Debugging Racing Conditions in Magento In JavaScript

Racing conditions in web development refer to scenarios where the outcome of a task depends on the timing or order in which certain events occur. These events could be related to server-side or client-side processing, and can impact the functionality or performance of a website or web application.

One common example of a racing condition in web development is when JavaScript files are loaded asynchronously without honoring their dependencies. This means that certain scripts may be loaded and executed before other scripts that they depend on have finished loading. This can cause errors or unexpected behavior, as the dependent scripts may not have the necessary resources or information available to execute correctly.

To prevent racing conditions and ensure that your website or web application functions as intended, it’s important to carefully manage the loading and execution of your scripts. This may involve using techniques like lazy loading or module bundling to control the order in which scripts are loaded, or using tools like Promises or async/await to manage the flow of execution. By proactively addressing racing conditions, you can improve the reliability and performance of your web applications and provide a better experience for your users.

What was my problem?

At one of my customer’s websites, there were occasional instances where some JavaScript on the site would fail. This happened infrequently, but when it did, it negatively impacted the usability of the webshop for some customers. From what I could tell, the issue seemed to be related to a module in the merged JavaScript that would crash because jQuery was not available at the time. Strangely, this issue never occurred in any local development environments, so I had to figure out a way to replicate the problem locally in order to find a solution.

How can a JavaScript loading racing condition be triggered in Magento2?

I had an idea to try and cause the JavaScript loading issue to occur in my local development environment by delaying the loading of the jQuery source file. I didn’t want to modify the webserver directly, so I looked for ways to manipulate how the file was being loaded without altering the internal workings of the web server. I discovered that I could use the .htaccess files to achieve this, which of course only works if you use Apache for your local development environment.

First, I checked the path to the jQuery file. In this case, it was static/frontend/shop/theme/de_DE/jquery.js. In the .htaccess file, there was a general rewrite rule that redirected all requests to files that could not be found directly to the pub/static.php file. So, I added the following line just before that RewriteRule:

RewriteCond %{REQUEST_FILENAME} static/frontend/shop/theme/de_DE/jquery.js

RewriteRule .* ../static.php?resource=$0 [L]
// ...

This makes sure that all requests to the jQuery source file go through pub/static.php, now.

I added the following lines to the file pub/static.php right after the opening PHP tag.

if ($_REQUEST['resource'] == 'frontend/shop/theme/de_DE/jquery.js') {

By delaying the loading of (only) jQuery, I was able to ensure that any plugins with faulty dependencies would immediately fail. This also works with other files. By causing the issue to occur every time, I was able to debug and fix the dependency declaration, confident that it would work consistently on the live system.