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Next.js

NPM version Build Status Coverage Status Join the community on Spectrum

Visit nextjs.org/learn to get started with Next.js.

The below readme is the documentation for the canary (prerelease) branch. To view the documentation for the latest stable Next.js version visit nextjs.org/docs

How to use

Setup

Install it:

npm install --save next react react-dom

and add a script to your package.json like this:

{
  "scripts": {
    "dev": "next",
    "build": "next build",
    "start": "next start"
  }
}

After that, the file-system is the main API. Every .js file becomes a route that gets automatically processed and rendered.

Populate ./pages/index.js inside your project:

export default () => <div>Welcome to next.js!</div>

and then just run npm run dev and go to http://localhost:3000. To use another port, you can run npm run dev -- -p <your port here>.

So far, we get:

  • Automatic transpilation and bundling (with webpack and babel)
  • Hot code reloading
  • Server rendering and indexing of ./pages
  • Static file serving. ./static/ is mapped to /static/ (given you create a ./static/ directory inside your project)

To see how simple this is, check out the sample app - nextgram

Automatic code splitting

Every import you declare gets bundled and served with each page. That means pages never load unnecessary code!

import cowsay from 'cowsay-browser'

export default () => (
  <pre>
    {cowsay.say({ text: 'hi there!' })}
  </pre>
)

CSS

Built-in CSS support

Examples

We bundle styled-jsx to provide support for isolated scoped CSS. The aim is to support "shadow CSS" similar to Web Components, which unfortunately do not support server-rendering and are JS-only.

export default () => (
  <div>
    Hello world
    <p>scoped!</p>
    <style jsx>{`
      p {
        color: blue;
      }
      div {
        background: red;
      }
      @media (max-width: 600px) {
        div {
          background: blue;
        }
      }
    `}</style>
    <style global jsx>{`
      body {
        background: black;
      }
    `}</style>
  </div>
)

Please see the styled-jsx documentation for more examples.

CSS-in-JS

Examples

It's possible to use any existing CSS-in-JS solution. The simplest one is inline styles:

export default () => <p style={{ color: 'red' }}>hi there</p>

To use more sophisticated CSS-in-JS solutions, you typically have to implement style flushing for server-side rendering. We enable this by allowing you to define your own custom <Document> component that wraps each page.

Importing CSS / Sass / Less / Stylus files

To support importing .css, .scss, .less or .styl files you can use these modules, which configure sensible defaults for server rendered applications.

Static file serving (e.g.: images)

Create a folder called static in your project root directory. From your code you can then reference those files with /static/ URLs:

export default () => <img src="/static/my-image.png" alt="my image" />

Note: Don't name the static directory anything else. The name is required and is the only directory that Next.js uses for serving static assets.

Populating <head>

Examples

We expose a built-in component for appending elements to the <head> of the page.

import Head from 'next/head'

export default () => (
  <div>
    <Head>
      <title>My page title</title>
      <meta name="viewport" content="initial-scale=1.0, width=device-width" />
    </Head>
    <p>Hello world!</p>
  </div>
)

To avoid duplicate tags in your <head> you can use the key property, which will make sure the tag is only rendered once:

import Head from 'next/head'

export default () => (
  <div>
    <Head>
      <title>My page title</title>
      <meta name="viewport" content="initial-scale=1.0, width=device-width" key="viewport" />
    </Head>
    <Head>
      <meta name="viewport" content="initial-scale=1.2, width=device-width" key="viewport" />
    </Head>
    <p>Hello world!</p>
  </div>
)

In this case only the second <meta name="viewport" /> is rendered.

Note: The contents of <head> get cleared upon unmounting the component, so make sure each page completely defines what it needs in <head>, without making assumptions about what other pages added

Fetching data and component lifecycle

Examples

When you need state, lifecycle hooks or initial data population you can export a React.Component (instead of a stateless function, like shown above):

import React from 'react'

export default class extends React.Component {
  static async getInitialProps({ req }) {
    const userAgent = req ? req.headers['user-agent'] : navigator.userAgent
    return { userAgent }
  }

  render() {
    return (
      <div>
        Hello World {this.props.userAgent}
      </div>
    )
  }
}

Notice that to load data when the page loads, we use getInitialProps which is an async static method. It can asynchronously fetch anything that resolves to a JavaScript plain Object, which populates props.

Data returned from getInitialProps is serialized when server rendering, similar to a JSON.stringify. Make sure the returned object from getInitialProps is a plain Object and not using Date, Map or Set.

For the initial page load, getInitialProps will execute on the server only. getInitialProps will only be executed on the client when navigating to a different route via the Link component or using the routing APIs.

Note: getInitialProps can not be used in children components. Only in pages.


If you are using some server only modules inside getInitialProps, make sure to import them properly. Otherwise, it'll slow down your app.


