Required text field validator using javascript

Write this javascript code inside to "HEAD" tag of .aspx file

function FormValidator(theForm)

{

if (theForm.TextBox1.value == "")

{

alert("You must enter somthing");

theForm.TextBox1.focus();

return (false);

}

}

call this function in form's onsubmit event

form id="form1" runat="server" onsubmit="return FormValidator(this)"