Javascript Google Maps

Understanding JavaScript Load Order

When developing web applications, ensuring that JavaScript files load in the correct order is essential to avoid errors and ensure that dependencies are met. This is particularly important when working with libraries like ActiveMQ (amq.js) and APIs like Google Maps. In this post, we will dive into an issue faced by many developers regarding the loading order of scripts and how to resolve it.

The Problem Explained

A developer recently encountered an issue when trying to integrate both ActiveMQ and Google Maps into their application. The scripts were loaded in a specific order, but the application still failed to subscribe to a Topic using AMQ.

Here’s the loading order they used:

<head>
    <meta http-equiv="content-type" content="text/html;charset=UTF-8" />
    <title>AMQ & Maps Demo</title>

    <!-- Stylesheet -->
    <link rel="stylesheet" type="text/css" href="style.css" />

    <!-- Google APIs -->
    <script type="text/javascript" src="http://www.google.com/jsapi?key=abcdefg"></script>

    <!-- Active MQ -->
    <script type="text/javascript" src="amq/amq.js"></script>
    <script type="text/javascript">amq.uri='amq';</script>

    <!-- Application -->
    <script type="text/javascript" src="application.js"></script>
</head>

Despite correctly loading the Google Maps API and ActiveMQ, the developer received an error stating that the “object is not defined,” specifically for a dependency (Prototype) required by amq.js. This raised a crucial question: “Is there a way to make sure both scripts load before I use them in my application.js?”

The Solution

1. Understanding the Load Order

It is critical to note that:

JavaScript files typically load sequentially and block subsequent scripts from executing until they are fully loaded.
Therefore, loading application.js after the other scripts should ideally suffice.

2. Using Callbacks for Initialization

Although both Google Maps and ActiveMQ should work under normal circumstances, using a callback approach can enhance reliability, particularly when dealing with asynchronous loading.

Google Maps Callback

Google Maps provides a built-in function named google.setOnLoadCallback(initialize); that ensures the Google Maps API is fully loaded before executing any further code. You can utilize similar techniques:

function initialize() {
    // Google Maps initialization code here
}

// Ensuring Google Maps loads before the application initializes
google.setOnLoadCallback(initialize);

Handling ActiveMQ Initialization

Unfortunately, amq.js does not inherently provide a similar callback function. Therefore, you may need to wrap your application logic in a function that checks if amq.js has properly loaded before proceeding.

function initAMQ() {
    if (typeof amq !== 'undefined') {
        // AMQ-related logic here (e.g., subscribing to topic)
    } else {
        console.error('ActiveMQ script not loaded.');
    }
}

// Calling initAMQ after verifying the script load
initAMQ();

3. Review of Non-blocking Techniques

If you wish to explore more advanced techniques, you can delve into non-blocking JavaScript downloads, which minimize blocking resource loading. Refer to the YUI Blog’s article on Non-blocking Script Downloads for more in-depth strategies.

Conclusion

In conclusion, managing JavaScript load order is critical when developing applications that rely on multiple libraries and APIs. By understanding that scripts load sequentially, utilizing callback functions where available, and checking for dependencies, you can ensure a smoother integration of ActiveMQ and Google Maps.

Avoid frustration while coding; remember to control your script’s loading sequence!