You can also define the getInitialProps lifecycle method for stateless components:

const Page = ({ stars }) =>
  <div>
    Next stars: {stars}
  </div>

Page.getInitialProps = async ({ req }) => {
  const res = await fetch('https://api.github.com/repos/zeit/next.js')
  const json = await res.json()
  return { stars: json.stargazers_count }
}

export default Page

getInitialProps receives a context object with the following properties:

  • pathname - path section of URL
  • query - query string section of URL parsed as an object
  • asPath - String of the actual path (including the query) shows in the browser
  • req - HTTP request object (server only)
  • res - HTTP response object (server only)
  • jsonPageRes - Fetch Response object (client only)
  • err - Error object if any error is encountered during the rendering

Routing

With <Link>

Examples

Client-side transitions between routes can be enabled via a <Link> component. Consider these two pages:

// pages/index.js
import Link from 'next/link'

export default () => (
  <div>
    Click{' '}
    <Link href="/about">
      <a>here</a>
    </Link>{' '}
    to read more
  </div>
)
// pages/about.js
export default () => <p>Welcome to About!</p>

Note: use <Link prefetch> for maximum performance, to link and prefetch in the background at the same time

Client-side routing behaves exactly like the browser:

  1. The component is fetched
  2. If it defines getInitialProps, data is fetched. If an error occurs, _error.js is rendered
  3. After 1 and 2 complete, pushState is performed and the new component is rendered

To inject the pathname, query or asPath in your component, you can use withRouter.

With URL object
Examples

The component <Link> can also receive an URL object and it will automatically format it to create the URL string.

// pages/index.js
import Link from 'next/link'

export default () => (
  <div>
    Click{' '}
    <Link href={{ pathname: '/about', query: { name: 'Zeit' } }}>
      <a>here</a>
    </Link>{' '}
    to read more
  </div>
)

That will generate the URL string /about?name=Zeit, you can use every property as defined in the Node.js URL module documentation.

Replace instead of push url

The default behaviour for the <Link> component is to push a new url into the stack. You can use the replace prop to prevent adding a new entry.

// pages/index.js
import Link from 'next/link'

export default () => (
  <div>
    Click{' '}
    <Link href="/about" replace>
      <a>here</a>
    </Link>{' '}
    to read more
  </div>
)
Using a component that supports onClick

<Link> supports any component that supports the onClick event. In case you don't provide an <a> tag, it will only add the onClick event handler and won't pass the href property.

// pages/index.js
import Link from 'next/link'

export default () => (
  <div>
    Click{' '}
    <Link href="/about">
      <img src="/static/image.png" alt="image" />
    </Link>
  </div>
)
Forcing the Link to expose href to its child

If child is an <a> tag and doesn't have a href attribute we specify it so that the repetition is not needed by the user. However, sometimes, you’ll want to pass an <a> tag inside of a wrapper and the Link won’t recognize it as a hyperlink, and, consequently, won’t transfer its href to the child. In cases like that, you should define a boolean passHref property to the Link, forcing it to expose its href property to the child.

Please note: using a tag other than a and failing to pass passHref may result in links that appear to navigate correctly, but, when being crawled by search engines, will not be recognized as links (owing to the lack of href attribute). This may result in negative effects on your sites SEO.

import Link from 'next/link'
import Unexpected_A from 'third-library'

export default ({ href, name }) => (
  <Link href={href} passHref>
    <Unexpected_A>
      {name}
    </Unexpected_A>
  </Link>
)
Disabling the scroll changes to top on page

The default behaviour of <Link> is to scroll to the top of the page. When there is a hash defined it will scroll to the specific id, just like a normal <a> tag. To prevent scrolling to the top / hash scroll={false} can be added to <Link>:

<Link scroll={false} href="/?counter=10"><a>Disables scrolling</a></Link>
<Link href="/?counter=10"><a>Changes with scrolling to top</a></Link>

Imperatively

Examples

You can also do client-side page transitions using the next/router

import Router from 'next/router'

export default () => (
  <div>
    Click <span onClick={() => Router.push('/about')}>here</span> to read more
  </div>
)

Intercepting popstate

In some cases (for example, if using a custom router), you may wish to listen to popstate and react before the router acts on it. For example, you could use this to manipulate the request, or force an SSR refresh.

import Router from 'next/router'

Router.beforePopState(({ url, as, options }) => {
  // I only want to allow these two routes!
  if (as !== "/" || as !== "/other") {
    // Have SSR render bad routes as a 404.
    window.location.href = as
    return false
  }

  return true
});

If you return a falsy value from beforePopState, Router will not handle popstate; you'll be responsible for handling it, in that case. See Disabling File-System Routing.

Above Router object comes with the following API:

  • route - String of the current route
  • pathname - String of the current path excluding the query string
  • query - Object with the parsed query string. Defaults to {}
  • asPath - String of the actual path (including the query) shows in the browser
  • push(url, as=url) - performs a pushState call with the given url
  • replace(url, as=url) - performs a replaceState call with the given url
  • beforePopState(cb=function) - intercept popstate before router processes the event.

The second as parameter for push and replace is an optional decoration of the URL. Useful if you configured custom routes on the server.

