Why the output of this code is '0,1,2,3,4,5'

[pikadun]

Promise.resolve().then(function () {
            console.log(0);
            return Promise.resolve(4);
        }).then(function (r) {
            console.log(r);
        })

        Promise.resolve().then(function () {
            console.log(1);
        }).then(function () {
            console.log(2);
        }).then(function () {
            console.log(3);
        }).then(function () {
            console.log(5);
        })

All 1, 2, 3, 5 's callback are MicroTask, they should be pushed into the MicroTask queue at the same time, so they should be executed in sequence at the same time. But why the output of this code is '0,1,2,3,4,5'?

[devsnek]

I patched https://engine262.js.org to print when a function is pushed to the microtask queue, and when the resolve function from a promise constructor is called, by which i mean new Promise((resolve) => { resolve(5) }), not Promise.resolve(5).

Promise.resolve('p0')
  .then(function h0() {
    console.log(0);
    return Promise.resolve(4);
  })
  .then(function h1(r) {
    console.log(r);
    return 'h2ret';
  });

Promise.resolve('p1')
  .then(function h2() {
    console.log(1);
    return 'h2ret';
  })
  .then(function h3() {
    console.log(2);
    return 'h3ret';
  })
  .then(function h4() {
    console.log(3);
    return 'h4ret';
  })
  .then(function h5() {
    console.log(5);
    return 'h5ret';
  });

resolved 'p0'
queue 'h0'
resolved 'p1'
queue 'h2'
resolved undefined
0
resolved 4
resolved Promise {
  [[PromiseState]]: fulfilled
  [[PromiseResult]]: 4
}
1
resolved 'h2ret'
queue 'h3'
queue ''
2
resolved 'h3ret'
queue 'h4'
resolved 4
queue 'h1'
resolved undefined
3
resolved 'h4ret'
queue 'h5'
4
resolved 'h2ret'
5
resolved 'h5ret'