Callback function in JavaScript

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.

Here is a quick example:

The above example is the synchronous callback, as it is executed immediately.

Note, however, that callbacks are often used to continue code execution after an asynchronous operation has completed — these are called asynchronous callbacks. A good example is the callback functions executed inside a .then() block chained onto the end of a promise after that promise fulfills or rejects. This structure is used in many modern web APIs, such as fetch().