Today I will share promises with you. The author will start with the dilemma of early asynchronous code, what problems the emergence of promises solved, the ultimate solution to asynchronous callback hell, and implement the core syntax of async await. In fact, async/await is just a variant of generator promise. That’s all.
promise. function requestData(url) {
setTimeout(() => {
if (url === 'iceweb.io') {
return '请求成功'
}
return '请求失败'
}, 3000)
}
const result = requestData('iceweb.io')
console.log(result) //undefinedThe execution order of the code is not something you take for granted. The code is actually not executed in the order you wrote it.
? function, I start executing the function. When encountering an asynchronous operation, it will not block the execution of the following code, because js is single-threaded, so the return you write does not return if successful or failed. In this function, I ignore the asynchronous operation and do not There is no return value, so the value is undefined.
function requestData(url, successCB, failureCB) {
setTimeout(() => {
if (url === 'iceweb.io') {
successCB('我成功了,把获取到的数据传出去', [{name:'ice', age:22}])
} else {
failureCB('url错误，请求失败')
}
}, 3000)
}
//3s后 回调successCB
//我成功了,把获取到的数据传出去 [ { name: 'ice', age: 22 } ]
requestData('iceweb.io', (res, data) => console.log(res, data), rej => console.log(rej))
//3s后回调failureCB
//url错误，请求失败
requestData('icexxx.io', res => console.log(res) ,rej => console.log(rej)) (promise), gives the caller a promise, and returns data to You can create a promise object
a promise, at this time we need to pass a callback function, this function is executed immediately, called For (executor)
, reject (the function can pass parameters) function is executed, the .then function of the promise object will be called back
function is executed, the .catche function of the promise object will be called back
new Promise((resolve, reject) => {
console.log(`executor 立即执行`)
}) is executed immediately function requestData(url) {
return new Promise((resolve, reject) => {
setTimeout(() => {
if (url === 'iceweb.io') {
//只能传递一个参数
resolve('我成功了,把获取到的数据传出去')
} else {
reject('url错误，请求失败')
}
}, 3000)
})
}
//1. 请求成功
requestData('iceweb.io').then(res => {
//我成功了,把获取到的数据传出去
console.log(res)
})
//2. 请求失败
//2.2 第一种写法
//url错误，请求失败
requestData('iceweb.org').then(res => {},rej => console.log(rej))
//2.2 第二种写法
//url错误，请求失败
requestData('iceweb.org').catch(e => console.log(e))(Will be automatically executed in the Promise class)
function when it is correct, and call the reject function when it fails to pass the required parameters.
method. You can also view Promise/A Specification , give it a promise, we can divide it into three stages function represents the cashed status
function represents the rejected status
const promise = new Promise((resolve, reject) => {
setTimeout(() => {
reject('失败')
resolve('成功')
}, 3000);
})
promise.then(res => console.log(res)).catch(err => console.log(err))
//失败reject之后，在调用resolve是无效的，因为状态已经发生改变，并且是不可逆的。resolve传入一个普通的值或者对象，只能传递接受一个参数，那么这个值会作为then回调的参数const promise = new Promise((resolve, reject) => {
resolve({name: 'ice', age: 22})
})
promise.then(res => console.log(res))
// {name: 'ice', age: 22}resolve中传入的是另外一个Promise，那么这个新Promise会决定原Promise的状态const promise = new Promise((resolve, reject) => {
resolve(new Promise((resolve, reject) => {
setTimeout(() => {
resolve('ice')
}, 3000);
}))
})
promise.then(res => console.log(res))
//3s后 iceresolve中传入的是一个对象，并且这个对象有实现then方法，那么会执行该then方法，then方法会传入resolve，reject函数。此时的promise状态取决于你调用了resolve，还是reject函数。这种模式也称之为: thenable
const promise = new Promise((resolve, reject) => {
resolve({
then(res, rej) {
res('hi ice')
}
})
})
promise.then(res => console.log(res))
// hi icePromise.prototype上的方法，也就是Promise的显示原型上，当我new Promise的时候，会把返回的改对象的 promise[[prototype]]（隐式原型） === Promise.prototype (显示原型)then方法可以接受参数，一个参数为成功的回调，另一个参数为失败的回调，前面重构requestData中有演练过。const promise = new Promise((resolve, reject) => {
resolve('request success')
// reject('request error')
})
promise.then(res => console.log(res), rej => console.log(rej))
//request successnull或""占位const promise = new Promise((resolve, reject) => {
