#352 Promise Chain Output 2

easy
javascript
promises

What will be the order of the following code:

Promise.resolve(1)
  .then((val) => {
    console.log(val);
    return val + 1;
  })
  .then((val) => {
    console.log(val);
  })
  .then((val) => {
    console.log(val);
    return Promise.resolve(3).then((val) => {
      console.log(val);
    });
  })
  .then((val) => {
    console.log(val);
    return Promise.reject(4);
  })
  .catch((val) => {
    console.log(val);
  })
  .finally((val) => {
    console.log(val);
    return 10;
  })
  .then((val) => {
    console.log(val);
  });

Select one of the following:

1
2
undefined
3
undefined
4
undefined
undefined

Here's why:

  1. 1 - First promise resolves with 1
  2. 2 - Second .then gets 1+1
  3. undefined - Third .then gets undefined (since previous .then had no return)
  4. 3 - From the nested Promise.resolve(3)
  5. undefined - Next .then gets undefined (nested promise had no return)
  6. 4 - From Promise.reject(4) caught in catch
  7. undefined - From finally (finally always receives undefined)
  8. undefined - Last .then gets undefined (finally's return value is ignored)

Key points:

  • If a .then doesn't return anything, the next .then receives undefined
  • finally() always receives undefined and its return value is ignored
  • The chain continues after catch() with undefined