Develop Reference

How to throw a javascript error during runtime via browser (Chrome)?

My objective: Test out my error handling functionality.
Temporary solution: Have a custom route: /error, which contains code which purposefully produces fatal error.
var a = undefined;
a.b.c // Breaks.
The above works, but I can't use it to test production site as the page is not required.
I was looking for a way to test it via the browser. I tried simply adding"
throw new Error("Custom error thrown here") to the console. That doesn't actually break it during runtime.
I tried adding a break point and adding the same code: throw new Error("Custom error thrown here"). That didn't work either.
Any other easier ways to do this rather than the above?
I was looking for a way where I can do it via browser only.
Thanks.

You did not clearly mention how and where the error should be thrown. I will assume that you can use a modified copy of your JavaScript file to throw errors. The modified file will reside on your computer and only be used when you're using Chrome developer tools. This feature is called Local Overrides. The steps are as follows:
Open the webpage
Open Chrome developer tools for that webpage
In Sources panel go to Overrides tab
Click Select folder for overrides and choose a folder on your computer
A warning appears on the webpage which reads "DevTools requests full access to ..." which you must allow
In Sources panel go to Page tab
Locate the file in which you need to inject the "throw error" code
Right click and choose Save for overrides
Now you can edit the copy of the file on your computer or from within developer tools. Insert the code that produces the error at the desired location. When you reload the page with developer tools open, Chrome will load the local copy of the JavaScript file and throw the error. The error thrown that way will contain the context from where it originated e.g. call stack. If the developer tools are closed then live copy will be used.

If I got your question right, this is How you can do it from the console:
var script_tag = document.createElement('script');
script_tag.type = 'text/javascript';
script_tag.text = 'throw new Error("Custom error thrown here")';
document.body.appendChild(script_tag);
Or if you want you can trigger it on click:
var script_tag = document.createElement('script');
script_tag.type = 'text/javascript';
script_tag.text = 'window.document.onclick = function() { throw new Error("Custom error thrown here")}';
document.body.appendChild(script_tag);
And then you click anywhere on the page, to throw the error;

I would use the exec function which actually takes string and runs the code within at compile time.
exec('a.b.c')

You won't be able to throw an error inside your application from the console, since you are out of scope of the app.
Having said that, one slightly awkward way you could do this is by adding a breakpoint at the start of the javascript file.
Reload the page and your app will pause at the breakpoint - you can then modify the code as you need - like adding a throw new Error("something...") - and save your edits.
Then allow the code to run and you will see your error.
A downside is if you reload the changes will be gone, but I believe it's as close as you can get to modifying code at runtime.

Add this code to your production code
window.addEventListener('err', () => {
throw new Error('break it');
})
and when you want to create an error simply
dispatchEvent(new Event('err'))
in the console

You can use a global variable, which is accessible from your app and from debug console.
if (window.shouldThrow) {
throw new Error("Custom error thrown here");
}
This way you can turn on/off the exception throwing using the window.shouldThrow variable.

Try this way to catch error detail on run time
try
{
var a = undefined;
a.b.c // Breaks.
}
catch ( e )
{
alert("Error: " + e.description );
}

redirect Javascript syntax errors and console.log to somewhere else

I'm trying to send whatever would normally appear in the javascript console to my own custom functions.
I've tried the following
window.console.error = window.console.debug = window.console.log
window.console.log=mylog;
function mylog(msg){
//send it somewhere just using alert as an example
alert("log="+msg);
}
console.log("yo");
var x=y;//a syntax error
Using the code above I only see the following alert
"yo"
In the console log I see
"x is not defined"
How do I redirect the syntax errors?

You can use onerror it is designed to catch syntax errors.
window.onerror = function(message, source, lineno, colno, error) {
alert(message);
}
Offical docs on MDN

Here's a thread on this. Note the answer that shows how to "hijack" console.log() and send it wherever you want: Capturing javascript console.log?

How to catch all javascript warnings and errors to an output div?

i'm using try, catch, for debugging, but warnings is not create exceptions. How to get all javascript warnings and errors to output div?
UPDATED:
If browser supports Afaik logging, how to get that log to string or output div?
UPDATED:
I found the way how to do that:
i can reload console.log function to my custom function an call native console.log function.

