Identifying a failed Ajax Request in JavaScript

How can I check if an Ajax request failed to load a file?

Here is the code I'm currently using:

var pro = undefined;
var xmlhttp;
if (window.XMLHttpRequest){
    xmlhttp = new XMLHttpRequest();
}
else{
    xmlhttp=new ActiveXObject("Microsoft.XMLHTTP");
}
xmlhttp.onreadystatechange = function(){
    if (xmlhttp.readyState==4 && xmlhttp.status==200){
        pro = JSON.parse(xmlhttp.responseText);
    }
}
xmlhttp.open("GET","data.json",true);
xmlhttp.send();

Please note that I am not using jQuery.

Thank you!

javascriptajaxxmlhttprequest

Answer №1

One way to determine if an error occurred in a request is by checking the HTTP status code. If the status code is 500 or greater, it typically indicates an error. You can learn more about HTTP status codes here.

if (xmlhttp.status >= 500){
    alert('error');
}

NOTE: You may also want to consider other HTTP status codes depending on your needs.

Answer №2

onerror callback is essential for effectively managing errors, as demonstrated here:

xmlhttp.onerror = function onError(e) {
    alert("Error " + e.target.status + " occurred while receiving the document.");
}

To my understanding, jQuery library also utilizes this approach. While I am unsure of its compatibility with all browsers, it has been confirmed to work seamlessly in Firefox.

Answer №3

I have a clever solution where I utilize a global variable called 'boomerang'.

When sending out the request, set boomerang=1 and upon success, set it to boomerang=0.

At a suitable time after sending the request, simply check if boomerang equals 1 or 0.

Answer №5

You really won't require anything extra. Make sure to include an action in the else statement besides just leaving a comment.

xmlhttp.onreadystatechange = function(){
    if (xmlhttp.readyState==4 && xmlhttp.status==200){
        pro = JSON.parse(xmlhttp.responseText);
    } else {
        if (xmlhttp.readyState==4) {
            // Attempt unsuccessful
        }
    }
}

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