业务开发中经常用Promise，但是第一题真不一定能作对。。。。。emmm，我说的是别犹豫的、能非常肯定的给出答案的哪种...
作为前端开发，相信日常开发中Promise的使用率应该时最高的，另外面试中js基础部分考察最多的也莫过于Promise，所以Promise的重要性咱就不多说了。
说的那么重要，是不是有点夸张了，想想不就几个api吗？但是真的了解他的运行机制吗？现在不管是大厂还是小厂，promise 已经成为了面试必考知识点；可是真正掌握了多少，真正面试的时候，又能有多少把握呢？
平时大家忙于业务开发，很多基础知识时间一长就容易淡忘，所以本文根据 Promise 的一些知识点总结
主要考察点
- 执行顺序
- 值透传
- 错误处理
- 返回值
- 链式调用
最终考察的还是我们对promise的理解程度。
目标
通关标准，能够给出答案，并且给出合理的解释。【为什么给出这个答案？】
#01
难易程度：⭐⭐⭐
Promise.resolve(1) .then((res) => { console.log(res) return 2 }) .catch((err) => { return 3 }) .then((res) => { console.log(res) })
#02
难易程度：⭐
const promise = new Promise((resolve, reject) => { console.log(1) resolve() console.log(2) }) promise.then(() => { console.log(3) }) console.log(4)
#03
难易程度：⭐⭐⭐
const promise1 = new Promise((resolve, reject) => { setTimeout(() => { resolve('success') }, 1000) }) const promise2 = promise1.then(() => { throw new Error('error!!!') }) console.log('promise1', promise1) console.log('promise2', promise2) setTimeout(() => { console.log('promise1', promise1) console.log('promise2', promise2) }, 2000)
#04
难易程度：⭐⭐
setTimeout(()=> console.log(5), 0); new Promise(resolve => { console.log(1); resolve(3); Promise.resolve().then(()=> console.log(4)) }).then(num => { console.log(num) }); console.log(2);
#05
难易程度：⭐⭐
const promise = new Promise((resolve, reject) => { resolve('success1') reject('error') resolve('success2') }) promise .then((res) => { console.log('then: ', res) }) .catch((err) => { console.log('catch: ', err) })
#06
难易程度：⭐⭐
const promise = new Promise((resolve, reject) => { setTimeout(() => { console.log('once') resolve('success') }, 1000) }) const start = Date.now() promise.then((res) => { console.log(res, Date.now() - start) }) promise.then((res) => { console.log(res, Date.now() - start) })
#07
难易程度：⭐⭐⭐
Promise.resolve() .then(() => { return new Error('error!!!') }) .then((res) => { console.log('then: ', res) }) .catch((err) => { console.log('catch: ', err) })
#08
难易程度：⭐⭐⭐⭐
const promise = Promise.resolve() .then(() => { return promise }) promise.catch(console.error)
#09
难易程度：⭐⭐⭐
Promise.resolve(1) .then(2) .then(Promise.resolve(3)) .then(console.log)
#10
难易程度：⭐⭐⭐
Promise.resolve() .then(function success (res) { throw new Error('error') }, function fail1 (e) { console.error('fail1: ', e) }) .catch(function fail2 (e) { console.error('fail2: ', e) })
变种后
Promise.resolve() .then(function success1 (res) { throw new Error('error') }, function fail1 (e) { console.error('fail1: ', e) }) .then(function success2 (res) { }, function fail2 (e) { console.error('fail2: ', e) })
#11
难易程度：⭐⭐⭐⭐
process.nextTick(() => { console.log('nextTick') }) Promise.resolve() .then(() => { console.log('then') }) setImmediate(() => { console.log('setImmediate') }) console.log('end')
#12
难易程度：⭐⭐⭐⭐
const first = () => (new Promise((resolve, reject) => { console.log(3); let p = new Promise((resolve, reject) => { console.log(7); setTimeout(() => { console.log(5); resolve(6); }, 0) resolve(1); }); resolve(2); p.then((arg) => { console.log(arg); }); })); first().then((arg) => { console.log(arg); }); console.log(4);
#13
难易程度：⭐⭐
var p = new Promise((resolve, reject) => { reject(Error('The Fails!')) }) p.catch(error => console.log(error.message)) p.catch(error => console.log(error.message))
#14
难易程度：⭐⭐⭐
var p = new Promise((resolve, reject) => { return Promise.reject(Error('The Fails!')) }) p.catch(error => console.log(error.message)) p.catch(error => console.log(error.message))
#15
难易程度：⭐⭐
var p = new Promise((resolve, reject) => { reject(Error('The Fails!')) }) .catch(error => console.log(error)) .then(error => console.log(error))
#16
难易程度：⭐⭐
new Promise((resolve, reject) => { resolve('Success!') }) .then(() => { throw Error('Oh noes!') }) .catch(error => { return "actually, that worked" }) .catch(error => console.log(error.message))
#17
难易程度：⭐⭐
Promise.resolve('Success!') .then(data => { return data.toUpperCase() }) .then(data => { console.log(data) return data }) .then(console.log)
#18
难易程度：⭐⭐
Promise.resolve('Success!') .then(() => { throw Error('Oh noes!') }) .catch(error => { return 'actually, that worked' }) .then(data => { throw Error('The fails!') }) .catch(error => console.log(error.message))
#19
难易程度：⭐⭐⭐⭐
const first = () => (new Promise((resolve,reject)=>{ console.log(3); let p = new Promise((resolve, reject)=>{ console.log(7); setTimeout(()=>{ console.log(5); resolve(6); },0) resolve(1); }); resolve(2); p.then((arg)=>{ console.log(arg); }); })); first().then((arg)=>{ console.log(arg); }); console.log(4);
#20
难易程度：⭐⭐⭐⭐⭐
async function async1() { console.log(1); const result = await async2(); console.log(3); } async function async2() { console.log(2); } Promise.resolve().then(() => { console.log(4); }); setTimeout(() => { console.log(5); }); async1(); console.log(6);
# 最后
以上19个代码题未贴答案，后面会单独发送。
也欢迎大家在留言区回复参与答题。
今天一提就到这里，希望对你有所帮助。