With URL object

You can use an URL object the same way you use it in a <Link> component to push and replace an URL.

import Router from 'next/router'

const handler = () => {
  Router.push({
    pathname: '/about',
    query: { name: 'Zeit' }
  })
}

export default () => (
  <div>
    Click <span onClick={handler}>here</span> to read more
  </div>
)

This uses the same exact parameters as in the <Link> component.

Router Events

You can also listen to different events happening inside the Router. Here's a list of supported events:

  • routeChangeStart(url) - Fires when a route starts to change
  • routeChangeComplete(url) - Fires when a route changed completely
  • routeChangeError(err, url) - Fires when there's an error when changing routes
  • beforeHistoryChange(url) - Fires just before changing the browser's history
  • hashChangeStart(url) - Fires when the hash will change but not the page
  • hashChangeComplete(url) - Fires when the hash has changed but not the page

Here url is the URL shown in the browser. If you call Router.push(url, as) (or similar), then the value of url will be as.

Here's how to properly listen to the router event routeChangeStart:

const handleRouteChange = url => {
  console.log('App is changing to: ', url)
}

Router.events.on('routeChangeStart', handleRouteChange)

If you no longer want to listen to that event, you can unsubscribe with the off method:

Router.events.off('routeChangeStart', handleRouteChange)

If a route load is cancelled (for example by clicking two links rapidly in succession), routeChangeError will fire. The passed err will contain a cancelled property set to true.

Router.events.on('routeChangeError', (err, url) => {
  if (err.cancelled) {
    console.log(`Route to ${url} was cancelled!`)
  }
})
Shallow Routing
Examples

Shallow routing allows you to change the URL without running getInitialProps. You'll receive the updated pathname and the query via the url prop of the same page that's loaded, without losing state.

You can do this by invoking either Router.push or Router.replace with the shallow: true option. Here's an example:

// Current URL is "/"
const href = '/?counter=10'
const as = href
Router.push(href, as, { shallow: true })

Now, the URL is updated to /?counter=10. You can see the updated URL with this.props.router.query inside the Component (make sure you are using withRouter around your Component to inject the router prop).

You can watch for URL changes via componentDidUpdate hook as shown below:

componentDidUpdate(prevProps) {
  const { pathname, query } = this.props.router
  // verify props have changed to avoid an infinite loop
  if (query.id !== prevProps.router.query.id) {
    // fetch data based on the new query
  }
}

NOTES:

Shallow routing works only for same page URL changes. For an example, let's assume we have another page called about, and you run this:

Router.push('/?counter=10', '/about?counter=10', { shallow: true })

Since that's a new page, it'll unload the current page, load the new one and call getInitialProps even though we asked to do shallow routing.

Using a Higher Order Component

Examples

If you want to access the router object inside any component in your app, you can use the withRouter Higher-Order Component. Here's how to use it:

import { withRouter } from 'next/router'

const ActiveLink = ({ children, router, href }) => {
  const style = {
    marginRight: 10,
    color: router.pathname === href? 'red' : 'black'
  }

  const handleClick = (e) => {
    e.preventDefault()
    router.push(href)
  }

  return (
    <a href={href} onClick={handleClick} style={style}>
      {children}
    </a>
  )
}

export default withRouter(ActiveLink)

The above router object comes with an API similar to next/router.

Prefetching Pages

⚠️ This is a production only feature ⚠️

Examples

Next.js has an API which allows you to prefetch pages.

Since Next.js server-renders your pages, this allows all the future interaction paths of your app to be instant. Effectively Next.js gives you the great initial download performance of a website, with the ahead-of-time download capabilities of an app. Read more.

With prefetching Next.js only downloads JS code. When the page is getting rendered, you may need to wait for the data.

With <Link>

You can add prefetch prop to any <Link> and Next.js will prefetch those pages in the background.

import Link from 'next/link'

// example header component
export default () => (
  <nav>
    <ul>
      <li>
        <Link prefetch href="/">
          <a>Home</a>
        </Link>
      </li>
      <li>
        <Link prefetch href="/about">
          <a>About</a>
        </Link>
      </li>
      <li>
        <Link prefetch href="/contact">
          <a>Contact</a>
        </Link>
      </li>
    </ul>
  </nav>
)

Imperatively

Most prefetching needs are addressed by <Link />, but we also expose an imperative API for advanced usage:

import { withRouter } from 'next/router'

export default withRouter(({ router }) => (
  <div>
    <a onClick={() => setTimeout(() => router.push('/dynamic'), 100)}>
      A route transition will happen after 100ms
    </a>
    {// but we can prefetch it!
    router.prefetch('/dynamic')}
  </div>
)

The router instance should be only used inside the client side of your app though. In order to prevent any error regarding this subject, when rendering the Router on the server side, use the imperatively prefetch method in the componentDidMount() lifecycle method.

import React from 'react'
import { withRouter } from 'next/router'

class MyLink extends React.Component {
  componentDidMount() {
    const { router } = this.props
    router.prefetch('/dynamic')
  }
  
  render() {
    const { router } = this.props
    
    return (
       <div>
        <a onClick={() => setTimeout(() => router.push('/dynamic'), 100)}>
          A route transition will happen after 100ms
        </a>
      </div>   
    )
  }
}

export default withRouter(MyLink)

Custom server and routing

Examples

Typically you start your next server with next start. It's possible, however, to start a server 100% programmatically in order to customize routes, use route patterns, etc.

