Why my if statements don't work properly?

Question

I am creating a program that has to check if the value in the input meets all the conditions if not display an error.

for(let i = 0; i < bannedNumbers.length; i++){
    signIn.addEventListener('click', ()=>{

        if(signInFirstName.value == ""){
            signInFirstName.nextSibling.textContent = "Fill in the field!"
        } else{
            if(signInFirstName.value.includes(bannedNumbers[i])){
                signInFirstName.nextSibling.textContent = "You can't use symbols or/and numbers in your first name"
            } else{
                signInFirstName.nextSibling.textContent = "vbvnv"
            }
        }
    })
        
}

Answer 1

You likely MEANT to do this

I do not however recommend to block people from entering special characters.

What about Marie-Louise and Jean-Jacques?

const re = /[^a-zA-Z]/g
const validName = (fld, name) =>   {
  const val = fld.value.trim();
  let error = "";
  if (val === "") {
    error = `Please enter ${name}`; 
  } else if (re.test(val)) {
    error = `You can't use symbols and/or numbers in ${name}`
  }
  fld.nextSibling.textContent = error ? error : "OK";
  return error === "";
};
document.getElementById("signInForm").addEventListener('submit', (e) => {
  const signInFirstName = this.firstName;
  if (!validName(signInFirstName,"your first name")) e.preventDefault(); // cancel submit
  const signInLastName = this.lastName;
  if (!validName(signInLastName,"your last name")) e.preventDefault(); // cancel submit
})

<form id="signInForm">
  <label>First name: <input id="firstName" name="firstName" type="text"><span class="error"></span></label><br/>
  <label>Last name: <input id="lastName" name="lastName" type="text"><span class="error"></span></label><br/>
  <input type="submit" />
</form>