A full page load is where the entire page is downloaded from the server. A page typically consists of several sections: header, footer, navigation, and content. In a classic web application without AJAX, a user clicks on a link to another page, and has to download the full page, even though only the main content is changing. The header, footer, and navigation all get downloaded again even though they don't change.

With AJAX there is the opportunity to only change the parts of the page that will change. When a user clicks on the link, JavaScript loads just the content for that link and inserts it into the current page. The header, footer, and navigation don't need to reload.

This introduces other problems that need attention.

• When AJAX inserts new content into the page, the URL doesn't change. That makes it difficult for users to bookmark or link to specific content. Well written AJAX applications use history.pushState() to update the URL when loading content via AJAX.

• There are then two paths to get to every piece of content. Users can either load the URL containing that content directly, or load the content into some other page by following a link. Web developers need to test and ensure both work.

• Search engines have trouble crawling AJAX powered sites. For best compatibility, you need to employ server side rendering (SSR) or pre-rendering to serve initial content on a page load that doesn't require JavaScript.

  Even for Googlebot (which executes JavaScript) care must be taken to make an AJAX powered site crawlable. Googlebot doesn't simulate user actions like clicking, scrolling, hovering, or moving the mouse.

  • Content needs to appear on page load without any user interaction
  • You must use <a href=...> links for navigation so that Googlebot can find other pages by scanning the document object model (DOM). For users, JavaScript can intercept clicks on those links and prevent a full page load by using return false from the onclick handler or event.preventDefault() in the click handler.