When using a custom server with a server file, for example called server.js, make sure you update the scripts key in package.json to:

{
  "scripts": {
    "dev": "node server.js",
    "build": "next build",
    "start": "NODE_ENV=production node server.js"
  }
}

This example makes /a resolve to ./pages/b, and /b resolve to ./pages/a:

// This file doesn't go through babel or webpack transformation.
// Make sure the syntax and sources this file requires are compatible with the current node version you are running
// See https://github.com/zeit/next.js/issues/1245 for discussions on Universal Webpack or universal Babel
const { createServer } = require('http')
const { parse } = require('url')
const next = require('next')

const dev = process.env.NODE_ENV !== 'production'
const app = next({ dev })
const handle = app.getRequestHandler()

app.prepare().then(() => {
  createServer((req, res) => {
    // Be sure to pass `true` as the second argument to `url.parse`.
    // This tells it to parse the query portion of the URL.
    const parsedUrl = parse(req.url, true)
    const { pathname, query } = parsedUrl

    if (pathname === '/a') {
      app.render(req, res, '/b', query)
    } else if (pathname === '/b') {
      app.render(req, res, '/a', query)
    } else {
      handle(req, res, parsedUrl)
    }
  }).listen(3000, err => {
    if (err) throw err
    console.log('> Ready on http://localhost:3000')
  })
})

The next API is as follows:

  • next(opts: object)

Supported options:

  • dev (bool) whether to launch Next.js in dev mode - default false
  • dir (string) where the Next project is located - default '.'
  • quiet (bool) Hide error messages containing server information - default false
  • conf (object) the same object you would use in next.config.js - default {}

Then, change your start script to NODE_ENV=production node server.js.

Disabling file-system routing

By default, Next will serve each file in /pages under a pathname matching the filename (eg, /pages/some-file.js is served at site.com/some-file.

If your project uses custom routing, this behavior may result in the same content being served from multiple paths, which can present problems with SEO and UX.

To disable this behavior & prevent routing based on files in /pages, simply set the following option in your next.config.js:

// next.config.js
module.exports = {
  useFileSystemPublicRoutes: false
}

Note that useFileSystemPublicRoutes simply disables filename routes from SSR; client-side routing may still access those paths. If using this option, you should guard against navigation to routes you do not want programmatically.

You may also wish to configure the client-side Router to disallow client-side redirects to filename routes; please refer to Intercepting popstate.

Dynamic assetPrefix

Sometimes we need to set the assetPrefix dynamically. This is useful when changing the assetPrefix based on incoming requests. For that, we can use app.setAssetPrefix.

Here's an example usage of it:

const next = require('next')
const micro = require('micro')

const dev = process.env.NODE_ENV !== 'production'
const app = next({ dev })
const handleNextRequests = app.getRequestHandler()

app.prepare().then(() => {
  const server = micro((req, res) => {
    // Add assetPrefix support based on the hostname
    if (req.headers.host === 'my-app.com') {
      app.setAssetPrefix('http://cdn.com/myapp')
    } else {
      app.setAssetPrefix('')
    }

    handleNextRequests(req, res)
  })

  server.listen(port, (err) => {
    if (err) {
      throw err
    }

    console.log(`> Ready on http://localhost:${port}`)
  })
})

Dynamic Import

Examples

Next.js supports TC39 dynamic import proposal for JavaScript. With that, you could import JavaScript modules (inc. React Components) dynamically and work with them.

You can think dynamic imports as another way to split your code into manageable chunks. Since Next.js supports dynamic imports with SSR, you could do amazing things with it.

Here are a few ways to use dynamic imports.

1. Basic Usage (Also does SSR)

import dynamic from 'next/dynamic'

const DynamicComponent = dynamic(() => import('../components/hello'))

export default () => (
  <div>
    <Header />
    <DynamicComponent />
    <p>HOME PAGE is here!</p>
  </div>
)

2. With Custom Loading Component

import dynamic from 'next/dynamic'

const DynamicComponentWithCustomLoading = dynamic(() => import('../components/hello2'), {
  loading: () => <p>...</p>
})

export default () => (
  <div>
    <Header />
    <DynamicComponentWithCustomLoading />
    <p>HOME PAGE is here!</p>
  </div>
)

3. With No SSR

import dynamic from 'next/dynamic'

const DynamicComponentWithNoSSR = dynamic(() => import('../components/hello3'), {
  ssr: false
})

export default () => (
  <div>
    <Header />
    <DynamicComponentWithNoSSR />
    <p>HOME PAGE is here!</p>
  </div>
)

