Async and await

First, problem without async/await:-

Imagine:

1. Get user from server
2. Get orders using user id
3. Get payment using order id

Using Promise chaining:

getUser()
  .then(user => {
    return getOrders(user.id);
  })
  .then(orders => {
    return getPayment(orders[0].id);
  })
  .then(payment => {
    console.log(payment);
  });

Works fine, but lots of .then().

Same thing:

async function getData() {
    const user = await getUser();
    const orders = await getOrders(user.id);
    const payment = await getPayment(orders[0].id);

    console.log(payment);
}

Looks like normal code.

What does async do?

async = This function will return a Promise.

async function hello() {
    return "Hi";
}

is equivalent to 

function hello() {
    return Promise.resolve("Hi");
}
What does await do?
“Wait here until Promise finishes, then give me the result.”
One important rule:
await only works inside an async function.

❌ Wrong:

const data = await getUser();

✅ Correct:

async function test() {
   const data = await getUser();
}
Promise  = "I will give result later"
async    = "This function returns a promise"
await    = "Wait until promise finishes"