你能答对几题?

题目一

const promise = new Promise((resolve, reject) => { console.log(1) resolve() console.log(2) }) promise.then(() => { console.log(3) }) console.log(4)

题目二

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)

题目三

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) })

题目四

Promise.resolve(1) .then((res) => { console.log(res) return 2 }) .catch((err) => { return 3 }) .then((res) => { console.log(res) })

题目五

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) })

题目六

Promise.resolve() .then(() => { return new Error('error!!!') }) .then((res) => { console.log('then: ', res) }) .catch((err) => { console.log('catch: ', err) })

题目七

const promise = Promise.resolve() .then(() => { return promise }) promise.catch(console.error)

题目八

Promise.resolve(1) .then(2) .then(Promise.resolve(3)) .then(console.log)

题目九

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) })

题目十

process.nextTick(() => { console.log('nextTick') }) Promise.resolve() .then(() => { console.log('then') }) setImmediate(() => { console.log('setImmediate') }) console.log('end') 总结:promise的状态一旦确定的话,就不能修改. then()的参数如果不合法,则跳过 promise构造函数的参数是个立即执行的函数,then是异步的