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()
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()
.