What are the key differences between using `req.query` and `req.params` in Express.js for retrieving data from a client request, and when is each appropriate?

`req.query` and `req.params` in Express.js are used to retrieve data from a client request, but they access different parts of the URL and serve distinct purposes. `req.query` is used to access query parameters in the URL. Query parameters are appended to the URL after a question mark (?) and are typically used for optional data, filtering, sorting, or pagination. They are key-value pairs separated by ampersands (&). Example: `/products?category=electronics&sort=price`. In this case, `req.query.category` would be 'electronics' and `req.query.sort` would be 'price'. `req.params` is used to access route parameters in the URL. Route parameters are defined as part of the route path using colons (:). They are typically used to identify a specific resource or entity. Example: `/products/:id`. In this case, if the URL is `/products/123`, then `req.params.id` would be '123'. Route parameters are mandatory; the route will only match if the parameter is present in the URL. Key differences and when to use each: 1. Source: `req.query` retrieves data from the query string, while `req.params` retrieves data from the route path. 2. Purpose: `req.query` is for optional data or filtering, while `req.params` is for identifying specific resources. 3. Optionality: Query parameters are optional, while route parameters are mandatory (for the route to match). Use `req.query` when you want to allow the client to provide optional data, such as filtering options or search terms. Use `req.params` when you need to identify a specific resource, such as a product ID or a user ID.