4. With Multiple Modules At Once

import dynamic from 'next/dynamic'

const HelloBundle = dynamic({
  modules: () => {
    const components = {
      Hello1: () => import('../components/hello1'),
      Hello2: () => import('../components/hello2')
    }

    return components
  },
  render: (props, { Hello1, Hello2 }) =>
    <div>
      <h1>
        {props.title}
      </h1>
      <Hello1 />
      <Hello2 />
    </div>
})

export default () => <HelloBundle title="Dynamic Bundle" />

Custom <App>

Examples

Next.js uses the App component to initialize pages. You can override it and control the page initialization. Which allows you to do amazing things like:

  • Persisting layout between page changes
  • Keeping state when navigating pages
  • Custom error handling using componentDidCatch
  • Inject additional data into pages (for example by processing GraphQL queries)

To override, create the ./pages/_app.js file and override the App class as shown below:

import React from 'react'
import App, { Container } from 'next/app'

export default class MyApp extends App {
  static async getInitialProps({ Component, router, ctx }) {
    let pageProps = {}

    if (Component.getInitialProps) {
      pageProps = await Component.getInitialProps(ctx)
    }

    return { pageProps }
  }

  render () {
    const { Component, pageProps } = this.props

    return (
      <Container>
        <Component {...pageProps} />
      </Container>
    )
  }
}

Custom <Document>

Examples

  • Is rendered on the server side
  • Is used to change the initial server side rendered document markup
  • Commonly used to implement server side rendering for css-in-js libraries like styled-components, glamorous or emotion. styled-jsx is included with Next.js by default.

Pages in Next.js skip the definition of the surrounding document's markup. For example, you never include <html>, <body>, etc. To override that default behavior, you must create a file at ./pages/_document.js, where you can extend the Document class:

// _document is only rendered on the server side and not on the client side
// Event handlers like onClick can't be added to this file

// ./pages/_document.js
import Document, { Head, Main, NextScript } from 'next/document'

export default class MyDocument extends Document {
  static async getInitialProps(ctx) {
    const initialProps = await Document.getInitialProps(ctx)
    return { ...initialProps }
  }

  render() {
    return (
      <html>
        <Head>
          <style>{`body { margin: 0 } /* custom! */`}</style>
        </Head>
        <body className="custom_class">
          <Main />
          <NextScript />
        </body>
      </html>
    )
  }
}

The ctx object is equivalent to the one received in all getInitialProps hooks, with one addition:

  • renderPage (Function) a callback that executes the actual React rendering logic (synchronously). It's useful to decorate this function in order to support server-rendering wrappers like Aphrodite's renderStatic

Note: React-components outside of <Main /> will not be initialised by the browser. Do not add application logic here. If you need shared components in all your pages (like a menu or a toolbar), take a look at the App component instead.

Custom error handling

404 or 500 errors are handled both client and server side by a default component error.js. If you wish to override it, define a _error.js in the pages folder:

import React from 'react'

export default class Error extends React.Component {
  static getInitialProps({ res, err }) {
    const statusCode = res ? res.statusCode : err ? err.statusCode : null;
    return { statusCode }
  }

  render() {
    return (
      <p>
        {this.props.statusCode
          ? `An error ${this.props.statusCode} occurred on server`
          : 'An error occurred on client'}
      </p>
    )
  }
}

Reusing the built-in error page

If you want to render the built-in error page you can by using next/error:

import React from 'react'
import Error from 'next/error'
import fetch from 'isomorphic-unfetch'

export default class Page extends React.Component {
  static async getInitialProps() {
    const res = await fetch('https://api.github.com/repos/zeit/next.js')
    const statusCode = res.statusCode > 200 ? res.statusCode : false
    const json = await res.json()

    return { statusCode, stars: json.stargazers_count }
  }

  render() {
    if (this.props.statusCode) {
      return <Error statusCode={this.props.statusCode} />
    }

    return (
      <div>
        Next stars: {this.props.stars}
      </div>
    )
  }
}

If you have created a custom error page you have to import your own _error component instead of next/error

Custom configuration

For custom advanced behavior of Next.js, you can create a next.config.js in the root of your project directory (next to pages/ and package.json).

Note: next.config.js is a regular Node.js module, not a JSON file. It gets used by the Next server and build phases, and not included in the browser build.

// next.config.js
module.exports = {
  /* config options here */
}

Or use a function:

module.exports = (phase, {defaultConfig}) => {
  //
  // https://github.com/zeit/
  return {
    /* config options here */
  }
}

phase is the current context in which the configuration is loaded. You can see all phases here: constants Phases can be imported from next/constants:

const {PHASE_DEVELOPMENT_SERVER} = require('next/constants')
module.exports = (phase, {defaultConfig}) => {
  if(phase === PHASE_DEVELOPMENT_SERVER) {
    return {
      /* development only config options here */
    }
  }

  return {
    /* config options for all phases except development here */
  }
}

Setting a custom build directory

You can specify a name to use for a custom build directory. For example, the following config will create a build folder instead of a .next folder. If no configuration is specified then next will create a .next folder.

// next.config.js
module.exports = {
  distDir: 'build'
}

Disabling etag generation

You can disable etag generation for HTML pages depending on your cache strategy. If no configuration is specified then Next will generate etags for every page.

