Rails validation form

By HTML5 and Javascript

eg, a submit form
Using setCustomValidity function by default js and oninput="checkPasscode(); , required on input field.

<form>
  <label for="passcode">Enter Passcode:</label>
  <input
    id="passcode"
    type="password"
    placeholder="Your passcode"
    oninput="checkPasscode();"
    required
  />
  <button type="submit">Submit</button>
</form>

function checkPasscode() {
  var passcode_input = document.querySelector("#passcode");

  if (passcode_input.value != "Ivy") {
    passcode_input.setCustomValidity("Wrong. It's 'Ivy'.");
  } else {
    passcode_input.setCustomValidity(""); // be sure to leave this empty!
    alert("Correct!");
  }
}