When JS deals with asynchronous, promise deals with 'Callback hell'

Without Promise

isBusHere(function(isBusHereResult) {
    isBusStop(function(isBusStopResult) {
        isDoorOpen(function(isDoorOpenResult) {
            findWallet(function(findWalletResult) {
                findCard(function(findCardResult) {
                    payBusFare(function(payBusFareResult) {
                        // pay success
                    }, failureCallback);
                }, failureCallback);
            }, failureCallback);
        }, failureCallback);
    }, failureCallback);
}, failureCallback);

With promise

isBusHere().then(function(isBusHereResult) {
    return isBusStop(isBusHereResult)
}).then(function(isBusStopResult) {
    return isDoorOpen(isDoorOpenResult)
}).then(function(isDoorOpenResult) {
    return findWallet(findWalletResult)
}).then(function(findWalletResult) {
    return findCard(findCardResult)
}).then(function(findCardResult) {
    return payBusFare(payBusFareResult)
}).catch(failureCallback);

With arrow function (to look better)

isBusHere()
    .then(isBusStopResult => isBusStop(isBusStopResult))
    .then(isDoorOpenResult => isDoorOpen(isDoorOpenResult))
    .then(findWalletResult => findWallet(findWalletResult))
    .then(findCardResult => findCard(findCardResult))
    .then(payBusFareResult => payBusFare(payBusFareResult))
    .catch(failureCallback);

Promise has 3 states.

1. Pending: When new promise produced, it's pending state.
2. Fulfilled: After executed resolve, state will be fulfilled.
3. Rejected: With Error.

const promise1 = new Promise((resolve, reject) => {});
console.log(promise1); // Promise { <pending> }

const promise2 = new Promise((resolve, reject) => {
    resolve();
});
console.log(promise2) // Promise { undefined }

const promise3 = new Promise((resolve, reject) => {
    //reject();
});
console.log(promise3) // Promise { <rejected> undefined }