Query parameters are used to send additional data in the URL.

They appear after a ? and are written as key-value pairs.

Example:

• '/users?active=true';
• '/products?category=books'.

In Express, query parameters are accessed using req.query.

app.get('/users', (req, res) => {
  const isActive = req.query.active;

  res.send(`Active: ${isActive}`);
});

If you open '/users?active=true', the value of isActive will be 'true'.

Query parameters are often used for filtering, searching, or sorting data.