What validation and sanitization methods can be applied to user input on the client side?

On the client side, the following methods can be used for validation and sanitization of user input:

• Format and content validation: checking required fields, length, format (e.g., email, phone), regular expressions.
• Data sanitization: removing or escaping dangerous characters to prevent XSS attacks.
• Using HTML5 validation: required, pattern, type attributes in input.
• Debouncing and deferred validation: to avoid overloading the system during input.
• Using specialized libraries: such as Joi, Validator.js for comprehensive validation.

Example of simple email validation in JavaScript:

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

// Sanitizing a string
function sanitizeInput(str) {
  const div = document.createElement('div');
  div.textContent = str;
  return div.innerHTML;
}

It is important to remember that client-side validation is for user convenience, but the main validation should be done on the server.