PROMISE FINALLY() DEFINITION

Formerly, there were only inelegant ways of defining behavior that would execute after a promise exits the “pending” state no matter the outcome. Usually, this would take the form of recycling the handler:

let resolveA, rejectB;

function finalHandler() {
 console.log('finished');
}

function resolveHandler(val) {
 console.log('resolved');
 finalHandler();
}

function rejectHandler(err) {
 console.log('rejected');
 finalHandler();
}

new Promise((resolve, reject) => {
 resolveA = resolve;
})
.then(resolveHandler, rejectHandler);

new Promise((resolve, reject) => {
 rejectB = reject;
})
.then(resolveHandler, rejectHandler);
 
resolveA();
rejectB();
// resolved
// finished
// rejected
// finished

With Promise.prototype.finally(), you are able to unify the shared handler. The finally() handler is not passed any arguments and does not know if it is handling a resolved or rejected promise. The preceding example...