摘要：分析題目先執行微任務后執行宏任務因此結果為按照這個思路做以下題目做出如下更改運行結果為更改如下更改如下運行結果為運行結果為運行結果為參考自

參考自https://github.com/Advanced-F...

async function async1() {
    console.log("async1 start");
    await async2();
    console.log("async1 end");
}
async function async2() {
    console.log("async2");
}
console.log("script start");
setTimeout(function() {
    console.log("setTimeout");
}, 0)
async1();
new Promise(function(resolve) {
    console.log("promise1");
    resolve();
}).then(function() {
    console.log("promise2");
});
console.log("script end");

運行結果為:

script start
async1 start
async2
promise1
script end
async1 end
promise2
setTimeout

這道題涉及到事件循環中函數執行順序的問題,其中包括async,await,setTimeout,Promise函數

1.event loop
JS主線程不斷的循環往復的從任務隊列中讀取任務,執行任務,這種運行機制為事件循環.

2.Microtasks, Macrotasks
Microtasks(微任務)和Macrotasks(宏任務)是異步任務的一種類型,Microtasks的優先級要高于Macrotasks,下面是它們所包含的api:

microtasks

process.nextTicks
promise
Object.observe
MutationObserver



macrotasks

setTimeout
setImmerdiate
I/O
UI渲染

1. 每一個 event loop 都有一個 microtask queue
2. 每個 event loop 會有一個或多個macrotaks queue ( 也可以稱為task queue )
3. 一個任務 task 可以放入 macrotask queue 也可以放入 microtask queue中
4. 每一次event loop，會首先執行 microtask queue， 執行完成后，會提取 macrotask queue 的一個任務加入 microtask queue， 接著繼續執行microtask queue，依次執行下去直至所有任務執行結束。

分析題目:

先執行微任務后執行宏任務,因此結果為

script start
async1 start
async2
promise1
script end
async1 end
promise2
setTimeout

按照這個思路,做以下題目:

async function async1() {
    console.log("async1 start");
    await async2();
    console.log("async1 end");
}
async function async2() {
    //async2做出如下更改：
    new Promise(function(resolve) {
    console.log("promise1");
    resolve();
}).then(function() {
    console.log("promise2");
    });
}
console.log("script start");

setTimeout(function() {
    console.log("setTimeout");
}, 0)
async1();

new Promise(function(resolve) {
    console.log("promise3");
    resolve();
}).then(function() {
    console.log("promise4");
});

console.log("script end");

運行結果為:

script start
async1 start
promise1
promise3
script end
promise2
async1 end
promise4
setTimeout

async function async1() {
    console.log("async1 start");
    await async2();
    //更改如下：
    setTimeout(function() {
        console.log("setTimeout1")
    },0)
}
async function async2() {
    //更改如下：
    setTimeout(function() {
        console.log("setTimeout2")
    },0)
}
console.log("script start");

setTimeout(function() {
    console.log("setTimeout3");
}, 0)
async1();

new Promise(function(resolve) {
    console.log("promise1");
    resolve();
}).then(function() {
    console.log("promise2");
});
console.log("script end");

運行結果為

script start
async1 start
promise1
script end
promise2
setTimeout3
setTimeout2
setTimeout1

async function a1 () {
    console.log("a1 start")
    await a2()
    console.log("a1 end")
}
async function a2 () {
    console.log("a2")
}

console.log("script start")

setTimeout(() => {
    console.log("setTimeout")
}, 0)

Promise.resolve().then(() => {
    console.log("promise1")
})

a1()

let promise2 = new Promise((resolve) => {
    resolve("promise2.then")
    console.log("promise2")
})

promise2.then((res) => {
    console.log(res)
    Promise.resolve().then(() => {
        console.log("promise3")
    })
})
console.log("script end")

運行結果為

script start
a1 start
a2
promise2
script end
promise1
a1 end
promise2.then
promise3
setTimeout

async function async1() {
    console.log("async1 start")
    await async2()
    console.log("async1 end")
}
async function async2() {
    console.log("async2")
}
console.log("script start")
setTimeout(function () {
    console.log("settimeout")
})
async1()
new Promise(function (resolve) {
    console.log("promise1")
    resolve()
}).then(function () {
    console.log("promise2")
})
console.log("script end")

運行結果為

script start
async1 start
async2
promise1
script end
async1 end
promise2
settimeout

參考自https://github.com/Advanced-F...