What is the role and purpose of guards in programming or security systems?

In programming and security systems, "guards" are mechanisms or checks that control access or the execution of certain actions.

In the context of Node.js, guards are often used for:

• Access rights verification before performing an operation (e.g., middleware for authentication and authorization).
• Input validation to prevent the execution of unwanted or malicious code.
• Restricting execution conditions, such as checking the state or parameters before calling a function.

Example of a guard in Express.js for authorization check:

function authGuard(req, res, next) {
  if (req.user && req.user.isAuthenticated) {
    next(); // allow execution
  } else {
    res.status(401).send('Unauthorized'); // block access
  }
}

app.get('/protected', authGuard, (req, res) => {
  res.send('Access granted');
});

Thus, guards ensure the security and correctness of applications, preventing unwanted actions.