Set a breakpoint in XHR in Chrome

Question

I have a page that sends an XHR request when a form is submitted and I would like to get Chrome to break when it receives a response. It seems like the best way to accomplish this would be if Chrome has a javascript function that I can call that breaks execution but I've been unable to find anything like that so far. Is there another solution?

Edit:

I don't actually have a callback defined for the request so I can't set a breakpoint that way. The request is being sent with this line of jquery code:

$.post(this.action, $(this).serialize(), null, "script"); 

where this is a form element. The null argument is where you would usually define a callback but with the "script" argument, raw javascript is returned by the server and then directly executed, so it seems the only way to break and step through the code is with the debugger; statement. This works, but when stepping through the code you can't actually see which line you are on so its a little awkward. I suspect that this is a limitation of Chrome's debugging tools.

Answer 1

drop down the chrome console (ctrl+shift+j) and type any of these:

Just rewrite the jquery ajax:

var prevajax = jQuery.ajax; jQuery.ajax = function () { debugger; return prevajax.apply(jQuery, arguments); }; 

or if you are not using jQuery, rewrite the xhr class:

var prevxhr = XMLHttpRequest; XMLHttpRequest = function (){debugger; prevxhr.apply(this, arguments);}; 

After it breaks, just press shift+f11 until you find the method which initiates the ajax request.

Answer 2

You can just set a breakpoint in the success callback and step through the debugger. To set a breakpoint:

1. Open "Developer Tools", and click the "Scripts" tab on the top.
2. Select the script that contains the success callback for your AJAX request.
3. Click the line number on the left hand side where you want the breakpoint. A blue arrow will show up indicating the breakpoint has been set.
4. Then make the AJAX request as you would, and the debugger will automatically stop at the breakpoint you set.

Alternatively, you can use the debugger statement to automatically invoke the debugger. So with this your success callback may look like:

success: function(data, textStatus, request) {     debugger; // pause execution and opens debugger at this point     ... } 

Also checkout this nice article for debugging JavaScript.

However, the first approach is better as it doesn't require you to modify your code.