Problem in invoking function

hi friends am new to javascript and stuck some where in my script and I don't know what the actual problem is....let me explain with code

I have written a function called "abcd" in script1.js file.

function abcd()
{
Ext.MessageBox.alert('am new to javascript');
}
[Am using Extjs framework for client side designing]

and I have written span tag in the same file

'<span style="cursor: pointer;" '+'onclick="abcd();">'+'<img src="assets/modules/main/images/gdocs1.jpg"/></span><br/>'

But strangely I was at the error "Object required" on the same line where the span tag is, when execute the script1.js

please anybody put me in the right direction
thank u....

 

And did you import the Extjs framework in/before script1.js file?