实现Promise
2022/1/19 promise
# Promise
- 简化版
const PENDING = "pending";
const FULFILLED = 'fulfilled';
const REJECTED = "rejected";
function myPromise(executor) {
let self = this;
self.status = PENDING; //定义状态改变前的初始状态
self.value = undefined; //定义状态为fulfilled的时候的状态
self.reason = undefined; //定义状态为rejected的时候的状态
self.resolveCb = [];
self.rejectCb = [];
function resolve(value) {
// 判断传入元素是否为Promise值，如果是，则状态改变必须等待前一个状态改变后再进行改变
if (value instanceof myPromise) {
return value.then(resolve, reject);
}
// 保证代码的执行顺序为本轮事件循环的末尾
setTimeout(() => {
// 只有状态为 pending 时才能转变，
if (self.status === PENDING) {
// 修改状态
self.status = FULFILLED;
// 设置传入的值
self.value = value;
// 执行回调函数
self.resolveCb.forEach(callback => {
callback(value);
});
}
}, 0);
}
function reject(reason) {
// 保证代码的执行顺序为本轮事件循环的末尾
setTimeout(() => {
// 只有状态为 pending 时才能转变
if (self.status === PENDING) {
// 修改状态
self.status = REJECTED;
// 设置传入的值
self.value = reason;
// 执行回调函数
self.rejectCb.forEach(callback => {
callback(reason);
});
}
}, 0);
}
//捕获构造异常
try {
executor(resolve, reject);
} catch (e) {
reject(e);
}
}
myPromise.prototype.then = function(onResolved, onRejected) {
const self = this;
// 首先判断两个参数是否为函数类型，因为这两个参数是可选参数
onResolved = typeof onResolved === "function"
? onResolved : function(value) { return value};
onRejected =typeof onRejected === "function"
? onRejected : function(error) { throw error };
// 判断当前状态是否为fulfilled
if (self.status == FULFILLED ) {
onResolved(self.value);
}
// 判断当前状态是否为rejected
if (self.status == REJECTED ) {
onRejected(self.reason);
}
// 判断当前状态是否为pending
if (self.status == PENDING ) {
self.resolveCb.push(() => onResolved(self.value));
self.rejectCb.push(() => onRejected(self.reason));
}
};
// demo
var p = new myPromise(function(resolve, reject) {
resolve(1);
});
p.then(function(x) {
console.log(x);
});
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# promise完整版
- ES5实现
const PENDING = 'pending';
const FULFILLED = 'fulfilled';
const REJECTED = 'rejected';
const resolvePromise = (promise2, x, resolve, reject) => {
// 自己等待自己完成是错误的实现，用一个类型错误，结束掉 promise Promise/A+ 2.3.1
if (promise2 === x) {
return reject(
new TypeError("Chaining cycle detected for promise #<Promise>"));
}
// Promise/A+ 2.3.3.3.3 只能调用一次
let called;
// 后续的条件要严格判断 保证代码能和别的库一起使用
if ((typeof x === "object" && x != null) || typeof x === "function") {
try {
// 为了判断 resolve 过的就不用再 reject 了（比如 reject 和 resolve 同时调用的时候） Promise/A+ 2.3.3.1
let then = x.then;
if (typeof then === "function") {
// 不要写成 x.then，直接 then.call 就可以了 因为 x.then 会再次取值，Object.defineProperty Promise/A+ 2.3.3.3
then.call(
x, (y) => {
// 根据 promise 的状态决定是成功还是失败
if (called) return;
called = true;
// 递归解析的过程（因为可能 promise 中还有 promise） Promise/A+ 2.3.3.3.1
resolvePromise(promise2, y, resolve, reject);
}, (r) => {
// 只要失败就失败 Promise/A+ 2.3.3.3.2
if (called) return;
called = true;
reject(r);
});
} else {
// 如果 x.then 是个普通值就直接返回 resolve 作为结果 Promise/A+ 2.3.3.4
resolve(x);
}
} catch (e) {
// Promise/A+ 2.3.3.2
if (called) return;
called = true;
reject(e);
}
} else {
// 如果 x 是个普通值就直接返回 resolve 作为结果 Promise/A+ 2.3.4
resolve(x);
}
};
function myPromise(executor) {
this.status = PENDING;
this.value = undefined;
this.reason = undefined;
this.onResolvedCallbacks = [];
this.onRejectedCallbacks = [];
let resolve = (value) => {
if (this.status === PENDING) {
this.status = FULFILLED;
this.value = value;
this.onResolvedCallbacks.forEach((fn) => fn());
}
};
let reject = (reason) => {
if (this.status === PENDING) {
this.status = REJECTED;