First of all, get rid of the try catch. Don't use try catch when you are debugging.
Second, you don't want to out errors to a div, use firebug or inspector for that - console.log();
Third, if you really want to do it: you could use try catch and in the catch, use something like
$('body').append($('div').html('variable for error message goes here'));
if you are using jquery
OR
document.getElementByTagName("body").appendChild( document.createTextNode("variable for error message goes here") );
if you have plain javascript
EDIT: try looking up ie debug bar , ie webDeveloper

I understand myself why someone may want something to actually happen when an error occours in the document. The answers above just say that you would use developer tools, but I needed things to actually happen, and after some searching, here's what I found...
If you wish to catch all errors that come through, you can put the following code into your file, best at the top:
window.onerror = function(errorMsg, url, lineNumber){
// any action you want goes here
// errorMsg is the error message itself.
// url should be the file presenting the error, though i have
// found that it only presents to me the address of the site.
// lineNumber is the line number the error occoured on.
// here is an example of what you could do with it:
alert("Error in " + url + " at " + lineNumber + ":\n" + errorMsg);
}
I, myself, like to output the errors to a div that contains them all, though you can do literally anything to this information that you could do with any other string passed to a function.
Here is an example of what may happen if you throw an error with a button using the code above:
Error in your.site.here at 1:
Uncaught ReferenceError: foo is not defined

For IE javascript debugging you can follow this guide:
http://msdn.microsoft.com/en-au/library/ie/gg699336(v=vs.85).aspx
Keep in mind that the developer tools window must be open prior to loading the page for the warnings and errors to appear in the console.
For webkit, (chrome, safari) developer console - here is a guide:
https://developers.google.com/chrome-developer-tools/docs/console
...Firefox also has a console

how to catch ALL javascript errors with window.onerror? (including dojo)

this question is a follow-up to javascript: how to display script errors in a popup alert? where it was explained how to catch regular javascript errors using:
<script type="text/javascript">
window.onerror = function(msg, url, linenumber) {
alert('Error message: '+msg+'\nURL: '+url+'\nLine Number: '+linenumber);
return true;
}
</script>
I tried it and found out that dojo erros like this one:
TypeError: this.canvas is undefined dojo.js (Row 446)
were not reported using this method, which leads me to my question:
How can I report all javascript errors using window.onerror (especially dojo errors)?

It could be Dojo is using proper Error handling methods (i.e. try-catch blocks) which prevents the exception from bubbling up and reaching the window container, on which you have registered the error handler.
If so, there is no way for you to do this. No error is going past the catch block, so no error handler is being called.
As pointed out by the comments, you can also use browser-specific debugging APIs like the Venkman hook and do break-on-error -- a solution that usually only works for privileged code (thanks to #Sam Hanes).
You can also do On(require, 'error', function () {}); to add error handling on DOJO's asynchronous script loader -- another point mentioned in the comments by #buggedcom

you can write code like this:
var goErrHandler=window.onerror;
goErrHandler= function(msg, url, linenumber) {
console.log('Error message: '+msg+'\nURL: '+url+'\nLine Number: '+linenumber);
return true;
}
goErrHandler();
so in console you'll see some thing like this :
Error message: undefined
URL: undefined
Line Number: undefined

The better solution is to use try/catch, e.g.
try{
if(a=='a'){
}
}catch(e){
alert(e);
//or send to server
new Image().src='errorReport.php?e='+e;
}
Google Plus seems to use this.

Firebug not showing Javascript errors for Rails applications

I have a Rails application, and when I have Javascript errors they are not showing in the Firebug console. I have 'Show javascript errors' and 'Show javascript warnings' selected.
When I insert javascript errors in a basic html file, the errors show as expected.
In the javascript of the Rails app, it only shows errors in rare cases.
For example i can insert nonsense like:
dfghaefb;
and no error is shown in Firebug. But if i insert a space in there Firebug does show the error:
dfgh aefb;
Any ideas? This is driving me nuts.
UPDATE:
Pumbaa80 was right, it's syntax vs runtime.
So I set up onerror:
onerror=errorHandler;
var error="";
function errorHandler(errMessage,url,line){
error="There is an error at this page.\n";
error+="Error: " + errMessage+ "\n";
error+="URL: " + url + "\n";
error+="Line: " + line + "\n\n";
error+="Click OK to continue viewing this page,\n";
alert(error);
return true;
}
And I have a method with an error:
function initForm() {
asdfs;
}
And it works when I call it outside of a method:
initForm();
but not in this case:
document.observe('dom:loaded', function() {
initForm();
});
Why is that?

dfghaefb;
produces a run-time error. Those errors may be suppressed by putting an onerror handler on the window or by trapping them in a try/catch in some way. In that case, Firebug won't show anything.
In contrast,
dfgh aefb;
is a syntax error, which is shown in the error console, regardless of try/catch and onerror.

Did you let the page load completely? I have a rails app running in front of me right now with firebug on and i entered dfghaefb; and yes it does throw an error!
ReferenceError: dfghaefb is not defined { message="dfghaefb is not defined", more...}
Running Firefox 3.6.3, Firebug 1.5.3 and MacOSX 10.5.6 :)

This may appear to be obvious, but be sure you wrap your inline JS using
<script type="text/javascript"> code </script>
Otherwise FireBug won't look at it at all.

I had the same, it wasn't showing anything and it was driving me crazy for ages, even if i had "Show javascript errors" in the console tab selected, it seems that it started working after I did "Enable all panels".

These kinds of problems cannot be answered without a test case.