how to pass callback function in javascript

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

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

processUserInput(greeting);function functionOne(x) { alert(x); }

function functionTwo(var1, callback) {
    callback(var1);		
}

functionTwo(2, functionOne);