We Keep Coding

What are the rights ways of debugging javascript?

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.

Multiple Different Javascript Errors Can't Duplicate

I am using a window.onerror=function(msg, url, linenumber) to send myself an email when a javascript error occurs. When I use my webpages (IE, FF, CHRM, ect) I do not receive any errors. However I am getting all kinds of errors emailed to me from other users that I cannot duplicate. I am probably getting an error happening on 1% of the users visiting the website so 1/100 page loads I get an error. Here are some:
Syntax error: http://www.google-analytics.com/ga.js
Error loading script: http://www.google-analytics.com/ga.js
Error loading script: https://seal.thawte.com/getthawteseal?host_name=XXX&size=M&lang=en
$("input.datepicker").datepicker is not a function
Uncaught SyntaxError: Unexpected token
Object doesn't support this property or method
Uncaught TypeError: Object [object Object] has no method 'dialog'
Syntax error: http://seal-cencal.bbb.org/logo/XXX
Error: Permission denied to access property 'toString'
What I don't understand is the website 99% of the time is running without any errors and I can't seem to get an error to happen for me on any browser I use. I am also tracking what browsers the users are using who send these errors and it's random, all sorts of browsers and they are all newer FF, IE versions mainly.
How do you stop syntax errors on external code (but they work most of the time)...?
How come google and thawte scripts fail to load some people so often but never happens to me, is there a way to have a fallback to this?
The datepicker one, I can only assume its error out because jquery UI failed to load because jquery loads (otherwise it wouldn't get called inside a document ready call) but I even have a localhost jquery UI script fallback include and it still happens..
Does anyone have a suggestion for me to help me further debug/fix these problems. The thing is, the website gets a lot of hits I am getting about 1 error emailed to me every minute (and it's not the same user causing each error). If you have any suggestions let me know (remember the code and website work fine 99% of the time so I don't think its a syntax problem on my end anywhere).
Thanks for your time.

How do you stop syntax errors on external code (but they work most of the time)...?
You can't really. Without a more detailed message investigation is quite impossible. Notice that sysntax errors might be caused by something else as well, their origin might not be in the external script. I doubt Google Analyticts serves invalid scripts.
How come google and thawte scripts fail to load some people so often but never happens to me
They might use adblockers or similiar against those. Not everyone wants his web usage logged by google. Also, some proxies might ignore/block them, like intranet firewalls.
is there a way to have a fallback to this?
You could try serving them from your own domain as a proxy. Or just ignore them, I don't think your app really needs a fallback for those.
The datepicker one, I can only assume its error out because jquery UI failed to load as jquery still loads
Yes, that sounds likely. Or it just didn't load before it was called.
but I even have a localhost jquery UI script fallback include and it still happens..
Localhost? I guess you mean a copy on your server. Still, it depends on the details how the script and its fallback version are loaded.
Does anyone have a suggestion for me to help me further debug/fix these problems.
A stack trace would help a lot to locate the user's action that caused the error.

Most of the errors on the list are caused by errors in browser extensions.
If you really want to log JavaScript errors, it is best you also log the stack trace if available. I also log details about functions that execute and the arguments sent to it. When the error is caught I cut X amount of stuff from the stack I built and send that up as well.

Programatically retrieve count of javascript errors on page

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

Getting data from the browser's console using javascript

I don't know if this has been asked before, but what i'd like to be able to do is get data from the error console within the browser itself(if it supports it) this would be for when a user sends off a bug report it'd pull up any errors related to pages at my website for things such as typos in code and other things that somehow managed to slip by. Also, in that regard is there a way to pass the errors from the console to a useable format? If this isn't possible, then i could just tell them to copy and paste what came up from the site itself.
I thought of this right now as i was thinking about how to make the bug reporting system run better since the entire thing is basically ran within the browser and for the backend I can easily just look at error logs but for the frontend ie javascript bits of things it's not goign to be as easy.
So to finish wrap all of this up in one little statement, is there an easy way to get the data from the error console and be able to send it along via javascript ie to a form, or something similar.

You can use the onerror event in JS to get the details of the error. Hoptoad do this for example and log the errors to their console, Their code re-uses lots of nice JS scripts including a printStackTrace function that is great.....
You can see how they do it here:
http://hoptoadapp.com/javascripts/notifier.js

Output the rendered contents of a page on a JavaScript error

I'm having problems with getting decent JavaScript error invormation in a Production environment.
When I'm developing I can just attach a debugger and (usually) fix the problem.
When I get the same error in a production environment however at best I see is an error report that looks like this:
Error: Object doesn't support this property or method
Url: SomePage
Line: 42
Char: 13
Which doesn't help me very much - I can't see the rendered page and so I have no idea what line 42 looks like.
Is there any way for me to log the entire rendered page contents whenever an error like this occurs? (So line 42 of the output is the line where the error occured)
While I'm at it, are there any other techniques that I can use to help with getting useful error information from JavaScript (without need to break into the debugger) - failing that is there any way that I can structure my JavaScript slightly differently to help getting decent debug information?
I'm predominantly interested in IE - this is the browser that tends to cause me most problems.

I don't think you'll be able to get the exact original HTML source of the page back in all pages and all browsers.
Regarding debugging, you could use a logging library such as log4javascript (disclaimer: I wrote it) and intersperse logging calls in your code. log4javascript enables you to send logging messages back to the server via Ajax.

Unfortunately, IE has by default the most utterly useless error reporting. The script and line number reported in the error are essentially guaranteed to be absolutely wrong. You can, however, install the IE developer tool bar (for IE7 and older, it's built into IE8) from Microsoft, which can help track down the error source.