If validation needs to occur on the fly — how to organize it?

For on-the-fly validation (i.e., during user data input), the following approaches are usually used:

• Input events: Track input, change, or keyup events on form fields.
• Debouncing: To avoid validating on every character, debouncing is applied — a delay before starting the check.
• Validation by rules: Check data against predefined rules (e.g., length, format, required).
• Error display: Show hints or error messages next to the field to the user.

Example in JavaScript using debounce:

function debounce(fn, delay) {
  let timeoutId;
  return function(...args) {
    clearTimeout(timeoutId);
    timeoutId = setTimeout(() => fn.apply(this, args), delay);
  };
}

const validateEmail = (email) => {
  const re = /^[^@\s]+@[^@\s]+\.[^@\s]+$/;
  return re.test(email);
};

const input = document.querySelector('#email');
const errorMsg = document.querySelector('#email-error');

input.addEventListener('input', debounce(() => {
  if (!validateEmail(input.value)) {
    errorMsg.textContent = 'Invalid email';
  } else {
    errorMsg.textContent = '';
  }
}, 300));

This approach allows quick response to user input without overloading the system with checks.