Simple Error Checking

Sorry this will be an ignorant question I know, but I'm just trying to implement some quick JS in my form to check if the fields were filled in:


function check_form() {

if (survey.form_firstname.value == "") {
firstname_error = "-Please enter your first name
";
}

if (survey.form_lastname.value == "") {
lastname_error = "-Please enter your last name
";
}

if (survey.form_phone.value == "") {
phone_error = "-Please enter your phone number
";
}

if (survey.form_email.value == "") {
email_error = "-Please enter your email
";
}

if (!survey.form_agree.checked) {
agree_error = "-You must be over 18 and provide accurate information
";
}

errors = firstname_error + lastname_error + phone_error + email_error + agree_error;

if(!errors) {
return true;
} else {
alert("There were errors:
" + errors);
delete firstname_error;
delete lastname_error;
delete phone_error;
delete email_error;
delete agree_error;
delete errors;
return false;
}
}

It only seems to work if no values are filled in, if one of the fields is filled in the error checking fails. Again, I'm extremely new to JS, i've only ever copy/pasted and this is the first time i've tried writing a piece.