Callback function

A callback function is a function passed into another function as an argument, which is then invoked inside the outer function to complete some kind of routine or action.

The consumer of a callback-based API writes a function that is passed into the API. The provider of the API (called the caller) takes the function and calls back (or executes) the function at some point inside the caller’s body. The caller is responsible for passing the right parameters into the callback function. The caller may also expect a particular return value from the callback function, which is used to instruct further behavior of the caller.