Definition

Promise is an object or function with a then method. it has to obey the specific rules of Promise A+

In other words, different implementations can get different results of Promise.

The Promise Resolution Procedure

Promise.resolve()
  .then(() => {
    console.log(0)
    return 2
  })
  .then((res) => {
    console.log(res)
  })

Promise.resolve()
  .then(() => {
    console.log(1)
  })
  .then(() => {
    console.log(3)
  })

// result: 0 1 2 3

Promise.resolve()
  .then(() => {
    console.log(0)
    return Promise.resolve(2)
  })
  .then((res) => {
    console.log(res)
  })

Promise.resolve()
  .then(() => {
    console.log(1)
  })
  .then(() => {
    console.log(3)
  })
// result: 0 1 3 2

The only difference you'll notice is that the second code returns promise.resolve(2), which is a Promise object

When value is a promise, it's a little complicated.

Promise A+ specification says, that when then returns a Promise or thenable， the promise's state after then will be determined by the state of the value returned by the callback function. However, the Promise A+ specification does not specify how this process is implemented, and different implementations may lead to different results.

In the implementation of V8, the then method of the Promise or thenable object is called and added to the microQueue.

let microQueue = [Promise.resolve(2).then()]

Therefore, this difference results in the output of the two pieces of code being different.