Callbacks, 'Then' and 'Async / Await'.

callbacks.js

function first () {
    console.log(1)
}

function second () {
    console.log(2)
}

first();
second();

// outputs
1
2

// add timeout to first function
function first () {
    setTimeout( () => { 
        console.log(1) 
    }, 500 )
}

function second () {
    console.log(2)
}

first();
second();
// outputs
2
1

// add callback to first function
function first (callback) {
    setTimeout( () => { 
        console.log(1); 
        callback() 
    }, 500 )
}

function second () {
    console.log(2)
}

first(second);
// outputs
1
2


first(second) // callbacks

first().then(second(result))
  .then(third(result))
  .then(forth(result))
  .catch(failureCallback); // using 'then' to handle promises

async function wrapper(){ // using 'await' to handle a promise 

    await first();
    await second();
    await third();

}