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.

Here is a quick example:

`function greeting(name) { alert('Hello ' + name); }

function processUserInput(callback) { var name = prompt('Please enter your name.'); callback(name); }

processUserInput(greeting);`

The above example is an asynchronous callback, as it is executed immediately.

Note, however, that callbacks are often used to continue code execution after an asynchronous operation has been completed — these are called asynchronous callbacks. A good example is the callback functions executed inside a [.then()](<https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise/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()](<https://developer.mozilla.org/en-US/docs/Web/API/WindowOrWorkerGlobalScope/fetch>).