I am giving you a test example for that
I have a external js file named test.js that i have kept in Script folder in my root directory
I have button on my page and on its click event client side code is written in test.js file
Main page code
<html>
<head id="Head1">
<title></title>
<script language="javascript" type="text/javascript" src="http://ajax.googleapis.com/ajax/libs/jquery/1.3.2/jquery.min.js"></script>
<script src="Scripts/test.js" type="text/javascript"></script>
</head>
<body>
<input id="TestButton" type="button" value="Test" />
</body>
</html>
External js code in Test.js file
$(function()
{
$('#TestButton').click(function()
{
alert('I am clicked.');
});
});