What's the significance of the (req, res, next) callback?

In a lot of Express HTTP verb methods, the argument of (req, res, next) is usually given for the callback function, eg:

app.get('/resources', (req, res, next) => {

Are these ‘standard’ bits of syntax and what do they generally refer to? I understand that ‘res’ is the server response and ‘req’ is probably short for ‘request’, but what’s their exact function here?

Yes, they are standard Express parameters. “req” is for the request object. This is used in your Express route handler function to get data from the client (such as a web-browser). For example: username or order number could be attached to the req object. “res” is for the response object and you will use this in your Express route handler function to send data back to the client. For example: a 200 success status code and some data (often in json format) that your client requested from the server (such as a weather report). “next” is used to call other “middle-ware” functions with Express which can handle other things such as logging or error handling. You can also name them something else such as (request, response) and sometimes you won’t need to use next so there may only be 2 arguments (req and res).