SOURCE

const p1 = new Promise((resolve, reject) => {
  console.log('promise1');
  resolve();
})
  .then(() => {
    console.log('then11');
    new Promise((resolve, reject) => {
      console.log('promise2');
      resolve();
    })
      .then(() => {
        console.log('then21');
      })
      .then(() => {
        console.log('then23');
      });
  })
  .then(() => {
    console.log('then12');
  });

const p2 = new Promise((resolve, reject) => {
  console.log('promise3');
  resolve();
}).then(() => {
  console.log('then31');
});