ASP.NET

btnSave.Attributes.Add("OnClick","javascript:return ValidateTextBox();");

<script language="javascript">

  function ValidateTextBox()
  {
     //for eg:let the textbox id be 'txtName'

     //First check whether the textbox exists 
     if(document.getElementById("txtName")!=null)
      {
        if(isEmptyCheck("txtName")==true)
	{
		alert("Text box value is empty,please enter value");
		return false;
	}
      }
  }

  //function to check whether text box value is empty or not

  function isEmptyCheck(X)
{
if((document.forms[0].elements[X].value=="") || (document.forms[0].elements[X].value.substring(0,1) == ' '))
	{
		return true;
	}
	return false;
}//end of function
  

 </script>   

btnSave.Attributes.Add("OnClick","javascript:return ValidateTextBox();");

<script language="javascript">

function ValidateTextBox()
{
//for eg:let the textbox id be 'txtName'

//First check whether the textbox exists 
if(document.getElementById("txtName")!=null)
{
if(isEmptyCheck("txtName")==true)
{
alert("Text box value is empty,please enter value");
return false;
}
}
}

//function to check whether text box value is empty or not

function isEmptyCheck(X)
{
if((document.forms[0].elements[X].value=="") || (document.forms[0].elements[X].value.substring(0,1) == ' '))
{
return true;
}
return false;
}//end of function

</script>