I'm trying to debug an issue that occur not on the first time, but on the second time I enter a page with Microsoft Edge.
This problem does not occur if I've got Developer Tools up; which makes it kind of tricky. I can't do anything with the page.
To complicate further I'm trying to debug an issue that occur on a page that integrate our solutions.
I know that console.log doesn't exist until you open Developer Tools, but I see nothing in the console. Are there other Developer Tools traps?
I doubt there's a single "right" answer here which might mean the question is off-topic for SO. I'll mark this answer CW; perhaps we can all add our suggestions here.
I know that console.log doesn't exist until you open Developer Tools, but I see nothing in the console.
That isn't true of Edge (or Chrome or Firefox), and wasn't true in at least the last couple of releases of IE. console.log always exists, and if you log something, then later open the dev tools, the previously-logged messages will be in the console. Of course, you can't see them until you open devtools. :-)
Some other things to try:
Perhaps the issue isn't hidden if devtools is open undocked? (Or if you normally use it undocked, try opening it undocked.)
Open a separate window and write to it via your own function.
Write messages via ajax to a server, observe them there.
In IE, React.js javascript is not working in thin client but its working in localhost. why?
I am working on React.Js JavaScript code. In other browsers its working fine. But in IE browser, Its not working. But when we open developer tools, then its getting updated.
I tried to update values in IE. It didn't work. But after opening developer tools, It's getting updated.
Please help me out.
This commonly happens because console isn't available in older versions of IE unless the dev tools are open - the development build of React has lots of console logging with helpful warnings and errors.
You can use something like https://github.com/paulmillr/console-polyfill to make sure your apps won't die when console logging is present.
See also answers to 'console' is undefined error for Internet Explorer.
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When I find that I have a problematic code snippet, how should I go about debugging it?
Firebug is one of the most popular tools for this purpose.
All modern browsers come with some form of a built-in JavaScript debugging application. The details of these will be covered on the relevant technologies web pages. My personal preference for debugging JavaScript is Firebug in Firefox. I'm not saying Firebug is better than any other; it depends on your personal preference and you should probably test your site in all browsers anyway (my personal first choice is always Firebug).
I'll cover some of the high-level solutions below, using Firebug as an example:
Firefox
Firefox comes with with its own inbuilt JavaScript debugging tool, but I would recommend you install the Firebug add on. This provides several additional features based on the basic version that are handy. I'm going to only talk about Firebug here.
Once Firebug is installed you can access it like below:
Firstly if you right click on any element you can Inspect Element with Firebug:
Clicking this will open up the Firebug pane at the bottom of the browser:
Firebug provides several features but the one we're interested in is the script tab. Clicking the script tab opens this window:
Obviously, to debug you need to click reload:
You can now add breakpoints by clicking the line to the left of the piece of JavaScript code you want to add the breakpoint to:
When your breakpoint is hit, it will look like below:
You can also add watch variables and generally do everything that you would expect in a modern debugging tool.
For more information on the various options offered in Firebug, check out the Firebug FAQ.
Chrome
Chrome also has its own in built JavaScript debugging option, which works in a very similar way, right click, inspect element, etc.. Have a look at Chrome Developer Tools. I generally find the stack traces in Chrome better than Firebug.
Internet Explorer
If you're developing in .NET and using Visual Studio using the web development environment you can debug JavaScript code directly by placing breakpoints, etc. Your JavaScript code looks exactly the same as if you were debugging your C# or VB.NET code.
If you don't have this, Internet Explorer also provides all of the tools shown above. Annoyingly, instead of having the right click inspect element features of Chrome or Firefox, you access the developer tools by pressing F12. This question covers most of the points.
Internet Explorer 8 (Developer Tools - F12). Anything else is second rate in Internet Explorer land
Firefox and Firebug. Hit F12 to display.
Safari (Show Menu Bar, Preferences -> Advanced -> Show Develop menu bar)
Google Chrome JavaScript Console (F12 or (Ctrl + Shift + J)). Mostly the same browser as Safari, but Safari is better IMHO.
