How to Set up Netlify CMS Previews with Next.js

As part of rebuilding my website to use Next.js and Netlify CMS, I wanted to reuse components I made for Next.js as the editor preview in Netlify CMS. Unfortunately for me, I couldn't find any resources about how to do so.

After lots of trial and error, I found a way to initialize the CMS as part of a Next.js page, so I was able to import other components to use as the preview. Here's how I did it.

Disclaimer

I am not claiming that this is the absolute best way to do this. It is simply the method I discovered. One particular thing to note is that the CMS can take a few seconds to load, since it only starts loading after the first render.

I'm going to assume that you have a Next.js site set up and that the CMS is set up to be available at /admin using the <script> tag method.

First of all, you'll want to remove the page that's currently hosting the CMS (/public/admin/index.html). Then, move the Netlify CMS config.yml to the top level of the public directory.

Next, install the netlify-cms-app package. We're going to use this to initialise and customise the CMS.

yarn add netlify-cms-app

Now create a new admin page in Next.js by creating a /pages/admin.js file. To start with, just put a blank component in there so that the page loads but doesn't display anything.

const Admin = () => <div />;

export default Admin;

To actually load and display the CMS, we need to import the netlify-cms-app module. But here's the issue: as soon as the module is imported, it'll try to access the global window object. That won't work because Next.js will try to prerender or server-side render the page, so window will be undefined and the page will crash.

Instead, we'll use a dynamic import to only import the module once we know window is defined. We'll do that using the React useEffect hook and an inline anonymous async function (since dynamic imports are asynchronous).

import { useEffect } from 'react';

const Admin = () => {
  useEffect(() => {
    (async () => {
      const CMS = (await import('netlify-cms-app')).default;
      CMS.init();
    })();
  }, []);

  return <div />;
}

To break this down a bit:

• We use useEffect since it will run the code after the first render, when window is defined.
• We define an anonymous async function and then call it immediately. This is because useEffect doesn't allow the callback itself to be asynchronous.
• We await the import, and then access the default export of the module; that's the CMS object we need.
• Once the module has been imported, we initialise the CMS by calling init(). This loads the CMS and causes it to replace the current page (which was blank).
• The dependencies of useEffect are an empty array [] because this should only be run once, after the first render.

At this point opening /admin in the browser should display the Netlify CMS. If you get an error about not being able to find the config, make sure you moved /public/admin/config.yml up to /public/config.yml.

So far all we have achieved is replicating the existing setup. Let's move on to actually creating an editor preview.

The important function we need is CMS.registerPreviewTemplate. This allows us to register a React component for rendering the preview for a given collection in Netlify CMS.

The CMS will pass a number of props to this component, but we're only interested one for now: entry. This is an Immutable.js Map containing all the data about the entry. It will contain a key/value pair for each field defined for the collection.

Since I assume you already know how you want to render a preview, I'm going to assume that you have a collection called article and you've already created a component called ArticlePreview at /components/ArticlePreview.js. You'll need to update this component to accept the entry prop and extract the metdata from it.

Assuming an article has three fields – title, date, and body – here's how the component would extract the data to render it:

const ArticlePreview = ({ entry }) => {
  const title = entry.getIn(['data', 'title']);
  const date = entry.getIn(['data', 'date']);
  const body = entry.getIn(['data', 'body']);

  return (
    <div>
      {/* Render the preview here... */}
    </div>
  );
}

Here's how to register that component as the article preview:

import { useEffect } from 'react';

import ArticlePreview from '../components/ArticlePreview';

const Admin = () => {
  useEffect(() => {
    (async () => {
      const CMS = (await import('netlify-cms-app')).default;
      CMS.init();

      CMS.registerPreviewTemplate('article', ArticlePreview);
    })();
  }, []);

  return <div />;
}

All interaction with the CMS object has to be done in the async function, otherwise it will try to execute before the CMS is initialised.

At this point, opening an entry in the CMS should display using the preview component you defined and registered! I'd recommend reading up on registerPreviewTemplate to see what other props it passes, if you need to make more complex previews.

For further reference, my admin page is here, and my preview component is here. Note my preview component is just a wrapper around the actual PostPage that I use to render posts on the real site (I like this approach because the preview will always match the final page).