how to not wait until function response js

function firstFunction(_callback){
    // do some asynchronous work
    // and when the asynchronous stuff is complete
    _callback();    
}

function secondFunction(){
    // call first function and pass in a callback function which
    // first function runs when it has completed
    firstFunction(function() {
        console.log('huzzah, I\'m done!');
    });    
}console.log('1')

setTimeout(function afterTwoSeconds() {
  console.log('2')
}, 2000)

console.log('3')