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Use next/image on Netlify

This repo is a proof of concept demonstrating how to use the next/image functionality on other platforms.

This implementation uses Netlify Functions to process images on the fly by setting up a rewrite rule to handle the special endpoint next/image sends requests to.

Expand for additional details about how `next/image` works under the hood

Images are sent to a special endpoint for processing

Let's assume the following next/image setup:

import Image from 'next/image';

export default function MyImage() {
  return (
    <Image
      src="/boop.jpg"
      alt="what a great boop"
      width="1368"
      height="1044"
      layout="responsive"
    />
  );
}

Next will render this using the following markup (formatting added for legibility):

<div style="display: block; overflow: hidden; position: relative; box-sizing: border-box; margin: 0px;">
  <div style="display: block; box-sizing: border-box; padding-top: 76.3158%;"></div>
  <img
    alt="what a great boop."
    src="/_next/image?url=%2Fboop.jpg&w=3840&q=75" 
    decoding="async"
    sizes="(max-width: 640px) 640px, (max-width: 750px) 750px, (max-width: 828px) 828px, (max-width: 1080px) 1080px, (max-width: 1200px) 1200px, (max-width: 1920px) 1920px, (max-width: 2048px) 2048px, 3840px"
    srcset="/_next/image?url=%2Fboop.jpg&w=640&q=75 640w, 
            /_next/image?url=%2Fboop.jpg&w=750&q=75 750w, 
            /_next/image?url=%2Fboop.jpg&w=828&q=75 828w, 
            /_next/image?url=%2Fboop.jpg&w=1080&q=75 1080w, 
            /_next/image?url=%2Fboop.jpg&w=1200&q=75 1200w, 
            /_next/image?url=%2Fboop.jpg&w=1920&q=75 1920w, 
            /_next/image?url=%2Fboop.jpg&w=2048&q=75 2048w, 
            /_next/image?url=%2Fboop.jpg&w=3840&q=75 3840w" 
    style="visibility: visible; position: absolute; inset: 0px; box-sizing: border-box; padding: 0px; border: none; margin: auto; display: block; width: 0px; height: 0px; min-width: 100%; max-width: 100%; min-height: 100%; max-height: 100%;"
  />
</div>

The image URL was changed from /boop.jpg to /_next/image?url=%2Fboop.jpg&w=750&q=75.

Let's break down what this does:

/_next/image?url=%2Fboop.jpg&w=750&q=75
^^endpoint^^     ^^^image^^^ ^^config^^ 
  • /_next/image — this is an endpoint where the image will be sent for processing
  • ?url=/boop.jpg — where the endpoint should load the image from
  • &w=750 — resize the image to 750px wide
  • &q=75 — resample the image at 75% quality to reduce the file size

Because this endpoint results in a unique URL, the result can be cached to ensure the image processing is only performed once.

How this repo adds next/image support for other platforms

To provide the next/image approach on any platform, including fully static exports, we're doing two things here:

1. Create a serverless function to act as the image processing endpoint

The code in functions/image.js loads the image and processes it using Jimp. The same url, w, and q parameters are supported for compatibility with the /_next/image endpoint API.

To test this, you can call the function directly like so:

https://next-image-on.netlify.app/.netlify/functions/image?url=/jason-rogers.jpg&w=400&q=75

This works with external images as well:

https://next-image-on.netlify.app/.netlify/functions/image?url=https://lengstorf.com/images/jason-lengstorf.jpg&w=400&q=75

2. Use a rewrite rule to send /_next/image to the serverless function

To tell next/image to use our serverless function, we could mess around with the configuration. However, since we already have a netlify.toml, we're going to use a rewrite rule to rewrite all traffic from /_next/image to /.netlify/functions/image.

[[redirects]]
  from = "/_next/image*"
  query = { url = ":url", w = ":width", q = ":quality" }
  to = "/.netlify/functions/image?url=:url&w=:width&q=:quality"
  status = 200

This allows us to use next/image without any modifications to the Next.js config, and the images will Just Work™.

As an added bonus, this means we can update the rewrite to use something like Cloudinary by changing one line in the rewrite rule:

  [[redirects]]
    from = "/_next/image*"
    query = { url = ":url", w = ":width", q = ":quality" }
-   to = "/.netlify/functions/image?url=:url&w=:width&q=:quality"
+   to = "https://res.cloudinary.com/jlengstorf/image/fetch/w_:width,q_auto,f_auto/https://next-image-on.netlify.app/:url"
    status = 200

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