io/gh/webpack/webpack-dev-middleware
[discussion]: https://img.shields.io/github/discussions/webpack/webpack
[discussion-url]: https://github.com/webpack/webpack/discussions
[size]: https://packagephobia.com/badge?p=webpack-dev-middleware
[size-url]: https://packagephobia.com/result?p=webpack-dev-middleware
[docs-url]: https://webpack.js.org/guides/development/#using-webpack-dev-middleware
[hash-url]: https://twitter.com/search?q=webpack
[middleware-url]: https://github.com/webpack/webpack-dev-middleware
[stack-url]: https://stackoverflow.com/questions/tagged/webpack-dev-middleware
[chat-url]: https://github.com/webpack/webpack/discussions
[wjo-url]: https://github.com/webpack/webpack.js.org