Promise.resolve-and-microticks.js

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');
  })

// out put 

// outer tick0
// inner tick0
// outer tick1
// outer tick2
// outer tick3
// inner tick1
//outer tick4

// my question is why return Promise.resove() takes 3 micktick ?