create callback function javascript

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

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

processUserInput(greeting);

function add(a, b, callback) {
  if (callback && typeof(callback) === "function") {
    callback(a + b);
  }
}

add(5, 3, function(answer) {
  console.log(answer);
});

const message = function() {  
    console.log("This message is shown after 3 seconds");
}
 
setTimeout(message, 3000);

Javascript CallBack