Background colors on Luhn Algorithm - JavaScript

Question

I am currently trying to learn to become a web developer and have a task where I need to use luhn algorithm to check credit cards are valid. If they are valid the box goes green, if they are not the box goes red. I’ve found the below javascript on GitHub that looks to do what I need, but I am unsure on how to add the code to make the boxes change colour. I believe I need an if/ else statement but I’m not sure how to implement it. I was thinking something like this as the code for the color change:

document.getElementById(‘cardInput’).style.backgroundColor = “green”;

Here is my html:

     <form name="form1" action="#">
       <ul>
         <li>Name:<input id="nameInput" onkeyup="letterscheck(this)" type='text' name='name' placeholder="Enter Your Name"/></li>
         <li>Email:<input id="emailInput" onkeyup="emailcheck(this)" type='text' name='email'placeholder="Enter Your Email"/></li>
         <li>Card:<input id="cardInput" onkeyup="cardnumber(this)" type='text' name='card' placeholder="Enter a Proxy Credit Card Number."/></li>
         <li class="submit"><input type="submit" name="submit" value="Submit"  /></li>
       </ul>
     </form>
   </div>

Here is the JS i found on GitHub:

function cardnumber(value) {
if (/[^0-9-\s]+/.test(value))
return false;
let nCheck = 0, bEven = false;
value = value.replace(/\D/g, “”);
for (var n = value.length - 1; n >= 0; n–) {
var cDigit = value.charAt(n),
nDigit = parseInt(cDigit, 10);
if (bEven && (nDigit *= 2) > 9) nDigit -= 9;
nCheck += nDigit;
bEven = !bEven;
}
return (nCheck % 10) == 0;
}

My JS for the other 2 fields

 function letterscheck(inputtxt)
  {
    var namechecker = /^[a-zA-Z]+(?:[\s.]+[a-zA-Z]+)*$/;
    if(inputtxt.value.match(namechecker))
          {
        document.getElementById('nameInput').style.backgroundColor = "green";
        return true;
          }
        else
          {
          document.getElementById('nameInput').style.backgroundColor = "red";;
          return false;
          }
  }


  function emailcheck(inputtxt)
    {
      var emailchecker = /^(([^<>()[\]\\.,;:\s@"]+(\.[^<>()[\]\\.,;:\s@"]+)*)|(".+"))@((\[[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\])|(([a-zA-Z\-0-9]+\.)+[a-zA-Z]{2,}))$/;
      if(inputtxt.value.match(emailchecker))
            {
          document.getElementById('emailInput').style.backgroundColor = "green";
          return true;
            }
          else
            {
            document.getElementById('emailInput').style.backgroundColor = "red";
            return false;
            }
    }

Hopefully, this is a really easy one for you all! Any help would be great.

Thanks!

Which box should we color, the li or input. Is cardnumber() the function that returns the boolean value for the right card number? — DecPK
– DecPK, Commented Mar 27, 2021 at 13:01
Hi, I've edited my original post to add in the full task. It must have a Name field, an email field and a card field using luhn validation. The other 2 boxes function correctly, the box you type in goes red or green depending on the input. I need the Card: box to do the same but i just don't have the knowledge to know how to implement it. — Bungle
– Bungle, Commented Mar 27, 2021 at 13:20
Please add the source of the cardnumber function algorithm — DecPK
– DecPK, Commented Mar 27, 2021 at 13:38

DecPK · Accepted Answer · 2021-03-27 13:56:50Z

The algorithm contains several errors so I've searched and found on Stackoverflow

You can change the background-color conditionally. I've changed the background-color inside the letterscheck for name field. You can return true or false and do it in the addEventListener like in email field.

const nameInput = document.querySelector("#nameInput")
const emailInput = document.querySelector("#emailInput")
const cardNumberInput = document.querySelector("#cardInput")

function valid_credit_card(value) {
  if (/[^0-9-\s]+/.test(value)) return false;

  var nCheck = 0,
    nDigit = 0,
    bEven = false;
  value = value.replace(/\D/g, "");

  for (var n = value.length - 1; n >= 0; n--) {
    var cDigit = value.charAt(n),
      nDigit = parseInt(cDigit, 10);

    if (bEven) {
      if ((nDigit *= 2) > 9) nDigit -= 9;
    }

    nCheck += nDigit;
    bEven = !bEven;
  }

  return (nCheck % 10) == 0;
}


function letterscheck(inputtxt) {
  var namechecker = /^[a-zA-Z]+(?:[\s.]+[a-zA-Z]+)*$/;
  if (inputtxt.match(namechecker)) {
    nameInput.style.backgroundColor = "green";
  } else {
    nameInput.style.backgroundColor = "red";;
  }
}


function emailcheck(inputtxt) {
  var emailchecker = /^(([^<>()[\]\\.,;:\s@"]+(\.[^<>()[\]\\.,;:\s@"]+)*)|(".+"))@((\[[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\])|(([a-zA-Z\-0-9]+\.)+[a-zA-Z]{2,}))$/;
  return inputtxt.match(emailchecker);
}

nameInput.addEventListener("keyup", e => {
  letterscheck(e.target.value);
})

emailInput.addEventListener("keyup", e => {
  const isEmailValid = emailcheck(e.target.value)
  if (isEmailValid) {
    e.target.style.backgroundColor = "green";
  } else {
    e.target.style.backgroundColor = "red";
  }
})

cardNumberInput.addEventListener("keyup", e => {
  const isCardValid = valid_credit_card(e.target.value);
  if (isCardValid) {
    cardNumberInput.style.backgroundColor = "green";
  } else {
    cardNumberInput.style.backgroundColor = "red";
  }
})

li {
  list-style-type: none;
}

input {
  margin: .25rem 1rem;
}

<form name="form1" action="#">
  <ul>
    <li>Name:<input id="nameInput" type='text' name='name' placeholder="Enter Your Name" /></li>
    <li>Email:<input id="emailInput" type='text' name='email' placeholder="Enter Your Email" /></li>
    <li>Card:<input id="cardInput" type='text' name='card' placeholder="Enter a Proxy Credit Card Number." /></li>
    <li class="submit"><input type="submit" name="submit" value="Submit" /></li>
  </ul>
</form>

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