（Javascript）如何删除警报框后的“未定义”

Question

/* Validating Captcha Function */
function ValidCaptcha() {
    var str1 = removeSpaces(document.getElementById('txtCaptcha').value);
    var str2 = removeSpaces(document.getElementById('txtCompare').value);

    if (str1 == str2) {
        alert("請按OKAY兩次 (Please Press OKAY Twice To Submit)");
        document.getElementById('contact-form').action = "/send_form_email.php";
    }
    else {
        alert("請輸入正確的號碼 (Please Enter The Correct Vertification Number)");
        event.preventDefault();
        event.stopPropagation();
    }
}

This is the code i use for a javascript captcha element on my website, this code works perfectly, however after the alert boxes pops up, an 'Undefined' will always follow. I have already tried return true and return false with no luck. MANY THANKS <3

BTW this is how i initiate the javascript form onsubmit in HTML

<form id="contact-form" class="form-horizontal"  method="post" onsubmit="return alert(ValidCaptcha());">

Answer 1

When you are calling submit

onsubmit="return alert(ValidCaptcha());"

You have an alert. Your ValidCaptcha does not return anything, hence why it is undefined. Also alert returns nothing so makes no sense.

Should just be

onsubmit="return ValidCaptcha();"

and now you should be returning a boolean.

function ValidCaptcha() {
  var str1 = removeSpaces(document.getElementById('txtCaptcha').value);
  var str2 = removeSpaces(document.getElementById('txtCompare').value);

  if (str1 == str2) {
    alert("請按OKAY兩次 (Please Press OKAY Twice To Submit)"); 
    document.getElementById('contact-form').action = "/send_form_email.php";
    return true
  } else {
    alert("請輸入正確的號碼 (Please Enter The Correct Verification Number)");  //<Vertification should be  Verification
    event.preventDefault();
    event.stopPropagation();
    return false;
  }
}

Answer 2

onsubmit was wrong function call change like this onsubmit="return ValidCaptcha(event);" .And also pass the event with onsubmit .Then only e.preventDeafult() is working

you could use trim() for remove unwanted spaces.

Javascript

function ValidCaptcha(e) {
  var str1 = document.getElementById('txtCaptcha').value.trim();
  var str2 = document.getElementById('txtCompare').value.trim();

  if (str1 == str2) {
    alert("請按OKAY兩次 (Please Press OKAY Twice To Submit)");
    document.getElementById('contact-form').action = "/send_form_email.php";
  } 
  else
  {
    alert("請輸入正確的號碼 (Please Enter The Correct Vertification Number)");
    e.preventDefault();
    e.stopPropagation();
  }
}

Html

<form id="contact-form" class="form-horizontal" method="post" onsubmit="return ValidCaptcha(event);">