Mastering TypeScript 3 - Third Edition

Function callbacks

One of the most powerful features of JavaScript and, in fact, the technology that Node was built on, is the concept of callback functions. A callback function is a function that is passed into another function, and is then generally invoked inside the function. This is how asynchronous programming is achieved. By supplying a callback function, we are saying to the function we are calling, go and do what you need to do, and then when you are finished, call this function. Just as we can pass a value into a function, we can also pass a function into a function.

This is best illustrated by taking a look at some sample JavaScript code:

var callbackFunction = function(text) { console.log('inside callbackFunction ' + text); } ...