Opera (Tools -> Advanced -> Developer Tools)
There is a debugger keyword in JavaScript to debug the JavaScript code. Put debugger; snippet in your JavaScript code. It will automatically start debugging the JavaScript code at that point.
For example:
Suppose this is your test.js file
function func(){
//Some stuff
debugger; //Debugging is automatically started from here
//Some stuff
}
func();
When the browser runs the web page in developer option with enabled debugger, then it automatically starts debugging from the debugger; point.
There should be opened the developer window the browser.
I use old good printf approach (an ancient technique which will work well in any time).
Look to magic %o:
console.log("this is %o, event is %o, host is %s", this, e, location.host);
%o dump clickable and deep-browsable, pretty-printed content of JS object. %s was shown just for a record.
And this:
console.log("%s", new Error().stack);
gives you Java-like stack trace to point of new Error() invocation (including path to file and line number!!).
Both %o and new Error().stack available in Chrome and Firefox.
With such powerful tools you make assumption whats going wrong in your JS, put debug output (don't forget wrap in if statement to reduce amount of data) and verify your assumption. Fix issue or make new assumption or put more debug output to bit problem.
Also for stack traces use:
console.trace();
as say Console
Happy hacking!
Start with Firebug and IE Debugger.
Be careful with debuggers in JavaScript though. Every once in a while they will affect the environment just enough to cause some of the errors you are trying to debug.
Examples:
For Internet Explorer, it's generally a gradual slowdown and is some kind of memory leak type deal. After a half hour or so I need to restart. It seems to be fairly regular.
For Firebug, it's probably been more than a year so it may have been an older version. As a result, I don't remember the specifics, but basically the code was not running correctly and after trying to debug it for a while I disabled Firebug and the code worked fine.
Although alert(msg); works in those "I just want to find out whats going on" scenarios... every developer has encountered that case where you end up in a (very large or endless) loop that you can't break out of.
I'd recommend that during development if you want a very in-your-face debug option, use a debug option that lets you break out. (PS Opera, Safari? and Chrome? all have this available in their native dialogs)
//global flag
_debug = true;
function debug(msg){
if(_debug){
if(!confirm(msg + '\n\nPress Cancel to stop debugging.')){
_debug = false;
}
}
}
With the above you can get your self into a large loop of popup debugging, where pressing Enter/Ok lets you jump through each message, but pressing Escape/Cancel lets you break out nicely.
I use WebKit's developer menu/console (Safari 4). It is almost identical to Firebug.
console.log() is the new black -- far better than alert().
My first step is always to validate the HTML and to check syntax with JSLint. If you have clean markup and valid JavaScript code then it is time for Firebug or another debugger.
Visual Studio 2008 has some very good JavaScript debugging tools. You can drop a breakpoint in your client side JavaScript code and step through it using the exact same tools as you would the server side code. There is no need to attach to a process or do anything tricky to enable it.
I use a few tools: Fiddler, Firebug, and Visual Studio. I hear Internet Explorer 8 has a good built-in debugger.
I used to use Firebug, until Internet Explorer 8 came out. I'm not a huge fan of Internet Explorer, but after spending some time with the built-in developer tools, which includes a really nice debugger, it seems pointless to use anything else. I have to tip my hat to Microsoft they did a fantastic job on this tool.
You might also check out YUI Logger. All you have to do to use it is include a couple of tags in your HTML. It is a helpful addition to Firebug, which is more or less a must.
I found the new version of Internet Explorer 8 (press F12) is very good to debug JavaScript code.
Of course, Firebug is good if you use Firefox.
Besides using Visual Studio's JavaScript debugger, I wrote my own simple panel that I include to a page. It's simply like the Immediate window of Visual Studio. I can change my variables' values, call my functions, and see variables' values. It simply evaluates the code written in the text field.
