Fix: Ajax Request Returns 200 OK But Error Event is Triggered

Introduction

We'll be looking at a peculiar issue that you may have encountered while working with AJAX and jQuery - the AJAX request returns a 200 OK status, but an error event is triggered. Things like this can be puzzling and frustrating, especially since the request returned a 200 OK status.

Understanding AJAX

Before we get into the problem, let's get a quick overview of AJAX. AJAX is a set of web development techniques used for creating asynchronous web applications. With AJAX, web applications can send and retrieve data from a server asynchronously, without blocking the main thread.

So basically, AJAX allows for the smooth exchange of data between server and browser, without the need for a complete page refresh. This is a much more user-friendly web experience.

Brief Overview of AJAX and jQuery

Now, let's talk about AJAX in the context of jQuery. jQuery, a fast, small, and feature-rich JavaScript library, gives us a simple interface for doing many DOM and web-related tasks, like sending AJAX requests. It abstracts many of the complexities of AJAX, making it easier to use.

For instance, we can use the $.ajax() method in jQuery to handle AJAX requests. It allows you to send HTTP requests and handle the responses in a more streamlined way. Here's a simple example:

$.ajax({
    url: 'http://example.com/api',
    type: 'GET',
    dataType: 'json',
    success: function(data) {
        console.log(data);
    },
    error: function(err) {
        console.error(err);
    }
});

In this example, we're sending a GET request to 'http://example.com/api', expecting a JSON response. If the request is successful, we log the response data. If an error occurs, we log the error.

Understanding the Error

Now, let's get to the heart of the matter, why AJAX would fire an error event even when the request returns 200 OK. So let's say you've set up your AJAX request, you're getting a 200 OK status from the server, but an error event is being fired. What's going on?

Let's say your AJAX code contains dataType: "json". In this case, jQuery tries to evaluate the response as JSON and return a JavaScript object. However, the JSON data is parsed in a strict manner. This means that any malformed JSON is rejected, and a parse error is thrown. An empty response is also rejected; the server should return a response of null or {} instead.

In another scenario, your server-side code might be returning an HTML snippet with a 200 OK status. jQuery, expecting valid JSON, fires the error callback, complaining about a parseerror.

These are just a few examples of why this error might occur. The key point here is that the error usually arises from a mismatch between what jQuery expects based on the dataType parameter and what the server actually returns. Understanding this is the first step towards fixing the problem.

In the next sections of this Byte, we'll explore why this error occurs in more depth, how to fix it, and how to debug AJAX errors in JavaScript.

Fixing the Error

Let's say your server-side code is returning an HTML snippet with a 200 OK status. Since jQuery is expecting valid JSON, it fires the error callback, complaining about a parseerror. To solve this, you can remove the dataType parameter from your jQuery code and have the server-side code return a different response.

Content-Type: application/javascript

alert("Record Deleted");

However, a better approach is to return a JSON response and display the message inside the success callback:

Content-Type: application/json

{"message": "Record deleted"}

Since this is a JSON-parseable string, the error event won't fire and you can instead handle the alert client-side.

How to Debug AJAX Errors in JavaScript

AJAX errors can be difficult to figure out, unless you have a more systematic approach to debugging. Here are a few tools that can save you hours of frustration:

1. Check the Network Tab: The Network tab in your browser's developer tools can provide insights into the AJAX request. You can check the request and response headers, the request method, the status code, and the response body.

2. Inspect the Console: The console is your best friend when debugging JavaScript. Look for any error messages or warnings that might be related to your AJAX request.

3. Use console.log: This is a simple yet powerful debugging tool. You can log the data returned by the server, the status code, and any error messages. This tends to be my favorite since it's so simple to use.

4. Use Breakpoints: If your AJAX code is more complex, consider using breakpoints in your developer tools to pause execution and inspect the variables and execution flow. Although this can be difficult to follow if you have minified/obscured code.

Conclusion

In this Byte we shed some light on a confusing issue in which an error event is fired, even though the AJAX call returned 200 OK. We also saw how to fix it and a few ways to improve your debugging of issues like this. In this case, the key is to understand what your AJAX code expects and what the server is returning.