// next.config.js
module.exports = {
  generateEtags: false
}

Configuring the onDemandEntries

Next exposes some options that give you some control over how the server will dispose or keep in memories pages built:

module.exports = {
  onDemandEntries: {
    // period (in ms) where the server will keep pages in the buffer
    maxInactiveAge: 25 * 1000,
    // number of pages that should be kept simultaneously without being disposed
    pagesBufferLength: 2,
  }
}

This is development-only feature. If you want to cache SSR pages in production, please see SSR-caching example.

Configuring extensions looked for when resolving pages in pages

Aimed at modules like @zeit/next-typescript, that add support for pages ending in .ts. pageExtensions allows you to configure the extensions looked for in the pages directory when resolving pages.

// next.config.js
module.exports = {
  pageExtensions: ['jsx', 'js']
}

Configuring the build ID

Next.js uses a constant generated at build time to identify which version of your application is being served. This can cause problems in multi-server deployments when next build is ran on every server. In order to keep a static build id between builds you can provide the generateBuildId function:

// next.config.js
module.exports = {
  generateBuildId: async () => {
    // For example get the latest git commit hash here
    return 'my-build-id'
  }
}

Customizing webpack config

Examples

Some commonly asked for features are available as modules:

Warning: The webpack function is executed twice, once for the server and once for the client. This allows you to distinguish between client and server configuration using the isServer property

Multiple configurations can be combined together with function composition. For example:

const withTypescript = require('@zeit/next-typescript')
const withSass = require('@zeit/next-sass')

module.exports = withTypescript(withSass({
  webpack(config, options) {
    // Further custom configuration here
    return config
  }
}))

In order to extend our usage of webpack, you can define a function that extends its config via next.config.js.

// next.config.js is not transformed by Babel. So you can only use javascript features supported by your version of Node.js.

module.exports = {
  webpack: (config, { buildId, dev, isServer, defaultLoaders }) => {
    // Perform customizations to webpack config
    // Important: return the modified config
    return config
  },
  webpackDevMiddleware: config => {
    // Perform customizations to webpack dev middleware config
    // Important: return the modified config
    return config
  }
}

The second argument to webpack is an object containing properties useful when customing the WebPack configuration:

  • buildId - String the build id used as a unique identifier between builds
  • dev - Boolean shows if the compilation is done in development mode
  • isServer - Boolean shows if the resulting configuration will be used for server side (true), or client size compilation (false).
  • defaultLoaders - Object Holds loader objects Next.js uses internally, so that you can use them in custom configuration
    • babel - Object the babel-loader configuration for Next.js.
    • hotSelfAccept - Object the hot-self-accept-loader configuration. This loader should only be used for advanced use cases. For example @zeit/next-typescript adds it for top-level typescript pages.

Example usage of defaultLoaders.babel:

// Example next.config.js for adding a loader that depends on babel-loader
// This source was taken from the @zeit/next-mdx plugin source: 
// https://github.com/zeit/next-plugins/blob/master/packages/next-mdx
module.exports = {
  webpack: (config, {}) => {
    config.module.rules.push({
      test: /\.mdx/,
      use: [
        options.defaultLoaders.babel,
        {
          loader: '@mdx-js/loader',
          options: pluginOptions.options
        }
      ]
    })

    return config
  }
}

Customizing babel config

Examples

In order to extend our usage of babel, you can simply define a .babelrc file at the root of your app. This file is optional.

If found, we're going to consider it the source of truth, therefore it needs to define what next needs as well, which is the next/babel preset.

This is designed so that you are not surprised by modifications we could make to the babel configurations.

Here's an example .babelrc file:

{
  "presets": ["next/babel"],
  "plugins": []
}

The next/babel preset includes everything needed to transpile React applications. This includes:

  • preset-env
  • preset-react
  • plugin-proposal-class-properties
  • plugin-proposal-object-rest-spread
  • plugin-transform-runtime
  • styled-jsx

These presets / plugins should not be added to your custom .babelrc. Instead, you can configure them on the next/babel preset:

{
  "presets": [
    ["next/babel", {
      "preset-env": {},
      "transform-runtime": {},
      "styled-jsx": {},
      "class-properties": {}
    }]
  ],
  "plugins": []
}

The modules option on "preset-env" should be kept to false otherwise webpack code splitting is disabled.

Exposing configuration to the server / client side

The next/config module gives your app access to runtime configuration stored in your next.config.js. Place any server-only runtime config under a serverRuntimeConfig property and anything accessible to both client and server-side code under publicRuntimeConfig.

// next.config.js
module.exports = {
  serverRuntimeConfig: { // Will only be available on the server side
    mySecret: 'secret'
  },
  publicRuntimeConfig: { // Will be available on both server and client
    staticFolder: '/static',
    mySecret: process.env.MY_SECRET // Pass through env variables
  }
}
// pages/index.js
import getConfig from 'next/config'
// Only holds serverRuntimeConfig and publicRuntimeConfig from next.config.js nothing else.
const {serverRuntimeConfig, publicRuntimeConfig} = getConfig()

console.log(serverRuntimeConfig.mySecret) // Will only be available on the server side
console.log(publicRuntimeConfig.staticFolder) // Will be available on both server and client

export default () => <div>
  <img src={`${publicRuntimeConfig.staticFolder}/logo.png`} alt="logo" />
</div>

Starting the server on alternative hostname

To start the development server using a different default hostname you can use --hostname hostname_here or -H hostname_here option with next dev. This will start a TCP server listening for connections on the provided host.

