目的
记录做错的题，就像上学期间的错题本一样，错题错过一次后还是容易错，只有不断反复练习才会真正掌握，持续更新错题本。
题目一
new Promise((res,rej)=>{
console.log(1);
res(2);
console.log(3)
throw Error('abc')
}).catch((err)=>{
console.log('123')
}).then((res)=>{
console.log(res)
})
console.log('333')
可以看看上面题的输出顺序是什么？这里有个坑，resolve后再throw还会reject吗？就是被这个点给迷惑住了，我在想executor执行的时候是捕获了异常的，所以肯定是走到了reject中，但是，reject执行的时候是会判断当前promise的状态是不是pending，如果是pending才会变化，否则是不会执行的。
解答：
1、3、333、2
来吧，上规范，promisesaplus.com/， 还是说人话，直接中文吧。 【译】 Promises/A+ 规范
这道题还是答错了，之前以为promise掌握的不错了，一做题还是崩啊，所以这知识真是越学发现自己不会的越多。
题目二
const promise1 = new Promise((resolve, reject) => {
console.log('promise1');
resolve('resolve1');
});
const promise2 = promise1.then(res => {
console.log(res);
});
console.log('1', promise1);
console.log('2', promise2);
知识点：resolve时状态更改和赋值是同步的，then的回调函数是异步的，所以promise1输出时是resolved的状态。
题目三
Promise.resolve(1)
.then(2)
.then(Promise.resolve(3))
.then(console.log)
知识点： Promise.resolve的方法如果是一个原始值，则返回一个新对象状态为resolved，同时把值传递给回调函数，中间的都忽略。
1
题目四
async function async1() {
console.log('async1 start');
// 注意这里没有resolve
await new Promise(resolve => {
console.log('promise1')
})
console.log('async1 success');
return 'async1 end';
}
console.log('script start')
async1().then(res => console.log(res))
console.log('script end');
易错点：await后的promise没有resolve，所以一直是pending状态
题目五
const first = () =>
new Promise((resolve, reject) => {
console.log(3);
let p = new Promise((resolve, reject) => {
console.log(7);
setTimeout(() => {
console.log(5);
resolve(6);
console.log(p);
}, 0);
resolve(1);
});
resolve(2);
p.then((arg) => {
console.log(arg);
});
});
first().then((arg) => {
console.log(arg);
});
console.log(4);
易错点：
- 代码多，需要仔细看
- 注意p和first的then调用顺序，p.then先调用所以它在前，
- resolve时会将promise状态变为success，并赋值，then是异步回调。
- 最后setTimeout里的console也是会打印出来。
题目六
const p1 = new Promise((resolve) => {
setTimeout(() => {
resolve("resolve3");
console.log("timer1");
}, 0);
resolve("resovle1");
resolve("resolve2");
})
.then((res) => {
console.log(res);
setTimeout(() => {
console.log(p1);
}, 1000);
// return 1
})
.finally((res) => {
console.log("finally", res);
});
易错点
- 关于finally返回值，res是拿不到任何值的，所以是undefined
- p1是finally返回的promise，finally返回值在没有抛出错误的情况下，默认是上一个promise的返回值，这道题上一个promise是then，它没有返回任何内容， 则是undefined，如果加上return 1，则返回1
题目七
Promise.resolve().then(() =>{
console.log('1');
throw 'Error';
}).then(() => {
console.log('2');
}).
catch(() => {
console.log('3');
throw 'Error';
}).then(() =>{
console.log('4');
}).
catch(() => {
console.log('5');
}).then(() =>{
console.log('6');
});
- catch中抛出错误，也是会被后面的catch住
1、3、5、6
题目八
setTimeout(() => {
console.log('timeout0');
new Promise((resolve, reject) => { resolve('resolved') }).then(res => console.log(res));
new Promise((resolve, reject) => {
setTimeout(()=>{
resolve('timeout resolved')
})
}).then(res => console.log(res));
process.nextTick(() => {
console.log('nextTick1');
process.nextTick(() => {
console.log('nextTick2');
});
});
process.nextTick(() => {
console.log('nextTick3');
});
console.log('sync');
setTimeout(() => {
console.log('timeout2');
}, 0);
}, 0);
答案：
> timeout0
sync
nextTick1
nextTick3
nextTick2
resolved
timeout resolved
timeout2
题目九
setImmediate(function(){
console.log("setImmediate");
setImmediate(function(){
console.log("嵌套setImmediate");
});
process.nextTick(function(){
console.log("nextTick");
});
setTimeout(() => {
console.log('settimeout')
})
});
题目十
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('setTimeout0')
},0)
setTimeout(function(){
console.log('setTimeout3')
},3)
setImmediate(() => console.log('setImmediate'));
process.nextTick(() => console.log('nextTick'));
async1();
new Promise(function(resolve){
console.log('promise1')
resolve();
console.log('promise2')
}).then(function(){
console.log('promise3')
})
console.log('script end')
最后三个setImmediate、setTimeout0、settimeout3 顺序不一定，每次运行可能不一样。
题目十一
setTimeout(() => {
console.log("setTimeout 100");
setTimeout(() => {
console.log("setTimeout 100 - 0");
process.nextTick(() => {
console.log("nextTick in setTimeout 100 - 0");
});
}, 0);
setImmediate(() => {
console.log("setImmediate in setTimeout 100");
process.nextTick(() => {
console.log("nextTick in setImmediate in setTimeout 100");
});
});
process.nextTick(() => {
console.log("nextTick in setTimeout100");
});
Promise.resolve().then(() => {
console.log("promise in setTimeout100");
});
}, 100);
// const fs = require('fs')
// fs.readFile('./1.poll.js', () => {
// console.log('poll 1');
// process.nextTick(() => {
// console.log('nextTick in poll ======');
// })
// })
setTimeout(() => {
console.log("setTimeout 0");
process.nextTick(() => {
console.log("nextTick in setTimeout");
});
}, 0);
setTimeout(() => {
console.log("setTimeout 1");
Promise.resolve().then(() => {
console.log("promise in setTimeout1");
});
process.nextTick(() => {
console.log("nextTick in setTimeout1");
});
}, 1);
setImmediate(() => {
console.log("setImmediate");
process.nextTick(() => {
console.log("nextTick in setImmediate");
});
});
process.nextTick(() => {
console.log("nextTick 1");
process.nextTick(() => {
console.log("nextTick 2");
});
});
console.log("global ------");
Promise.resolve().then(() => {
console.log("promise 1");
process.nextTick(() => {
console.log("nextTick in promise");
});
});
这里微任务执行完后，就会执行settimeout0、settimeout1、setImmediate在后面执行。
同时微任务是每执行完一个宏任务后就执行所有的微任务。