this.reason = reason;
this.onRejectedCallbacks.forEach((fn) => fn());
}
};
try {
executor(resolve, reject);
} catch (error) {
reject(error);
}
}
myPromise.prototype.then = function(onFulfilled, onRejected) {
// 解决 onFufilled，onRejected 没有传值的问题
onFulfilled = typeof onFulfilled === "function" ? onFulfilled : (v) => v;
// 因为错误的值要让后面访问到，所以这里也要抛出错误，不然会在之后 then 的 resolve 中捕获
onRejected = typeof onRejected === "function" ? onRejected : (err) => {
throw err;
};
// 每次调用 then 都返回一个新的 promise
let promise2 = new myPromise((resolve, reject) => {
if (this.status === FULFILLED) {
//Promise/A+ 2.2.4 --- setTimeout
setTimeout(() => {
try {
let x = onFulfilled(this.value);
// x可能是一个proimise
resolvePromise(promise2, x, resolve, reject);
} catch (e) {
reject(e);
}
}, 0);
}
if (this.status === REJECTED) {
//Promise/A+ 2.2.3
setTimeout(() => {
try {
let x = onRejected(this.reason);
resolvePromise(promise2, x, resolve, reject);
} catch (e) {
reject(e);
}
}, 0);
}
if (this.status === 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;
}
myPromise.prototype.catch = function(onRejected) {
return this.then(null, onRejected)
}
myPromise.defer = myPromise.deferred = function() {
let dfd = {}
dfd.promise = new myPromise((resolve, reject) => {
dfd.resolve = resolve;
dfd.reject = reject;
});
return dfd;
}
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See More
- ES6实现Promise
const PENDING = 'pending';
const FULFILLED = 'fulfilled';
const REJECTED = 'rejected';
class Promise {
constructor(executor) {
this.status = PENDING;
this.value = undefined;
this.reason = undefined;
this.onResolvedCallbacks = [];
this.onRejectedCallbacks = [];
let resolve = (value) => {
if (this.status === PENDING) {
this.status = FULFILLED;
this.value = value;
this.onResolvedCallbacks.forEach((fn) => fn());
}
};
let reject = (reason) => {
if (this.status === PENDING) {
this.status = REJECTED;
this.reason = reason;
this.onRejectedCallbacks.forEach((fn) => fn());
}
};
try {
executor(resolve, reject);
} catch (error) {
reject(error);
}
}
then(onFulfilled, onRejected) {
// 解决 onFufilled，onRejected 没有传值的问题
onFulfilled = typeof onFulfilled === "function" ? onFulfilled : (v) => v;
// 因为错误的值要让后面访问到，所以这里也要抛出错误，不然会在之后 then 的 resolve 中捕获
onRejected = typeof onRejected === "function" ? onRejected : (err) => {
throw err;
};
// 每次调用 then 都返回一个新的 promise
let promise2 = new Promise((resolve, reject) => {
if (this.status === FULFILLED) {
//Promise/A+ 2.2.4 --- setTimeout
setTimeout(() => {
try {
let x = onFulfilled(this.value);
// x可能是一个proimise
resolvePromise(promise2, x, resolve, reject);
} catch (e) {
reject(e);
}
}, 0);
}
if (this.status === REJECTED) {
//Promise/A+ 2.2.3
setTimeout(() => {
try {
let x = onRejected(this.reason);
resolvePromise(promise2, x, resolve, reject);
} catch (e) {
reject(e);
}
}, 0);
}
if (this.status === 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;
}
catch(onReject){
return this.then(null, onReject)
}
}
const resolvePromise = (promise2, x, resolve, reject) => {
// 自己等待自己完成是错误的实现，用一个类型错误，结束掉 promise Promise/A+ 2.3.1
if (promise2 === x) {
return reject(
new TypeError("Chaining cycle detected for promise #<Promise>"));
}
// Promise/A+ 2.3.3.3.3 只能调用一次
let called;
// 后续的条件要严格判断 保证代码能和别的库一起使用
if ((typeof x === "object" && x != null) || typeof x === "function") {
try {
// 为了判断 resolve 过的就不用再 reject 了（比如 reject 和 resolve 同时调用的时候） Promise/A+ 2.3.3.1
let then = x.then;
if (typeof then === "function") {
// 不要写成 x.then，直接 then.call 就可以了 因为 x.then 会再次取值，Object.defineProperty Promise/A+ 2.3.3.3
