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API Migrating legacy JavaScripts

Legacy code often contains JavaScripts that expect a full page load whenever the user interacts with the page. When you configure Unpoly to handle all interaction, there will not be additional page loads as the user clicks a link or submits a form.

This may cause some of the following issues:

  1. The window will not emit another DOMContentLoaded event when you update the page body through Unpoly. This may cause your legacy JavaScript to not run when you expect it to.
  2. Legacy JavaScript often enhances elements on the entire page instead of scoping its changes to a region of the DOM. When we're updating fragments we only want to enhance elements within the new fragment, and don't want to touch elements that we have already enhanced.
  3. Legacy JavaScript may have memory leaks, e.g. by scheduling timers with setInterval() but never clearing these timers when they're no longer needed. When you're resetting the page with every click you may never notice the leaks. However, when all links are handled through Unpoly the JavaScript VM may persist for minutes or hours and those leaks are going to stack up.

The cleanest solution to these issues is to call all your JavaScript from an Unpoly compiler.

Migrating legacy scripts to a compiler

The legacy code below waits for the page to load, then selects all links with a .lightbox class and calls lightboxify() for each of these links:

document.addEventListener('DOMContentLoaded', function(event) {
  document.querySelectorAll('a.lightbox').forEach(function(element) {

Since the code only runs after the initial page load, links contained in a fragment update will not be lightboxified.

You can fix this by moving your code to a compiler:

up.compiler('a.lightbox', function(element) {

When the page initially loads, Unpoly will call this compiler for every element matching a.lightbox. When a fragment is updated later, Unpoly will call this compiler for new matches within the new fragment.

Running inline <script> tags

By default Unpoly does not execute <script> tags in an updated fragment to prevent unwanted side effects. This means that your <script> tag will only be executed during the initial page load, but not when navigating to a new page via an a[up-follow] link.

It is recommended to move all your JavaScript from <script> tags into a compiler. This ensures your code is called after both page loads and fragment updats. A compiler also enables you to scope the side effects of your code to the DOM fragment that was updated.

If you cannot migrate your <script> tags to a compiler, you you can also configure Unpoly to run <script> tags in updated fragments:

up.fragment.config.runScripts = true

If you do this, mind that the <body> element is a default main target. If you are including your global application scripts at the end of your <body> for performance reasons, swapping the <body> will re-execute these scripts. In that case you must configure a different main target that does not include your application scripts.

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