为什么Promise.then'onFulfilled'函数在setTimeout回调之前触发？

Question

有人可以解释为什么承诺'then'函数在setTimeout函数之前触发？我原以为setTimeout函数会先在事件循环上调度，所以它会在promise.then函数之前运行。

setTimeout(() => {
    console.log('timer done, thought this would print second')
}, 0)

Promise.resolve().then(() => {
    console.log('promise done, thought this would print third')
})

console.log('synchronous, should print first')

输出：

'同步，应首先打印'

'承诺完成，认为这将打印第三个'

'计时器完成，认为这会打印第二个'