then.call(
x, (y) => {
// 根据 promise 的状态决定是成功还是失败
if (called) return;
called = true;
// 递归解析的过程（因为可能 promise 中还有 promise） Promise/A+ 2.3.3.3.1
resolvePromise(promise2, y, resolve, reject);
}, (r) => {
// 只要失败就失败 Promise/A+ 2.3.3.3.2
if (called) return;
called = true;
reject(r);
});
} else {
// 如果 x.then 是个普通值就直接返回 resolve 作为结果 Promise/A+ 2.3.3.4
resolve(x);
}
} catch (e) {
// Promise/A+ 2.3.3.2
if (called) return;
called = true;
reject(e);
}
} else {
// 如果 x 是个普通值就直接返回 resolve 作为结果 Promise/A+ 2.3.4
resolve(x);
}
};
Promise.defer = Promise.deferred = function () {
let dfd = {}
dfd.promise = new Promise((resolve,reject)=>{
dfd.resolve = resolve;
dfd.reject = reject;
});
return dfd;
}
module.exports = Promise;
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# Promise.resolve
Promise.resolve = function(value) {
// 如果是 Promsie，则直接输出它
if(value instanceof Promise){
return value
}
return new Promise(resolve => resolve(value))
}
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# Promise.reject
Promise.reject = function(reason) {
return new Promise((resolve, reject) => reject(reason))
}
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# Promise.all
Promise.all = function(promiseArr) {
let index = 0, result = []
return new Promise((resolve, reject) => {
promiseArr.forEach((p, i) => {
Promise.resolve(p).then(val => {
index++
result[i] = val
if (index === promiseArr.length) {
resolve(result)
}
}, err => {
reject(err)
})
})
})
}
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# Promise.race
Promise.race = function(promiseArr) {
return new Promise((resolve, reject) => {
promiseArr.forEach(p => {
Promise.resolve(p).then(val => {
resolve(val)
}, err => {
reject(err)
})
})
})
}
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# Promise.allSettled
Promise.allSettled = function(promiseArr) {
let result = []
return new Promise((resolve, reject) => {
promiseArr.forEach((p, i) => {
Promise.resolve(p).then(val => {
result.push({
status: 'fulfilled',
value: val
})
if (result.length === promiseArr.length) {
resolve(result)
}
}, err => {
result.push({
status: 'rejected',
reason: err
})
if (result.length === promiseArr.length) {
resolve(result)
}
})
})
})
}
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# Promise.any
Promise.any = function(promiseArr) {
let index = 0
return new Promise((resolve, reject) => {
if (promiseArr.length === 0) return
promiseArr.forEach((p, i) => {
Promise.resolve(p).then(val => {
resolve(val)
}, err => {
index++
if (index === promiseArr.length) {
reject(new AggregateError('All promises were rejected'))
}
})
})
})
}
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# Promise.prototype.finally
https://es6.ruanyifeng.com/#docs/promise#Promise-prototype-finally
Promise.prototype.finally = function (callback) {
let P = this.constructor;
return this.then(
value => P.resolve(callback()).then(() => value),
reason => P.resolve(callback()).then(() => { throw reason })
);
};
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# Promise.retry
/*
* @param {function} fn - 方法名
* @param {number} delay - 延迟的时间
* @param {number} times - 重发的次数
*/
function retry(fn, delay, times) {
return new Promise((resolve, reject) => {
function func() {
Promise.resolve(fn()).then(res => {
resolve(res);
})
.catch(err => {
// 接口失败后，判断剩余次数不为0时，继续重发
if (times !== 0) {
setTimeout(func, delay);
times--;
} else {
reject(err);
}
});
}
func();
});
}
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