SimpleNext.js

How to optimize your Next.js app build

Marouane Reda

March 16, 2021

If you need to understand the basics of Next.js, i recommend this course. (Disclaimer : this is an affiliate link that may earn me a small commission, but with no extra cost to you if you choose to enroll)

In phase with its philosophy, Next.js has built-in support of many optimization features , some of which we will see in this article.

Font optimization

By default, Next.js will automatically inline font CSS at build time, eliminating an extra round trip to fetch font declarations. This results in improvements to First Contentful Paint (FCP) and Largest Contentful Paint (LCP). For example:

// Before
<link
  href="https://fonts.googleapis.com/css2?family=Inter"
  rel="stylesheet"
/>

// After
<style data-href="https://fonts.googleapis.com/css2?family=Inter">
  @font-face{font-family:'Inter';font-style:normal...
</style>

How to import font in next.js

To add a web font to your Next.js application, override next/head. For example, you can add a font to a specific page:

// pages/index.js

import Head from 'next/head'

export default function IndexPage() {
  return (
    <div>
      <Head>
        <link
          href="https://fonts.googleapis.com/css2?family=Inter"
          rel="stylesheet"
        />
      </Head>
      <p>Hello world!</p>
    </div>
  )
}

or to your entire application with a Custom Document.

// pages/_document.js

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

class MyDocument extends Document {
  render() {
    return (
      <Html>
        <Head>
          <link
            href="https://fonts.googleapis.com/css2?family=Inter"
            rel="stylesheet"
          />
        </Head>
        <body>
          <Main />
          <NextScript />
        </body>
      </Html>
    )
  }
}

export default MyDocument

Automatic Webfont Optimization

currently supports Google Fonts and Typekit with support for other font providers coming soon. We're also planning to add control over loading strategies and font-display values.

Image optimization

Since version 10.0.0, Next.js has a built-in Image Component and Automatic Image Optimization.

The Next.js Image Component, next/image, is an extension of the HTML <img> element, evolved for the modern web.

The Automatic Image Optimization allows for resizing, optimizing, and serving images in modern formats like WebP when the browser supports it. This avoids shipping large images to devices with a smaller viewport. It also allows Next.js to automatically adopt future image formats and serve them to browsers that support those formats.

How to use the Next.js Image component

To add an image to your application, import the next/image component:

import Image from 'next/image'

function Home() {
  return (
    <>
      <h1>My Homepage</h1>
      <Image
        src="/me.png"
        alt="Picture of the author"
        width={500}
        height={500}
      />
      <p>Welcome to my homepage!</p>
    </>
  )
}

export default Home

Domains

To enable Image Optimization for images hosted on an external website, use an absolute url for the Image src and specify which domains are allowed to be optimized. This is needed to ensure that external urls can't be abused. When loader is set to an external image service, this option is ignored.

module.exports = {
  images: {
    domains: ['example.com'],
  },
}