Promise chaining

Promise chaining means executing multiple asynchronous tasks one after another, where the result of one Promise is used in the next Promise.

Simple idea:

Task-1 finishes
      ↓
Use result for Task-2
      ↓
Task-2 finishes
      ↓
Use result for Task-3
Example:
Get user from server
      ↓
Using user id, get user orders
      ↓
Using order id, get payment details

Step 1: Get User
function getUser() {
  return new Promise((resolve, reject) => {
    setTimeout(() => {
      resolve({
        id: 1,
        name: "Lokman"
      });
    }, 1000);
  });
}
This sends:
{ id: 1, name: "Lokman" }
Step 2: Get Orders using User ID
function getOrders(userId) {
  return new Promise((resolve, reject) => {

    setTimeout(() => {
      resolve({
        orderId: 101,
        product: "Laptop"
      });
    }, 1000);
  });
}
This sends:
{ orderId: 101, product: "Laptop" }
Step 3: Get Payment using Order ID
function getPayment(orderId) {
  return new Promise((resolve, reject) => {

    setTimeout(() => {
      resolve("Payment successful");
    }, 1000);
  });
}
Promise Chain
getUser()
  .then(user => {
      console.log(user);
      return getOrders(user.id);   // return Promise 2
  })
  .then(order => {
      console.log(order);
      return getPayment(order.orderId);   // return Promise 3
  })
  .then(payment => {
      console.log(payment);
  })
  .catch(error => {
      console.log(error);
  });