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1.Promise 产生背景及标准
家喻户晓，Promise 是 ES6 引入的新个性，旨在解决回调天堂。上面是一个简略的例子：管制接口调用程序：
apiA–>apiB–>apiC。简单的业务，开发人员会裂开。后生在此向老前辈致敬。
// 回调天堂
apiA({handleSuccess(resA){
apiB({handleSuccess(resB){
apiC({handleSuccess(resC){}})
}
})
}
})因而 Promise/A+ 标准应运而生，ES6 的 Promise 就是遵循标准开发进去的。
2. 同步 Promise
浏览标准可得上面几点根本要求:
- Promise 存在三个状态：pending（期待态）、fulfilled（胜利态）、rejected（失败态）
- pending 为初始态，并能够转化为 fulfilled 和 rejected
- 胜利时，不可转为其余状态，且必须有一个不可扭转的值（value）
- 失败时，不可转为其余状态，且必须有一个不可扭转的起因（reason）
- new Promise(executor=(resolve,reject)=>{resolve(value)})，resolve(value) 将状态置为 fulfilled
- new Promise(executor=(resolve,reject)=>{reject(reson)})，reject(reson) 将状态置为 rejected
- 若是 executor 运行异样执行 reject()
-
thenable：then(onFulfilled, onRejected)
- onFulfilled：status 为 fulfilled，执行 onFulfilled，传入 value
- onRejected：status 为 rejected，执行 onRejected，传入 reason
// 1.Promise 存在三个状态：pending（期待态）、fulfilled（胜利态）、rejected（失败态）const STATUS_PENDING = 'pending'
const STATUS_FULFILLED = 'fulfilled'
const STATUS_REJECTED = 'rejected'
class myPromise {constructor(executor) {
// pending 为初始态，并能够转化为 fulfilled 和 rejected
this.status = STATUS_PENDING
this.value = '' // 3
this.reason = '' // 4
let resolve = value => {
// 5.
if (this.status === STATUS_PENDING) {
this.status = STATUS_FULFILLED
this.value = value
}
}
let reject = reason => {
//6.
if (this.status === STATUS_PENDING) {
this.status = STATUS_REJECTED
this.reason = reason
}
}
// 7.
try {executor(resolve, reject);
} catch (err) {reject(err);
}
}
// 8.
then(onFulfilled = () => {}, onRejected = () => {}) {
// 8.1
if (this.status === STATUS_FULFILLED) {onFulfilled(this.value)
}
// 8.2
if (this.status === STATUS_REJECTED) {onRejected(this.reason)
}
}
}
new myPromise(resolve => {console.log('before resolve')
resolve(1)
}).then(res => {console.log(res)
})
new myPromise((resolve, reject) => {console.log('before reject')
reject('reject error')
}).then(res => {console.log(res)
}, error => {console.log(error)
})3. 异步 Promise
new myPromise(resolve => {console.log('before resolve')
setTimeout(()=>{resolve(1)
},1000)
}).then(res => {console.log(res)
})promise 的状态只能在 resolve 或者 reject 的时候扭转，同步代码执行到 then 回调的时候 promise 的状态还是 pending，明细不合乎咱们的冀望。
如果换做是你，你会怎么解决这个问题？举个栗子，你正在解决一堆事件（pending 状态），而后（then），老板曰: 一秒内做完手上的事来一下我办公室，做不完滚蛋。你怕遗记，个别会用清单记录 onResolvedCallbacks = [‘ 做完了手上的事, 去老板办公室 ’]，onRejectedCallbacks = [‘ 做不完, 滚蛋 ’]，1 秒后看实现后果，再根据抉择下一步。
欠缺后的代码如下：参考 前端进阶面试题具体解答
const STATUS_PENDING = 'pending'
const STATUS_FULFILLED = 'fulfilled'
const STATUS_REJECTED = 'rejected'
class myPromise {constructor(executor) {
this.status = STATUS_PENDING
this.value = ''this.reason =''
// 胜利寄存的数组
this.onResolvedCallbacks = [];
// 失败寄存法数组
this.onRejectedCallbacks = [];
let resolve = value => {if (this.status === STATUS_PENDING) {
this.status = STATUS_FULFILLED
this.value = value
// pending->fulfilled 依照胜利清单执行
this.onResolvedCallbacks.forEach(fn => fn())
}
}
let reject = reason => {if (this.status === STATUS_PENDING) {
this.status = STATUS_REJECTED
this.reason = reason
// pending->rejected 依照异样清单执行
this.onRejectedCallbacks.forEach(fn => fn());
}
}
try {executor(resolve, reject);
} catch (err) {reject(err);
}
}
then(onFulfilled = () => {}, onRejected = () => {}) {if (this.status === STATUS_FULFILLED) {onFulfilled(this.value)
}
if (this.status === STATUS_REJECTED) {onRejected(this.reason)
}
// 繁忙状态, 先记录老板嘱咐的内容
if (this.status === STATUS_PENDING) {
// onFulfilled 传入到胜利数组
this.onResolvedCallbacks.push(() => onFulfilled(this.value))
// onRejected 传入到失败数组
this.onRejectedCallbacks.push(() => onRejected(this.reason))
}
}
}
// 异步
new myPromise((resolve, reject) => {console.log('老板曰: 一秒做完手上的事来一下我办公室, 做不完滚蛋')
setTimeout(() => {if (false) { // 臣妾做不到啊
resolve('做完了手上的事, 去老板办公室')
} else {reject('做不完, 滚蛋')
}
}, 1000)
}).then(res => {console.log(`1s 后:${res}`)
}, error => {console.log(`1s 后:${error}`)
})
// 老板曰: 一秒做完手上的事来一下我办公室, 做不完滚蛋
// 1s 后: 做不完, 滚蛋 4. new Promise().then().then()…
这个思路倒是挺简略，就是 then 函数返回值为另一个 Promise 实例。依据标准批改后如下：
