Blog Post

by James Edwards

if(typeof input == 'undefined')
{
	throw('Missing input');
}
else if(parseFloat(input) != input)
{
	throw('Input must be a number');
}

var err = new Error();
err.name = 'My API Input Error';
err.message = 'Input must be a number';
throw(err);

function err(type, message)
{
	var err = new Error();
	err.name = 'My API ' + type + ' Error';
	err.message = message;
	throw(err);
}

// the delete in this example won't happen
if(typeof this.input == 'undefined')
{
	err('Input', 'Missing input');
	delete this.input;
}

// but this delete will happen fine
if(typeof this.input == 'undefined')
{
	delete this.input;
	err('Input', 'Missing input');
}

This post has 13 responses so far

1. Klas Says:
   August 22nd, 2008 at 2:44 pm

   Firebug Lite is cross-browser…

   http://getfirebug.com/lite.html

    
2. Phil Says:
   August 26th, 2008 at 7:04 pm

   This blog is so off the mark I don’t know where to start. Throw is not a debugging tool. Use it to delegate unrecoverable errors to further up the chain of execution, where it may be handled more gracefully with a try catch. You should never intend for your thrown error to reach the surface.

    
3. aj[fp] Says:
   August 27th, 2008 at 8:49 am

   Phil Says:

   This blog is so off the mark I don’t know where to start. Throw is not a debugging tool. Use it to delegate unrecoverable errors to further up the chain of execution, where it may be handled more gracefully with a try catch. You should never intend for your thrown error to reach the surface.

   I second this… you are meant to handle the error with the try / catch, not just throw. Throw literally “throws” the exception string to catch, for the try / catch to handle gracefully.

    
4. active8 Says:
   August 27th, 2008 at 10:31 am

   Yeah. I’m better at C++ but since learning that JS (ima newb) allows try catch blocks this was the first thing to come to mind when I read this. Seems to me you throw the error message and print it in the catch block. Correct me if I’m wrong. Even C++ error handling seems to be a foggy memory.

    
5. Breton Says:
   August 27th, 2008 at 11:33 am

   Here’s a much better alternative. Continue to use the alert function as normal, but overwrite the alert function with a simple console logger. I find this to be quite useful, as there’s no legitimate usage of alerts in applications other than to annoy your users. I find it satisfying to destroy it it outright. When you’re done debugging, just replace alert with a stub function that does nothing. (or even silently ajax the alerts to a serverside error logger!) Here’s a simple one I threw together JUST NOW. No warrantees, only tested in firefox 3, not garaunteed to work anywhere. But give it a try, you might like it:
   alert = (function () { var console = document.createElement("div"); console.setAttribute("id", "console"); console.setAttribute("style", "width:300px; height:300px; font-family: monospace; font-size: 10px; border: 1px solid black; background: white; position: absolute; right: 0; top: 0; overflow: scroll;"); document.body.appendChild(console); return function (s) { var p = document.createElement("p") p.appendChild(document.createTextNode(s)); console.appendChild(p); p.scrollIntoView(); } })()

    
6. Breton Says:
   August 27th, 2008 at 2:49 pm

   Apologies for the “inline” code. I didn’t notice the code block button!

    
7. brothercake Says:
   August 27th, 2008 at 6:36 pm

   I never said that throw was useful as a debugging tool. I said it’s useful as a high-level caretaker for other people using something you wrote.

   I guess the post’s title is a bit off the mark.

    
8. naterkane Says:
   August 28th, 2008 at 5:48 am

   As Klas was saying… Firebug Lite is cross browser, he didn’t mention that you don’t need to run it on your server if you use a Bookmarklet to call it up. Save the javascript url below as a bookmarklet, and enjoy firebug anywhere, on any site.

   Firebug Lite Bookmarklet

   The only downside to this approach, is that you have to manually load firebug and anything that’s running before it’s loaded is not available (console, net transfers, etc…) which doesn’t help you if you have a window load or dom load based app to test.

    
9. naterkane Says:
   August 28th, 2008 at 5:48 am

   here’s the url for that bookmarklet since it looks like sitepoint’s comment system ate it.

   javascript:var%20firebug=document.createElement(’script’);firebug.setAttribute(’src’,'http://getfirebug.com/releases/lite/1.2/firebug-lite-compressed.js’);document.body.appendChild(firebug);(function(){if(window.pi&&window.firebug){firebug.init();}else{setTimeout(arguments.callee);}})();void(firebug);

    
10. Red T-Rex Says:
    August 28th, 2008 at 11:16 am

    The DOJO framework provides a simple cross browser mechanism that creates a console in browsers like IE that do not natively support it. Then you can use something like:

    console.debug(”Deleted record: ” + recordId);

     
11. nathj07 Says:
    August 28th, 2008 at 4:54 pm

    The blog was useful in serving as a start for great discussion. so from that point of view - thanks; I’ve learnt some stuff here.

    As for my 2cents worth well, I use Firebug to debug any pre-release code I have and then once I know it’s working correctly there some cross platform testing sees me right. All this means that the released code is working fine. JS is supposed to be cross-browser.

    As for the earlier post from Breton that’s a great idea. We should never throw our error messages to users on the web. They are unliekly to report the problem and will simply get what they are after somewhere else. So if you must handle exceptions (my JS is a ll fairly simple) which is a good idea in more complex systems then a logging tool such as this is a great idea.

     
12. greg Says:
    August 28th, 2008 at 6:02 pm

    Instead of throw() function, I would rather use throw statement:
    throw { name: 'My API Input Error', message: 'Input must be a number' };
    Compare it with more clumsy throw() function:
    var err = new Error(); err.name = 'My API Input Error'; err.message = 'Input must be a number'; throw(err);

     
13. hj Says:
    September 11th, 2008 at 6:24 am

    Greg said:

    Instead of throw() function, I would rather use throw statement:
    throw { name: ‘My API Input Error’, message: ‘Input must be a number’ };
    Compare it with more clumsy throw() function:
    var err = new Error(); err.name = ‘My API Input Error’; err.message = ‘Input must be a number’; throw(err);

    Umm, you do realize that throw is a statement? Like return (which takes an optional argument), throw takes one argument, which is expected to be an object constructed by Error. So your assertion that throw {object} is simpler, is simply wrong — you’re saying that

    return 4;

    is simpler than

    var i = 2; return(i+i);

    You can say, if you’d like:

    throw new Error('My API Input Error','Input must be a number')
     

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