CDN support with Asset Prefix

To set up a CDN, you can set up the assetPrefix setting and configure your CDN's origin to resolve to the domain that Next.js is hosted on.

const isProd = process.env.NODE_ENV === 'production'
module.exports = {
  // You may only need to add assetPrefix in the production.
  assetPrefix: isProd ? 'https://cdn.mydomain.com' : ''
}

Note: Next.js will automatically use that prefix in the scripts it loads, but this has no effect whatsoever on /static. If you want to serve those assets over the CDN, you'll have to introduce the prefix yourself. One way of introducing a prefix that works inside your components and varies by environment is documented in this example.

Production deployment

To deploy, instead of running next, you want to build for production usage ahead of time. Therefore, building and starting are separate commands:

next build
next start

For example, to deploy with now a package.json like follows is recommended:

{
  "name": "my-app",
  "dependencies": {
    "next": "latest"
  },
  "scripts": {
    "dev": "next",
    "build": "next build",
    "start": "next start"
  }
}

Then run now and enjoy!

Next.js can be deployed to other hosting solutions too. Please have a look at the 'Deployment' section of the wiki.

Note: NODE_ENV is properly configured by the next subcommands, if absent, to maximize performance. if you’re using Next.js programmatically, it’s your responsibility to set NODE_ENV=production manually!

Note: we recommend putting .next, or your custom dist folder, in .gitignore or .npmignore. Otherwise, use files or now.files to opt-into a whitelist of files you want to deploy, excluding .next or your custom dist folder.

Browser support

Next.js supports IE11 and all modern browsers out of the box using @babel/preset-env. In order to support IE11 Next.js adds a global Promise polyfill. In cases where your own code or any external NPM dependencies you are using requires features not supported by your target browsers you will need to implement polyfills.

The polyfills example demonstrates the recommended approach to implement polyfills.

Static HTML export

Examples

next export is a way to run your Next.js app as a standalone static app without the need for a Node.js server. The exported app supports almost every feature of Next.js, including dynamic urls, prefetching, preloading and dynamic imports.

The way next export works is by pre-rendering all pages possible to HTML. It does so based on a mapping of pathname key to page object. This mapping is called the exportPathMap.

The page object has 2 values:

  • page - String the page inside the pages directory to render
  • query - Object the query object passed to getInitialProps when pre-rendering. Defaults to {}

Usage

Simply develop your app as you normally do with Next.js. Then run:

next build
next export

By default next export doesn't require any configuration. It will generate a default exportPathMap containing the routes to pages inside the pages directory. This default mapping is available as defaultPathMap in the example below.

If your application has dynamic routes you can add a dynamic exportPathMap in next.config.js. This function is asynchronous and gets the default exportPathMap as a parameter.

// next.config.js
module.exports = {
  exportPathMap: async function (defaultPathMap) {
    return {
      '/': { page: '/' },
      '/about': { page: '/about' },
      '/readme.md': { page: '/readme' },
      '/p/hello-nextjs': { page: '/post', query: { title: 'hello-nextjs' } },
      '/p/learn-nextjs': { page: '/post', query: { title: 'learn-nextjs' } },
      '/p/deploy-nextjs': { page: '/post', query: { title: 'deploy-nextjs' } }
    }
  }
}

Note that if the path ends with a directory, it will be exported as /dir-name/index.html, but if it ends with an extension, it will be exported as the specified filename, e.g. /readme.md above. If you use a file extension other than .html, you may need to set the Content-Type header to text/html when serving this content.

Then simply run these commands:

next build
next export

For that you may need to add a NPM script to package.json like this:

{
  "scripts": {
    "build": "next build",
    "export": "npm run build && next export"
  }
}

And run it at once with:

npm run export

Then you have a static version of your app in the out directory.

You can also customize the output directory. For that run next export -h for the help.

Now you can deploy the out directory to any static hosting service. Note that there is an additional step for deploying to GitHub Pages, documented here.