I'm using Venkman, a JavaScript debugger for XUL applications.
In addition to Firebug and browser-native developer extensions JetBrains WebStorm IDE comes with remote debug support for Firefox and Chrome (Extension required) built in.
Also supports:
coffescript: how to debug coffeescript in node.js with webstorm 6 source maps
node.js
Options to test this for free are the 30 trial or using an Early Access Version.
If you are using Visual Studio, just put debugger; above the code you want to debug. During execution the control will pause at that place, and you can debug step by step from there on.
As with most answers, it really depends: What are you trying to achieve with your debugging? Basic development, fixing performance issues? For basic development, all the previous answers are more than adequate.
For performance testing specifically, I recommend Firebug. Being able to profile which methods are the most expensive in terms of time has been invaluable for a number of projects I have worked on. As client-side libraries become more and more robust, and more responsibility is placed client-side in general, this type of debugging and profiling will only become more useful.
Firebug Console API:
http://getfirebug.com/console.html
By pressing F12 web developers can quickly debug JavaScript code without leaving the browser. It is built into every installation of Windows.
In Internet Explorer 11, F12 tools provides debugging tools such as breakpoints, watch and local variable viewing, and a console
for messages and immediate code execution.
Firefox has recently released a new "Firefox Developer Edition" providing all development features natively.
I always use chrome for my developer needs and thought of giving it a try. Searched through but found no way to edit JavaScript live & re-compile the script like in Chrome.
Of course, there is scratchpad but it's not changing the existing script and IMHO is no different then executing a script in console.
Is Firefox still way behind Chrome developer tools ? Or I missed how to live-edit javascript in Firefox ?
References :
https://developer.mozilla.org/en/docs/Tools
https://developer.mozilla.org/en-US/Firefox/Developer_Edition
The Firefox devtools are actually in standard Firefox too.
Is Firefox still way behind Chrome developer tools ?
No, Firefox devtool is way ahead of Chrome in a few ways, it has web-audio tab, canvas tab, shader editor for webgl, a style editor that lets you easily click to show/disable stylesheets, etc.
I missed how to live-edit javascript in Firefox ?
Unfortunately the JS debugger is definitely way behind Chrome. Not only does it not have live editing, even basic handling of callbacks can be problematic at the moment and there are many reports of bugs (Chrome has devtools bugs too, but are not usually as noticeable).
Use the ScratchPad. For example:
References
Scratchpad - Firefox Developer Tools | MDN
Firefox Developer Tools - Scratchpad - YouTube
I work in an application that is JavaScript intense. So to debug , I end up using many alerts. Are there other better ways to debug ? What methods do you use ?
For JavaScript, debugging is a sinch in most browsers:
IE - Where you really need to debug, F12 is the console. You can call console.log, console.debug, console.error and a few others, and it will print out good data. When you call console.log on an object, good consoles will print out the property break-down of the object. There's also an active dom inspector so that you can see what's going on as the script is running.
Firefox - Get the Firebug addon. It is my favorite console of them all. Does everything I've ever dreamed of needing, and a few more features.
Chrome - Built-in console, inspect element on the page to see the breakdown of the DOM live.
Opera - Built-in console
Safari - Add Firebug Lite to the page, and you'll have a JS driven version of Firebug. Safari has built-in dev tools similar to Chrome, however they need to be enabled.
For all browsers, you can add Firebug Lite, but I really only use it for IE and Safari.
Most modern browsers include a console, which can help with displaying syntax errors and the like. Of course, there's Firebug.
Use Firefox as your main development platform and open the Web Console. Error messages are printed there.
Chrome also has a console, but it doesn't have Firebug, a Firefox add-on used for web development. While the Web Console that comes with Firefox will display errors in Javascript, Firebug will also help with inspecting the HTML for your page, and there are even extensions to Firebug like Flashbug for working with Flash components on your page.
Obviously you'll want to check your web apps in all browsers eventually, but do most of your developing in Firefox.