I have written a simple java script in an external .js file named indexcheck.js

function check()
{
if(document.form1.txt.value == "")
{
alert("User ID cannot be blank")
return
}
}

In the html page i have i have included it under the HEAD tag using the code:

<script type="text/javascript" src="indexcheck.js"></script>

in a form named as form1 i gave a button as:

<input type="button" name="Button" value="Submit" onClick="check()">

But my code is not working.
Can anybody tell me the proper code or can u please tell me how can i call a function written in another file on the occurring of an event.
please help me