What is callback in callback

You can nest one callback inside in another callback to execute the actions sequentially one by one. This is known as callbacks in callbacks. Beware, too many levels of nesting lead to Callback hell

loadScript("/script1.js", function (script) {
console.log("first script is loaded");

loadScript("/script2.js", function (script) {
console.log("second script is loaded");

loadScript("/script3.js", function (script) {
console.log("third script is loaded");
// after all scripts are loaded
});
});
});