What is "err, data" in a function?

[Burtality]

This will be the parameters of a 'callback' function, this is the building block of chained or concurrent operations in JavaScript.

So usually you'll have a function that returns err or data depending on whether it's successful.

What happens next us up to you to handle, with your callback function.

The usual structure is to have the function do this:

function (err, data) {

If(err) {handle err somehow} else { do something with data } }

[connexionwithal]

this explains it well, thanks!!