For an example, simply visit the out directory and run following command to deploy your app to ZEIT Now.

now

Copying custom files

In case you have to copy custom files like a robots.txt or generate a sitemap.xml you can do this inside of exportPathMap. exportPathMap gets a few contextual parameter to aid you with creating/copying files:

  • dev - true when exportPathMap is being called in development. false when running next export. In development exportPathMap is used to define routes and behavior like copying files is not required.
  • dir - Absolute path to the project directory
  • outDir - Absolute path to the out directory (configurable with -o or --outdir). When dev is true the value of outDir will be null.
  • distDir - Absolute path to the .next directory (configurable using the distDir config key)
  • buildId - The buildId the export is running for
// next.config.js
const fs = require('fs')
const { join } = require('path')
const { promisify } = require('util')
const copyFile = promisify(fs.copyFile)

module.exports = {
  exportPathMap: async function (defaultPathMap, {dev, dir, outDir, distDir, buildId}) {
    if (dev) {
      return defaultPathMap
    }
    // This will copy robots.txt from your project root into the out directory
    await copyFile(join(dir, 'robots.txt'), join(outDir, 'robots.txt'))
    return defaultPathMap
  }
}

Limitation

With next export, we build a HTML version of your app. At export time we will run getInitialProps of your pages.

The req and res fields of the context object passed to getInitialProps are not available as there is no server running.

You won't be able to render HTML dynamically when static exporting, as we pre-build the HTML files. If you want to do dynamic rendering use next start or the custom server API

Multi Zones

Examples

A zone is a single deployment of a Next.js app. Just like that, you can have multiple zones. Then you can merge them as a single app.

For an example, you can have two zones like this:

With multi zones support, you can merge both these apps into a single one. Which allows your customers to browse it using a single URL. But you can develop and deploy both apps independently.

This is exactly the same concept as microservices, but for frontend apps.

How to define a zone

There are no special zones related APIs. You only need to do following things:

  • Make sure to keep only the pages you need in your app. (For an example, https://ui.my-app.com should not contain pages for /docs/**)
  • Make sure your app has an assetPrefix. (You can also define the assetPrefix dynamically.)

How to merge them

You can merge zones using any HTTP proxy.

You can use micro proxy as your local proxy server. It allows you to easily define routing rules like below:

{
  "rules": [
    {"pathname": "/docs**", "method":["GET", "POST", "OPTIONS"], "dest": "https://docs.my-app.com"},
    {"pathname": "/**", "dest": "https://ui.my-app.com"}
  ]
}

For the production deployment, you can use the path alias feature if you are using ZEIT now. Otherwise, you can configure your existing proxy server to route HTML pages using a set of rules as show above.

Recipes

FAQ

Is this production ready? Next.js has been powering https://zeit.co since its inception.

We’re ecstatic about both the developer experience and end-user performance, so we decided to share it with the community.

How big is it?

The client side bundle size should be measured in a per-app basis. A small Next main bundle is around 65kb gzipped.

Is this like `create-react-app`?

Yes and No.

Yes in that both make your life easier.

No in that it enforces a structure so that we can do more advanced things like:

  • Server side rendering
  • Automatic code splitting

In addition, Next.js provides two built-in features that are critical for every single website:

  • Routing with lazy component loading: <Link> (by importing next/link)
  • A way for components to alter <head>: <Head> (by importing next/head)

If you want to create re-usable React components that you can embed in your Next.js app or other React applications, using create-react-app is a great idea. You can later import it and keep your codebase clean!

How do I use CSS-in-JS solutions?

Next.js bundles styled-jsx supporting scoped css. However you can use any CSS-in-JS solution in your Next app by just including your favorite library as mentioned before in the document.

What syntactic features are transpiled? How do I change them?

We track V8. Since V8 has wide support for ES6 and async and await, we transpile those. Since V8 doesn’t support class decorators, we don’t transpile those.

See the documentation about customizing the babel config and next/preset for more information.

Why a new Router?

Next.js is special in that:

  • Routes don’t need to be known ahead of time
  • Routes are always lazy-loadable
  • Top-level components can define getInitialProps that should block the loading of the route (either when server-rendering or lazy-loading)

As a result, we were able to introduce a very simple approach to routing that consists of two pieces:

  • Every top level component receives a url object to inspect the url or perform modifications to the history
  • A <Link /> component is used to wrap elements like anchors (<a/>) to perform client-side transitions

We tested the flexibility of the routing with some interesting scenarios. For an example, check out nextgram.

How do I define a custom fancy route?

We added the ability to map between an arbitrary URL and any component by supplying a request handler.

On the client side, we have a parameter call as on <Link> that decorates the URL differently from the URL it fetches.

How do I fetch data?

It’s up to you. getInitialProps is an async function (or a regular function that returns a Promise). It can retrieve data from anywhere.

Can I use it with GraphQL?

Yes! Here's an example with Apollo.

Can I use it with Redux?

Yes! Here's an example

Can I use Next with my favorite Javascript library or toolkit?

Since our first release we've had many example contributions, you can check them out in the examples directory

What is this inspired by?

Many of the goals we set out to accomplish were the ones listed in The 7 principles of Rich Web Applications by Guillermo Rauch.

The ease-of-use of PHP is a great inspiration. We feel Next.js is a suitable replacement for many scenarios where you otherwise would use PHP to output HTML.

Unlike PHP, we benefit from the ES6 module system and every file exports a component or function that can be easily imported for lazy evaluation or testing.

As we were researching options for server-rendering React that didn’t involve a large number of steps, we came across react-page (now deprecated), a similar approach to Next.js by the creator of React Jordan Walke.

Contributing

Please see our contributing.md

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