[![npm][npm]][npm-url]
[![node][node]][node-url]
[![deps][deps]][deps-url]
[![tests][tests]][tests-url]
[![coverage][cover]][cover-url]
[![chat][chat]][chat-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
`

_Note: We do not recommend installing this module globally._

Requirements

webpack-dev-middleware requires Node v6 or higher, and must be used with a
server that accepts express-style middleware.

Usage

`js
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

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

_Note: The publicPath property is required, whereas all other options are optional_

$3

Type: Object
Default: undefined

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

$3

Type: String
Default: undefined

"index.html",
// The index path for web server, defaults to "index.html".
// If falsy (but not undefined), the server will not respond to requests to the root URL.




$3

Type: Boolean
Default: undefined

This option instructs the module to operate in 'lazy' mode, meaning that it won't
recompile when files change, but rather on each request.

$3

Type: Object
Default: webpack-log

In the rare event that a user would like to provide a custom logging interface,
this property allows the user to assign one. The module leverages
webpack-log
for creating the loglevelnext
logging management by default. Any custom logger must adhere to the same
exports for compatibility. Specifically, all custom loggers must have the
following exported methods at a minimum:

- log.trace
- log.debug
- log.info
- log.warn
- log.error

Please see the documentation for loglevel for more information.

$3

Type: String
Default: 'info'

This property defines the level of messages that the module will log. Valid levels
include:

- trace
- debug
- info
- warn
- error
- silent

Setting a log level means that all other levels below it will be visible in the
console. Setting logLevel: 'silent' will hide all console output. The module
leverages webpack-log
for logging management, and more information can be found on its page.

$3

Type: Boolean
Default: false

If true the log output of the module will be prefixed by a timestamp in the
HH:mm:ss format.

$3

Type: Object
Default: null

This property allows a user to register custom mime types or extension mappings.
eg. { 'text/html': [ 'phtml' ] }. Please see the documentation for
node-mime for more information.

$3

Type: String
_Required_

The public path that the middleware is bound to. _Best Practice: use the same
publicPath defined in your webpack config._

$3

Type: Object
Default: undefined

Allows users to provide a custom reporter to handle logging within the module.
Please see the default reporter
for an example.

$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: Object
Default: { context: process.cwd() }

Options for formatting statistics displayed during and after compile. For more
information and property details, please see the
webpack documentation.

$3

Type: Number
Default: 11000

Watching (by means of lazy: false) will frequently cause multiple compilations
as the bundle changes during compilation. This is due in part to cross-platform
differences in file watchers, so that webpack doesn't loose file changes when
watched files change rapidly. Since that scenario is more an edge case than not,
this option serves as a means to prevent multiple needless, identical compilations
by advancing start-time of a watcher by a number of seconds, which keeps generated
files from triggering the watch cycle.

_To disable this prevention, set this option to a value of 0._

$3

Type: Object
Default: { aggregateTimeout: 200 }

The module accepts an Object containing options for file watching, which is
passed directly to the compiler provided. For more information on watch options
please see the webpack documentation

API

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

$3

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

$3

#### callback

Type: Function

A function executed once the middleware has stopped watching.

$3

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

`js
const webpack = require('webpack');
const compiler = webpack({ ... });
const middleware = require('webpack-dev-middleware');
const instance = middleware(compiler);

app.use(instance);

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

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

$3

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

$3

#### callback

Type: Function

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 webpack = require('webpack');
const compiler = webpack({ ... });
const middleware = require('webpack-dev-middleware');
const instance = middleware(compiler);

app.use(instance);

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

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 stat to
res.locals.webpackStats 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 webpack = require('webpack');
const compiler = webpack({ ... });
const middleware = require('webpack-dev-middleware');

// This function makes server rendering of asset references consistent with different webpack chunk/entry configurations
function normalizeAssets(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 assetsByChunkName = res.locals.webpackStats.toJson().assetsByChunkName

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



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

${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 Gitter chat][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, of 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._

Contributing

We welcome your contributions! Please have a read of CONTRIBUTING.md for more information on how to get involved.

Maintainers

Kees Kluskens

Andrew Powell

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

[deps]: https://david-dm.org/webpack/webpack-dev-middleware.svg
[deps-url]: https://david-dm.org/webpack/webpack-dev-middleware

[tests]: http://img.shields.io/travis/webpack/webpack-dev-middleware.svg
[tests-url]: https://travis-ci.org/webpack/webpack-dev-middleware

[cover]: https://codecov.io/gh/webpack/webpack-dev-middleware/branch/master/graph/badge.svg
[cover-url]: https://codecov.io/gh/webpack/webpack-dev-middleware

[chat]: https://badges.gitter.im/webpack/webpack.svg
[chat-url]: https://gitter.im/webpack/webpack

[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
[uglify-url]: https://github.com/webpack-contrib/uglifyjs-webpack-plugin
[wjo-url]: https://github.com/webpack/webpack.js.org