How to Wait for a Dynamically Loaded JavaScript File to be Ready

Learn how to effectively handle dynamic JavaScript loading with Promises to ensure your functions execute at the right time.
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Understanding the Challenge of Dynamically Loading JavaScript Files

As web developers, we often find ourselves needing to include JavaScript files dynamically—whether for performance or modularity purposes. However, there's a common pitfall: the asynchronous nature of JavaScript file loading can cause issues if your script tries to use a function or variable before it has been loaded. This can lead to errors such as $ is not defined when working with libraries like jQuery.

The Problem in Action

Here's what typically happens when loading a JavaScript file dynamically:

You attempt to include a file.

Immediately after, you try to use a function from that file.

However, the file might not have completed loading yet, resulting in errors.

Example Scenario

Below is a simplified example illustrating this issue:

[[See Video to Reveal this Text or Code Snippet]]

In this code, when the script tries to run the jQuery ready function, jQuery may not have loaded completely yet, resulting in an error.

The Solution: Using Promises for Safe Loading

To avoid this issue, we can leverage JavaScript Promises to handle the loading of our dynamically included script files. Here’s how it works:

Step-by-Step Implementation

Create a Promise: When the script is being included, create a Promise that resolves when the script is fully loaded.

Use the Promise: After the script has loaded, use the .then() method to execute the code that relies on the included script.

Example Code

Here's how you can modify the previous example to use Promises for loading jQuery safely:

[[See Video to Reveal this Text or Code Snippet]]

Breakdown of the Code

Promise Creation: A new Promise is created, which will resolve when the script is loaded.

onload and onerror Handling: These event handlers ensure that the Promise is resolved or rejected based on whether the script loaded successfully.

Using .then(): After the Promise resolves, we can safely call any jQuery methods since it's guaranteed that jQuery is available.

Conclusion

Using Promises to manage the loading of JavaScript files not only prevents errors but also makes your code cleaner and easier to understand. By adhering to this approach, you ensure that your scripts run in the correct order, enhancing the reliability of your web applications.

Feel free to implement this pattern in your projects where dynamic JavaScript loading is involved!