javascript判断文本框输入是否为空

<script  type="text/javascript" language="javascript">
function checkValue(){
var result = true;
var username = document.registerForm.username.value;
username = username.replace(/^\s*|\s*$/g,"")   
 if(username == ""){

    result = false;   
    alert("user name should not be null!");
} else if (document.registerForm.password.value == ""){
    result = false;
    alert("password should not be null!");
 } else if (document.registerForm.passwordRepeat.value == ""){
    result = false;
    alert("password repeat should not be null!");
 } else    if(document.registerForm.passwordRepeat.value != document.registerForm.password.value){
    result = false;
    alert("password is not equal!");
 } else if(document.registerForm.email.value == ""){
    result = false;
    alert("email should not be null!");
 }
 return result;
}
</script>