How to Fix the undefined Issue in XMLHttpRequest with Tampermonkey

Learn how to effectively handle XMLHttpRequest results in Tampermonkey using Promises and async/await to prevent `undefined` errors and improve your JavaScript code.
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Handling XMLHttpRequest Results in Tampermonkey

If you're working with JavaScript in Tampermonkey and encountering issues where your function is logging undefined after making an XMLHttpRequest, this post is for you. Many users run into the problem of asynchronous calls, where a function tries to use data before it's available. Here, we’ll dive into how to properly handle these requests using Promises and async/await, ensuring your functions work harmoniously.

The Problem Statement

When you make an XMLHttpRequest in a Tampermonkey script, you might find that the result of your request isn't available immediately. Instead, your code may log undefined, which can be confusing. For example, you could have a function like kaminoan that calls another function weatherPull to fetch weather data. However, if you directly use the result from weatherPull without waiting for it to finish, it leads to logging undefined instead of the fetched data.

Understanding Asynchronous Behavior

AJAX requests, such as those made with GM_xmlhttpRequest, are inherently asynchronous. This means that the JavaScript engine doesn't wait for the request to complete before moving on to execute the next line of code. Here's where the confusion often arises for developers.

Key Questions

Do I have to use async/await?

How can I return the result from a function after the request is complete?

Solution Overview

The solution to these problems lies in wrapping your XMLHttpRequest call in a Promise. This allows you to handle the asynchronous nature of the request effectively.

Step 1: Wrapping the XMLHttpRequest in a Promise

You will create a new function, weatherPull, that returns a Promise. This Promise will resolve with the data you want when the XMLHttpRequest completes.

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

Step 2: Consuming the Result

Now that your function returns a Promise, you can handle the result in another function, kaminoan. You can do this using the .then method to get your data or catch any errors that might occur.

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

Alternative: Using Async/Await

If you'd rather use async/await syntax, which many find cleaner and more readable, you can achieve the same result as follows:

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

Conclusion

By wrapping your XMLHttpRequest in a Promise, you can effectively manage asynchronous behavior in JavaScript. This allows your functions to wait for the data they need before proceeding, thus avoiding the dreaded undefined. Whether you prefer using Promises with .then or the cleaner async/await syntax, both approaches will help you retrieve data from your requests in Tampermonkey smoothly.

Takeaway

Always remember: Handle asynchronous operations thoughtfully to avoid errors in your scripts.

Experiment with both Promises and async/await to find out which style you prefer!