Next.js is a minimalistic framework for server-rendered React applications.

**NOTE! the README on the master branch might not match that of the latest stable release! **

How to use

Install it:

npm install next --save

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

{
  "scripts": {
    "dev": "next"
  }
}

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

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/

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

Basic css

We bundle styled-jsx to provide support for isolated scoped CSS. The aim is to support "shadow CSS" resembling of 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>
  </div>
)

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

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" />
)

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>
)

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

Data fetch

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 }) {
    return req
      ? { userAgent: req.headers['user-agent'] }
      : { userAgent: navigator.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.

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.

getInitialProps receives a context object with the following properties:

pathname - path section of URL
query - query string section of URL parsed as an object
req - HTTP request object (server only)
res - HTTP response object (server only)
xhr - XMLHttpRequest object (client only)
err - Error object if any error is encountered during the rendering

Routing

With `<Link>`

Examples

Hello World

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 next/prefetch for maximum performance, to link and prefetch at the same time in the background

Client-side routing behaves exactly like the browser:

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

Each top-level component receives a url property with the following API:

pathname - String of the current path excluding the query string
query - Object with the parsed query string. Defaults to {}
push(url, as=url) - performs a pushState call with the given url
replace(url, as=url) - performs a replaceState call with the given url

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

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>
)

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 {}
push(url, as=url) - performs a pushState call with the given url
replace(url, as=url) - performs a replaceState call with the given url

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

Note: in order to programmatically change the route without triggering navigation and component-fetching, use props.url.push and props.url.replace within a 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

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:

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

If you are no longer want to listen to that event, you can simply unset the event listener like this:

Router.onRouteChangeStart = null

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

Router.onRouteChangeError = (err, url) => {
  if (err.cancelled) {
    console.log(`Route to ${url} was cancelled!`)
  }
}

Prefetching Pages

Examples

Prefetching

Next.js exposes a module that configures a ServiceWorker automatically to prefetch pages: next/prefetch.

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 `<Link>`

You can substitute your usage of <Link> with the default export of next/prefetch. For example:

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

When this higher-level <Link> component is first used, the ServiceWorker gets installed. To turn off prefetching on a per-<Link> basis, you can use the prefetch attribute:

<Link href='/contact' prefetch={false}><a>Home</a></Link>

Imperatively

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

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

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

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

const { createServer } = require('http')
const { parse } = require('url')
const next = require('next')

const app = next({ dev: true })
const handle = app.getRequestHandler()

app.prepare().then(() => {
  createServer((req, res) => {
    const { pathname, query } = parse(req.url, true)

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

The next API is as follows:

next(path: string, opts: object) - path is
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) Display error messages with server information - default false

Custom `<Document>`

Examples

Styled components custom document

Pages in Next.js skip the definition of the surrounding document's markup. For example, you never include <html>, <body>, etc. But we still make it possible to override that:

import Document, { Head, Main, NextScript } from 'next/document'

export default class MyDocument extends Document {
  static async getInitialProps (ctx) {
    const props = await Document.getInitialProps(ctx)
    return { ...props, customValue: 'hi there!' }
  }

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

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:

import React from 'react'
export default class Error extends React.Component {
  static getInitialProps ({ res, xhr }) {
    const statusCode = res ? res.statusCode : (xhr ? xhr.status : null)
    return { statusCode }
  }

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

Custom configuration

Examples

Custom babel 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 */
}

Customizing webpack config

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

// This file is not going through babel transformation.
// So, we write it in vanilla JS
// (But you could use ES2015 features supported by your Node.js version)

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

Warning: Adding loaders to support new file types (css, less, svg, etc.) is not recommended because only the client code gets bundled via webpack and thus it won't work on the initial server rendering. Babel plugins are a good alternative because they're applied consistently between server/client rendering (e.g. babel-plugin-inline-react-svg).

Customizing babel config

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",
    "stage-0"
  ],
}

Production deployment

To deploy, instead of running next, you probably want to build 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!

Note: we recommend putting .next in .npmignore or .gitignore. Otherwise, use files or now.files to opt-into a whitelist of files you want to deploy (and obviously exclude .next)

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: `

(by importingnext/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 a 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 this and this

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 Redux?

Yes! Here's an example

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.

Roadmap

Our Roadmap towards 2.0.0 is public.

Authors

Naoyuki Kanezawa (@nkzawa) – ▲ZEIT
Tony Kovanen (@tonykovanen) – ▲ZEIT
Guillermo Rauch (@rauchg) – ▲ZEIT
Dan Zajdband (@impronunciable) – Knight-Mozilla / Coral Project
Tim Neutkens (@timneutkens)

License

max/next.js

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