// resolve('request success')
reject('request error')
})
promise.then(null, rej => console.log(rej))
//request errorconst promise = new Promise((resolve, reject) => {
resolve('hi ice')
})
promise.then(res => ({name:'ice', age:22}))
.then(res => console.log(res))
//{name:'ice', age:22}then方法是有返回值的，它的返回值是promise，但是是promise那它的状态如何决定呢？接下来让我们一探究竟。const promise = new Promise((resolve, reject) => {
resolve('hi ice')
})
promise.then(res => ({name:'ice', age:22}))
.then(res => console.log(res))
//{name:'ice', age:22}Promise.resolve，并且把返回值作为实参传递到then方法中。undefined
const promise = new Promise((resolve, reject) => {
resolve('hi ice')
})
promise.then(res => {
return new Promise((resolve, reject) => {
resolve('then 的返回值')
})
}).then(res => console.log(res))
//then 的返回值promise对象，状态和你调用resolve，还是reject有关const promise = new Promise((resolve, reject) => {
resolve('hi ice')
})
promise.then(res => {
return {
then(resolve, reject) {
resolve('hi webice')
}
}
}).then(res => console.log(res))
//hi webiceresolve,还是reject
const promise = new Promise((resolve, reject) => {
reject('ice error')
})
promise.catch(err => console.log(err))
promise.catch(err => console.log(err))
promise.catch(err => console.log(err))
//ice error
//ice error
//ice errorresolve还是reject
const promise = new Promise((resolve, reject) => {
reject('ice error')
})
promise.catch(err => ({name:'ice', age: 22})).then(res => console.log(res))
//{name:'ice', age: 22}const promise = new Promise((resolve, reject) => {
reject('ice error')
})
promise.catch(err => {
return new Promise((resolve, reject) => {
reject('ice error promise')
})
}).catch(res => console.log(res))
//ice error promisenew Promise() 调用了reject函数，则会被catch捕获到const promise = new Promise((resolve, reject) => {
reject('ice error')
})
promise.catch(err => {
return {
then(resolve, reject) {
reject('ice error then')
}
}
}).catch(res => console.log(res))
//ice error thenfinally方法const promise = new Promise((resolve, reject) => {
resolve('hi ice')
})
promise.then(res => console.log(res)).finally(() => console.log('finally execute'))
//finally executePromise.resolve('ice')
//等价于
new Promise((resolve, reject) => resolve('ice'))Promise.reject('ice error')
//等价于
new Promise((resolve, reject) => reject('ice error'))fulfilled 状态
const promise1 = new Promise((resolve, reject) => {
setTimeout(() => {
resolve('hi ice')
}, 1000);
})
const promise2 = new Promise((resolve, reject) => {
setTimeout(() => {
resolve('hi panda')
}, 2000);
})
const promise3 = new Promise((resolve, reject) => {
setTimeout(() => {
resolve('hi grizzly')
}, 3000);
})
Promise.all([promise1, promise2, promise3]).then(res => console.log(res))
//[ 'hi ice', 'hi panda', 'hi grizzly' ]resolve状态的时候才会调用.then方法。.catch方法rejected状态
const promise1 = new Promise((resolve, reject) => {
setTimeout(() => {
resolve('hi ice')
}, 1000);
})
const promise2 = new Promise((resolve, reject) => {
setTimeout(() => {
reject('hi panda')
}, 2000);
})
const promise3 = new Promise((resolve, reject) => {
setTimeout(() => {
resolve('hi grizzly')
}, 3000);
})
Promise.all([promise1, promise2, promise3]).then(res => console.log(res)).catch(err => console.log(err))
//hi pandaPromise.all有一个缺陷，就是当遇到一个rejected的状态，那么对于后面是resolve或者reject的结果我们是拿不到的Promise.allSettled，无论状态是fulfilled/rejected都会把参数返回给我们所有promise都有结果
const promise1 = new Promise((resolve, reject) => {
setTimeout(() => {
reject('hi ice')
}, 1000);
})
const promise2 = new Promise((resolve, reject) => {
setTimeout(() => {
resolve('hi panda')
}, 2000);
})
const promise3 = new Promise((resolve, reject) => {
setTimeout(() => {
reject('hi grizzly')
}, 3000);
})
Promise.allSettled([promise1, promise2, promise3]).then(res => console.log(res))
/* [
{ status: 'rejected', reason: 'hi ice' },
{ status: 'fulfilled', value: 'hi panda' },
{ status: 'rejected', reason: 'hi grizzly' }
] */其中一个promise没有结果
const promise1 = new Promise((resolve, reject) => {
setTimeout(() => {
reject('hi ice')
}, 1000);
})
const promise2 = new Promise((resolve, reject) => {
setTimeout(() => {
resolve('hi panda')
}, 2000);
})
const promise3 = new Promise((resolve, reject) => {})
