I'm working on a new project which has some complex javascript. I can't post any code so that's not what my question is about.
I have a script which works in Firefox 3.0. It was pointed out that the script did not work in Firefox 3.5, so I'm trying to make it work. Indeed the script didn't produce the expected results, so I installed the latest version of Firebug, enabled the console and refreshed the page.
And wow, it worked.
No errors, warnings nothing.
So I disabled the console, and then it didn't work anymore...
What's going on here? The Firebug console somehow changes something in Firefox that makes my script work? Any advice on what next? (besides asking future visitors to install Firebug...)
Could it be something as simple as forgetting to comment a call to console.log() somewhere in your javascript?
If you have hanging references, and the user doesn't have Firebug installed, you're going to get a runtime error that will halt execution of the script.
It sounds to me like there's a chance you have a threading problem, and FireBug is analyzing and possibly slowing down one of the threads so that it has time to complete before the next step is resolved.
Are you possibly utilizing ajax, and something is waiting on that response? Or possibly you're doing something on or after the load of an object that is depending on something else in the DOM?
UPDATE:
For those stumbling upon this now, "threads" in JavaScript really only exist in abstraction (web workers, etc). I was mis-using the term. I was really thinking of an asynchronous action that returned before another one was ready.
Check in your code for console.log(), console.debug().Calling window.console objects methods throws an error if console is undefined (as expected).
In most cases you can easily delete or comment that lines.
I wrote a simple wrapper for firebug (I just use debug but it should give you what you need to duplicate the other methods) that only writes when the console is there so I can use firebug, don't need to go comment out my debug statements and it doesn't break sites for people without it.
If you use this code then use fbconsole.debug instead of console.debug you will never have this problem:
function fbconsole () {
this.debug = function (val) {
if(typeof(console) !== 'undefined' && console != null) {
console.debug(val);
}
}
}
var fbconsole = new fbconsole();
Related
I want to see the warnings in order to do a better debugging of my application which is made in Asp.Net, so it has a lot of code generated in javascript. The problem is that pages reload fast and I can't take a look at the warning messages. I want to know if there is a way to break on warnings in Google Chrome, as in this question is described how to break on errors. Is there a way to do this, or what alternatives do I have?
As #Nitsew has already said, preserving the log is a great solution. But just for fun, and because I thought your question was interesting, I decided to work out a solution to do what you asked; cause a debug to occur when warn is used:
function testBreak() {
console.warn('This is a warning', 1, 2);
return 1;
}
var oldWarningFunction = console.warn;
console.warn = function () {
debugger;
oldWarningFunction.apply(console, arguments);
};
console.log(testBreak());
Essentially, I have monkey-patched the original console.warn to output a debugger statement. When this causes the debug to occur, you will probably need to view the stack trace to see where you were in the code:
Once you click back one on that stack trace, you can see the function that made the call.
This all seems rather silly since you could just write your own debugger statement right in your code where you want them. And one other thing, to whoever may read this and tries it: Do not leave this in your production code
If you are just trying to look at the console messages, turning on "Preserve log" might be an easier solution.
I made a function called test() in javascript file.Placed a simple alert into it.
In html file, called the method on click of a button. But,it was not being invoked.
Problem was in the 11th function, nowhere related to mine !!!! But, how can a person making his first javascript function suppose to find that out ???
I am looking for best ways to debug javascript.
You can debug javascript using many modern browsers. See this question for details on how to debug in Google Chrome:
How do you launch the JavaScript debugger in Google Chrome?
Furthermore, you shouldn't use alert() for debugging as this can give different results to a production version due to alert() causing a pause in the script.
It is best practice to use console.log() and view the output in the browsers Console.
You can also put debugger in your javascript code to force a breakpoint. However I prefer not to use this as forgetting to remove this before deployment will cause your script to pause, which can be quite embarrassing!
You should use the debug console provided by the browser.
Chrome has it inbuilt, press CTRL + SHIFT + j. In Firefox, install Firebug plugin.
In your code, add alert() to show flow and get values of variables.
Also, use console.log() which will only output to the debug console.
Depending on your browser choice there are debugging options - I tend to use Firefox, so Firebug in my case. There is a question that list options for other browsers - What is console.log and how do I use it?
Unless the project you're working on has already adopted a mechanism for debugging, console.log() tends to be a simple and useful option when tracking down a problem.
Whilst debugging you could take the approach to log out a line when entering a function, like so:
var myFunc = function(el) {
console.log('Inside myFunc');
// Existing code
};
This will enable you to see which functions have been called and give you a rough idea of the order of execution.
You can also use console.log() to show the contents of variables - console.log(el);
Be mindful to remove/disable console.log() calls once you're done as it will likely cause some issues in production.
To answer your question within question,
how can a person making his first javascript function suppose to find that out ???
Well, when something is wrong in JavaScript, for example, you made a syntax error - the script will stop working from there. However, this won't stop HTML from rendering on, so it might look as if everything is correct (especially if your JS is not changing the look of the page) but all the functionality of JS will be dead.
That's why we use the debug tools (listed in the other answers here) to see what's wrong, and in cases like this, it's very easy to notice which function has errors and is causing the whole script to break. This would probably have save a few minutes to your seniors as well.
The best approach would be to test frequently so that whenever you run into errors, you can fix them right away.
