Difference with native Promise

[neewbee]

new Promise((resolve, reject) => {
  resolve();
})
  .then(() => {
    console.log('outer tick0');
    new Promise((resolve, reject) => {
      resolve();
    })
      .then(() => {
        console.log('inner tick0');
        return Promise.resolve();
      })
      .then(() => {
        console.log('inner tick1');
      })
  })
  .then(() => {
    console.log('outer tick1');
  })
  .then(() => {
    console.log('outer tick2');
  })
  .then(() => {
    console.log('outer tick3');
  })
  .then(() => {
    console.log('outer tick4');
  })

// native promise output
// outer tick0
// inner tick0
// outer tick1
// outer tick2
// outer tick3
// inner tick1
// outer tick4

// then/Promise output
// outer tick0
// inner tick0
// outer tick1
// inner tick1
// outer tick2
// outer tick3
// outer tick4

What's the reason behind this?

[edef1c]

I'm fairly sure the order in which the callbacks are invoked is undefined, and is thus an implementation detail you should not rely on.

[neewbee]

I'm fairly sure the order in which the callbacks are invoked is undefined, and is thus an implementation detail you should not rely on.

Is the order in which the callbacks are invoked defined in the ECMAScript Specification or any other place?