为什么Promise中的Promise.resolve将在Promise之前执行？

Question

new Promise(resolve => {
    resolve(1)
}).then(a => console.log(a))
Promise.resolve().then(() => console.log(2))

将打印1、2

Promise.resolve().then(() => console.log(2))
new Promise(resolve => {
    resolve(1)
}).then(a => console.log(a))

将打印2，1

但我不明白为什么

new Promise(resolve => {
    resolve(1)
    Promise.resolve().then(() => console.log(2))
}).then(a => console.log(a))

将打印2，1

Answer 1

Promises通过使用事件循环来工作，因此任何调用Promise的代码都将按照解析顺序进行，无论创建顺序如何。

const test = Promise.resolve(1).then(console.log) // pushes chain A to loop, [A]
Promise.resolve(2).then(console.log) // pushes chain B onto loop, [A,B]
test.then(() => 3).then(console.log) // pushes chain C onto loop, [A, B, C]
//first loop resolution, 1, then console.log
//second loop resolution, 2, then console.log
//third loop resolution, 3, then console.log

即使test是链之前作出B ，链C发生AFTER B因为事件循环处于先进先出模型。 这是因为Promise是asynchronous这意味着它们将按照事件循环接收的顺序进行解析，并且由于resolve无需等待就可以推送到事件循环中，因此它就在那里。

fetch('https://google.com').then(() => console.log(1)) // waits for networks
Promise.resolve(2).then(console.log) // pushes chain B onto loop [B]
// B resolves, 2, console.log, loop empty []
// fetch returns finally, pushes A onto loop [A]
// A resolves, 1, console.log, loop empty []

您所看到的是立即兑现承诺的竞争条件，这意味着它们将按照通过链条执行的命令性顺序解决。

Answer 2

Promise.resolve将返回立即解决的new Promise(resolve => { resolve() }) ，并且调用new Promise(resolve => { resolve() })也将立即解决。 仅仅因为它的承诺不会使它异步，如果您不对其执行异步操作，例如将其抛出超时，间隔等情况，那么：

// this resolves when called and logs first (1)
new Promise(resolve => {
    resolve(1)
}).then(a => console.log(a))
// this resolves and logs immediately (2)
Promise.resolve().then(() => console.log(2))

// this resolves immediately (2)
Promise.resolve().then(() => console.log(2))
// this resolves when called (1)
new Promise(resolve => {
    resolve(1)
}).then(a => console.log(a))

new Promise(resolve => {
    // this passes 1 to the then chained to this promise,
    // which won't run until this function body is done.
    resolve(1)
    // this resolves immediately and logs (2)
    Promise.resolve().then(() => console.log(2))
    // now we drop into then and it logs (1)
}).then(a => console.log(a))

有关Promise.resolve更多详细信息，请参见此处 。