Promise.allSettled([promise1, promise2, promise3]).then(res => console.log(res))
// 什么都不打印const promise1 = new Promise((resolve, reject) => {
setTimeout(() => {
reject('hi error')
}, 1000);
})
const promise2 = new Promise((resolve, reject) => {
setTimeout(() => {
resolve('hi panda')
}, 2000);
})
Promise.race([promise1, promise2])
.then(res => console.log(res))
.catch(e => console.log(e))
//hi errorAggregateError
const promise1 = new Promise((resolve, reject) => {
setTimeout(() => {
reject('hi error')
}, 1000);
})
const promise2 = new Promise((resolve, reject) => {
setTimeout(() => {
resolve('hi panda')
}, 2000);
})
Promise.any([promise1, promise2])
.then(res => console.log(res))
.catch(e => console.log(e))
//hi pandafunction requestData(url) {
return new Promise((resolve, reject) => {
setTimeout(() => {
if (url.includes('iceweb')) {
resolve(url)
} else {
reject('请求错误')
}
}, 1000);
})
}
requestData('iceweb.io').then(res => {
requestData(`iceweb.org ${res}`).then(res => {
requestData(`iceweb.com ${res}`).then(res => {
console.log(res)
})
})
})
//iceweb.com iceweb.org iceweb.iofunction requestData(url) {
return new Promise((resolve, reject) => {
setTimeout(() => {
if (url.includes('iceweb')) {
resolve(url)
} else {
reject('请求错误')
}
}, 1000);
})
}
requestData('iceweb.io').then(res => {
return requestData(`iceweb.org ${res}`)
}).then(res => {
return requestData(`iceweb.com ${res}`)
}).then(res => {
console.log(res)
})
//iceweb.com iceweb.org iceweb.iofunction requestData(url) {
return new Promise((resolve, reject) => {
setTimeout(() => {
if (url.includes('iceweb')) {
resolve(url)
} else {
reject('请求错误')
}
}, 1000);
})
}
function* getData(url) {
const res1 = yield requestData(url)
const res2 = yield requestData(res1)
const res3 = yield requestData(res2)
console.log(res3)
}
const generator = getData('iceweb.io')
generator.next().value.then(res1 => {
generator.next(`iceweb.org ${res1}`).value.then(res2 => {
generator.next(`iceweb.com ${res2}`).value.then(res3 => {
generator.next(res3)
})
})
})
//iceweb.com iceweb.org iceweb.iogetData已经变为同步的形式，可以拿到我最终的结果了。那么很多同学会问，generator一直调用.next不是也产生了回调地狱吗？function requestData(url) {
return new Promise((resolve, reject) => {
setTimeout(() => {
if (url.includes('iceweb')) {
resolve(url)
} else {
reject('请求错误')
}
}, 1000);
})
}
function* getData() {
const res1 = yield requestData('iceweb.io')
const res2 = yield requestData(`iceweb.org ${res1}`)
const res3 = yield requestData(`iceweb.com ${res2}`)
console.log(res3)
}
//自动化执行 async await相当于自动帮我们执行.next
function asyncAutomation(genFn) {
const generator = genFn()
const _automation = (result) => {
let nextData = generator.next(result)
if(nextData.done) return
nextData.value.then(res => {
_automation(res)
})
}
_automation()
}
asyncAutomation(getData)
//iceweb.com iceweb.org iceweb.ioasync await的一个变种而已.next方法function requestData(url) {
return new Promise((resolve, reject) => {
setTimeout(() => {
if (url.includes('iceweb')) {
resolve(url)
} else {
reject('请求错误')
}
}, 1000);
})
}
async function getData() {
const res1 = await requestData('iceweb.io')
const res2 = await requestData(`iceweb.org ${res1}`)
const res3 = await requestData(`iceweb.com ${res2}`)
console.log(res3)
}
getData()
//iceweb.com iceweb.org iceweb.iogetData生成器函数函数，改为async函数，yeild的关键字替换为await就可以实现异步代码同步写法了。async function sayHi() {
console.log('hi ice')
}
sayHi()
//hi ice异步函数的返回值和普通返回值有所区别
undefined
Promise.resolve(返回值)resolve,或者reject有关异步函数中可以使用await关键字，现在在全局也可以进行await，但是不推荐。会阻塞主进程的代码执行
async function sayHi() {
console.log(res)
}
sayHi().catch(e => console.log(e))
//或者
async function sayHi() {
try {
console.log(res)
}catch(e) {
console.log(e)
}
}
sayHi()
//ReferenceError: res is not definedawait关键字，普通函数不行resolve或者reject
await后续的代码，所以await后面的代码，相当于包括在.then方法的回调中，如果状态变为rejected，你则需要在函数内部try catch，或者进行链式调用进行.catch操作function requestData(url) {
return new Promise((resolve, reject) => {
setTimeout(() => {
if (url.includes('iceweb')) {
resolve(url)
} else {
reject('请求错误')
}
}, 1000);
})
}
async function getData() {
const res = await requestData('iceweb.io')
console.log(res)
}
getData()
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