I'd like to write a test case (using Selenium, but not the point of this question) to validate that my web application has no script errors\warnings or unhanded exceptions at certain points in time (like after initializing a major library).
This information can easily be seen in the debug consoles of most browsers. Is it possible to execute a javascript statement to get this information programatically?
It's okay if it's different for each browser, I can deal with that.
not so far read about your issue (as far as I understood your problem) here
The idea be the following:
I found, however, that I was often getting JavaScript errors when the page first loaded (because I was working on the JS and was introducing errors), so I was looking for a quick way to add an assert to my test to check whether any JS errors occurred. After some Googling I came to the conclusion that there is nothing built into Selenium to support this, but there are a number of hacks that can be used to accomplish it. I'm going to describe one of them here. Let me state again, for the record, that this is pretty hacky. I'd love to hear from others who may have better solutions.
I simply add a script to my page that will catch any JS errors by intercepting the window.onerror event:
<script type="text/javascript">
window.onerror=function(msg){
$("body").attr("JSError",msg);
}
</script>
This will cause an attribute called JSError with a value corresponding to the JavaScript error message to be added to the body tag of my document if a JavaScript error occurs. Note that I'm using jQuery to do this, so this specific example won't work if jQuery fails to load. Then, in my Selenium test, I just use the command assertElementNotPresent with a target of //body[#JSError]. Now, if any JavaScript errors occur on the page my test will fail and I'll know I have to address them first. If, for some strange reason, I want to check for a particular JavaScript error, I could use the assertElementPresent command with a target of //body[#JSError='the error message'].
Hope this fresh idea helps you :)
try {
//code
} catch(exception) {
//send ajax request: exception.message, exception.stack, etc.
}
More info - MDN Documentation
I get the following error in firebug in Firefox 3 with both MooTools and jQuery:
"p.onStatusChange is not a function".
I've noticed this error frequently in firebug since one of the latest updates of FF3. However, it has started appearing with code that hasn't been changed in some time and that was not reporting errors previously. The errors happens when ajax results are returned. It shows up in different applications that use separate javascript libraries, MooTools and jQuery.
Does anyone have any idea why these errors are appearing? My intuition tells me that it is something in Firefox that changed, but I can't find any information online currently. The ajax calls still work fine, but I am wary of just going with my intuition and leaving script errors in my code.
Thanks,
Jason
I get it in tabBrowser instead:
chrome://browser/content/tabbrowser.xml
(4) errors occur:
p.onStatusChange
p.onProgressChange
p.onStateChange
p.onSecurityChange
What I found was that the add-on "PDF Download" was causing these errors. The best way for me to check was to go to a page that produced the errors, turn off all the add-ons, and turn them on one-by-one (starting with Firebug). Instead of going one-by-one, I actually turned them on in lots of 3 to help identify the problem sooner.
Here is the reference for the function NsIDownloadProgressListener. It looks like it has been deprecated.
When a javascript error occures in IE (or in other browsers) you get a popup saying that javascript error has occurred - usually this comes with a line number and some hint.
Sometimes it comes with line 0 and with no way of knowing what the problem is.
Javscript can come from HTML itself, from a js file or from JSP (and more).
Microsoft has a script debugger that helps a lot in finding where js errors are, however sometimes when a js error occurs the script debugger cannot find the code portion and thus its difficult of finding where is the root cause of the problem.
My question is whether anyone knows any way of making script debugger find the code any way (mostly happen with js code that is in JSP file), or at least include in the IE popup the method or js file where the error has occurred. (it only display the line number, and many times its line 0...).
Thanks,
Tal.
The error object which is created when an error is thrown by JavaScript is very unreliable when it comes to the source line, especially within IE. Browsers like Firefox and Safari are better at line numbers, but they are generally pointless due to minification of the files.
What is obviously of more use is getting the call stack, but due to the anonymous nature of JavaScript functions (well, that they can be anonymous) a call stack can often be hard to work out.
If you're doing a try/ catch you can do arguments.callee which will return you the method which called the current method which failed.
Here's a good example of doing a complete stack in JavaScript - http://eriwen.com/javascript/js-stack-trace/
Also developer tools included with Internet Explorer 8 is something good to trace and debug your javascript code
There is a version of Firebug called Firebug Lite that will work with Internet Explorer. It's performance is going to be based on how complex your pages are; however, for relatively lightweight pages, it should provide some insight.
I recommend this tool rather than simply using Firebug and Firefox because not all errors that occur in Internet Explorer will occur in Firefox, and so performing any debugging in that browser may not yield any results.
Firebug on Firefox is usually considered one of the best debugging tools.
On Firefox, go to
http://getfirebug.com
to get it.
This will print you a stack trace:
function Stack()
{
try
{
throw Error()
}
catch(ex)
{
return ex.stack
}
};
print( Stack() );
If all else fails (and when dealing with IE it sometimes does) you can always walk through your code with alerts. It's crude and tedious, but sometimes it's all you can do:
Simply:
var count = 0;
then sprinkle some:
alert(count++);
at strategic lines along your code and note where it stops alerting.
Lather rinse repeat until you have your line.
If using Firefox you can press Ctrl + Shift + J to bring up the JavaScript error console that is built into Firefox, which will tell you exactly what went wrong.