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:

npm install webpack-dev-middleware --save-dev

Warning

We do not recommend installing this module globally.

Usage

const webpack = require("webpack");
const middleware = require("webpack-dev-middleware");
const compiler = webpack({
  // webpack options
});
const express = require("express");
const app = express();

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

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

Options

Name	Type	Default	Description
index	Boolean|String	index.html	If false (but not undefined), the server will not respond to requests to the root URL.
publicPath	String	output.publicPath (from a configuration)	The public path that the middleware is bound to.
writeToDisk	Boolean|Function	false	Instructs the module to write files to the configured location on disk as specified in your webpack configuration.

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

index

Type: Boolean|String Default: index.html

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

lastModified

Type: Boolean Default: undefined

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

publicPath

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.

writeToDisk

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.

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

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

API

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

close(callback)

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.

const express = require("express");
const webpack = require("webpack");
const compiler = webpack({
  /* Webpack configuration */
});
const middleware = require("webpack-dev-middleware");
const instance = middleware(compiler);

const app = new express();

app.use(instance);

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

invalidate(callback)

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.

const express = require("express");
const webpack = require("webpack");
const compiler = webpack({
  /* Webpack configuration */
});
const middleware = require("webpack-dev-middleware");
const instance = middleware(compiler);

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);

waitUntilValid(callback)

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.

const express = require("express");
const webpack = require("webpack");
const compiler = webpack({
  /* Webpack configuration */
});
const middleware = require("webpack-dev-middleware");
const instance = middleware(compiler);

const app = new express();

app.use(instance);

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

getFilenameFromUrl(url)

Get filename from URL.

Parameters

url

Type: String Required: Yes

URL for the requested file.

const express = require("express");
const webpack = require("webpack");
const compiler = webpack({
  /* Webpack configuration */
});
const middleware = require("webpack-dev-middleware");
const instance = middleware(compiler);

const app = new express();

app.use(instance);

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

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

FAQ

Avoid blocking requests to non-webpack resources.

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:

const webpack = require("webpack");
const middleware = require("webpack-dev-middleware");
const compiler = webpack({
  // webpack options
});
const express = require("express");
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!"));

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 for the module. If you see an opportunity for improvement of those docs, please head over to the webpack.js.org repo and open a pull request.

From there, we encourage users to visit the webpack discussions and talk to the fine folks there. If your quest for answers comes up dry in chat, head over to StackOverflow 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 with your question and someone should be able to reach out and lend a hand.

If you have discovered a 🐛, 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.

Connect

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

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

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

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

Router

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

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

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

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

server.listen(3000);

Express

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

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!"));

Contributing

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

CONTRIBUTING

License

MIT