step1：console.log('main1');
step2：setTimeout(function() {
    console.log('setTimeout');
}, 0);

step3：requestAnimationFrame(function () {
    console.log('requestAnimationFrame');
});

step4：new Promise(function(resolve, reject) {
    console.log('promise');
    resolve();
step5：}).then(function() {
    console.log('promise then');
});

step6：console.log('main2');

// output
main1
promise
main2
promise then
requestAnimationFrame
setTimeout

step1：console.log('main1');

step2：process.nextTick(function () {
    console.log('process.nextTick1');
});

step3：setImmediate(function () {
    console.log('setImmediate');
}, 0);

step4：setTimeout(function() {
    console.log('setTimeout');
    step5：process.nextTick(function() {
        console.log('process.nextTick2');
    });
}, 0);

step6：new Promise(function(resolve, reject) {
    console.log('promise');
    resolve();
step7：}).then(function() {
    console.log('promise then');
});

step8：console.log('main2');

// output
main1
promise
main2
process.nextTick1
promise then
setTimeout
process.nextTick2
setImmediate