Before we talk about data fetching, let’s talk about one of the most important concepts in Next.js: Pre-rendering.

By default, Next.js pre-renders every page. This means that Next.js generates HTML for each page in advance, instead of having it all done by client-side JavaScript. Pre-rendering can result in better performance and SEO.

Each generated HTML is associated with minimal JavaScript code necessary for that page. When a page is loaded by the browser, its JavaScript code runs and makes the page fully interactive. (This process is called hydration.)

Check That Pre-rendering Is Happening

You can check that pre-rendering is happening by taking the following steps:

1. Disable JavaScript in your browser. (Here’s how in Chrome).
2. Try accessing this page (the final result of this tutorial).

You should see that your app is rendered without JavaScript. That’s because Next.js has pre-rendered the app into static HTML, allowing you to see the app UI without running JavaScript.

Note: You can also try the above steps on localhost, but CSS won’t be loaded if you disable JavaScript.

If your app is a plain React.js app (without Next.js), there’s no pre-rendering, so you won’t be able to see the app if you disable JavaScript. For example:

• Enable JavaScript in your browser and check out this page. This is a plain React.js app built with Create React App.
• Now, disable JavaScript and access the same page again.
• You won’t see the app anymore — instead, it’ll say "You need to enable JavaScript to run this app." This is because the app is not pre-rendered into static HTML.

Summary: Pre-rendering vs. No Pre-rendering

Here’s a quick graphical summary:

Next, let’s talk about two forms of pre-rendering in Next.js.