const PENDING = 'pending'
const FULFILLED = 'fulfilled'
const REJECTED = 'rejected'
function resolvePromise(promise2, x, resolve, reject) {
// 循环援用报错
if (x === promise2) {
// reject 报错
return reject(new TypeError('Chaining cycle detected for promise'));
}
// 避免屡次调用
let called;
// x 不是 null 且 x 是对象或者函数
if (x != null && (typeof x === 'object' || typeof x === 'function')) {
try {
// A+ 规定，申明 then = x 的 then 办法
let then = x.then;
// 如果 then 是函数，就默认是 promise 了
if (typeof then === 'function') {
// 就让 then 执行 第一个参数是 this 前面是胜利的回调 和 失败的回调
then.call(x, y => {
// 胜利和失败只能调用一个
if (called) return;
called = true;
// resolve 的后果仍旧是 promise 那就持续解析
resolvePromise(promise2, y, resolve, reject);
}, err => {
// 胜利和失败只能调用一个
if (called) return;
called = true;
reject(err); // 失败了就失败了
})
} else {resolve(x); // 间接胜利即可
}
} catch (e) {
// 也属于失败
if (called) return;
called = true;
// 取 then 出错了那就不要在继续执行了
reject(e);
}
} else {resolve(x);
}
}
class Promise {constructor(executor) {
this.state = PENDING
this.value = ''this.reason =''
// 胜利寄存的数组
this.onResolvedCallbacks = [];
// 失败寄存法数组
this.onRejectedCallbacks = [];
let resolve = value => {if (this.state === PENDING) {
this.state = FULFILLED
this.value = value
// pending->fulfilled 依照胜利清单执行
this.onResolvedCallbacks.forEach(fn => fn())
}
}
let reject = reason => {if (this.state === PENDING) {
this.state = REJECTED
this.reason = reason
// pending->rejected 依照异样清单执行
this.onRejectedCallbacks.forEach(fn => fn());
}
}
try {executor(resolve, reject);
} catch (err) {reject(err);
}
}
then(onFulfilled, onRejected) {
// onFulfilled 如果不是函数，就疏忽 onFulfilled，间接返回 value
onFulfilled = typeof onFulfilled === 'function' ? onFulfilled : value => value;
// onRejected 如果不是函数，就疏忽 onRejected，扔出谬误
onRejected = typeof onRejected === 'function' ? onRejected : err => {throw err};
let promise2 = new Promise((resolve, reject) => {if (this.state === FULFILLED) {
// 异步解决：// onRejected 返回一个一般的值，失败时如果间接等于 value => value，// 则会跑到下一个 then 中的 onFulfilled 中，setTimeout(() => {
try {let x = onFulfilled(this.value);
resolvePromise(promise2, x, resolve, reject);
} catch (e) {reject(e);
}
}, 0);
};
if (this.state === REJECTED) {setTimeout(() => {
try {let x = onRejected(this.reason);
resolvePromise(promise2, x, resolve, reject);
} catch (e) {reject(e);
}
}, 0);
};
if (this.state === PENDING) {this.onResolvedCallbacks.push(() => {setTimeout(() => {
try {let x = onFulfilled(this.value);
resolvePromise(promise2, x, resolve, reject);
} catch (e) {reject(e);
}
}, 0);
});
this.onRejectedCallbacks.push(() => {setTimeout(() => {
try {let x = onRejected(this.reason);
resolvePromise(promise2, x, resolve, reject);
} catch (e) {reject(e);
}
}, 0)
});
};
});
return promise2;
}
}
new Promise(resolve => {console.log(0)
setTimeout(() => resolve(1), 3000)
})
.then(res => {console.log(res)
return new Promise(resolve => {console.log(2)
setTimeout(() => {resolve(3)
}, 3000)
})
})
.then(res => {console.log(res)
})5. catch、resolve、reject、race 和 all
1. catch(非凡的 then 办法)
catch(fn){return this.then(null,fn)
}2. resolve(resolve 一个值)
Promise.resolve = val => new Promise(resolve=> resolve(val))3. reject(reject 一个值)
Promise.reject = val => new Promise((resolve,reject)=> reject(val))4. race
Promise.race([p1, p2, p3]) 外面哪个后果取得的快，就返回那个后果，不论后果自身是胜利状态还是失败状态。
Promise.race = promises =>
new Promise((resolve, reject) =>
promises.forEach(pro => pro.then(resolve, reject))
)5. all
Promise.all 能够将多个 Promise 实例包装成一个新的 Promise 实例。同时，胜利和失败的返回值是不同的，胜利的时候返回的是一个后果数组，而失败的时候则返回最先被 reject 失败状态的值。
Promise.all = function (promises) {return new Promise((resolve, reject) => {
let index = 0;
let result = [];
if (promises.length === 0) {resolve(result);
} else {function processValue(i, data) {result[i] = data;
if (++index === promises.length) {resolve(result);
}
}
for (let i = 0; i < promises.length; i++) {//promises[i] 可能是一般值
Promise.resolve(promises[i]).then((data) => {processValue(i, data);
}, (err) => {reject(err);
return;
});
}
}
});
}本文旨在记录学习过程，通过手打代码加深印象
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