promise 实现（es6 完整源码）

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概览
const PENDING = Symbol(‘PENDING’);
const FULFILLED = Symbol(‘FULFILLED’);
const REJECTED = Symbol(‘REJECTED’);

class MyPromise {
constructor(fn) {}
then(successFn, failFn) {}
catch(failFn) {}
finally(finalFn){}
static resolve(val) {}
static reject(val) {}
static all(promiseArr) {}
static race(promiseArr) {}
}
Promise 内部维护着三种状态 pending、fulfilled 和 rejected，状态只能从 pending 转变到 fulfilled，或从 pending 转变到 rejected 且该转变不可逆。
Promise 主要提供了三个实例方法 then，catch，finally，和 4 个静态方法 resolve，reject，all，race。所有方法都都返回一个 Promise 对象。
构造函数
constructor(fn) {
this.fulfilledQueue = [];
this.rejectedQueue = [];
this._status = PENDING;
this._value = null;

// 执行成功队列中的回调函数
const handleFulfilledQueue = () => {
while(this.fulfilledQueue.length) {
let fulfiledFn = this.fulfilledQueue.shift();
fulfiledFn(this._value);
};
};

// 执行失败队列中的回调函数
const handleRejectedQueue = () => {
while(this.rejectedQueue.length) {
let rejectedFn = this.rejectedQueue.shift();
rejectedFn(this._value);
};
};

// 完成状态转变，执行回调队列中的回调函数
const _resolve = (val) => {
const fn = () => {
if(this._status !== PENDING) {
return;
}
if(val instanceof MyPromise) {
val.then((res) => {
this._status = FULFILLED;
this._value = res;
handleFulfilledQueue();
}, (err) => {
this._status = REJECTED;
this._value = err;
handleRejectedQueue();
});
} else {
this._status = FULFILLED;
this._value = val;
handleFulfilledQueue();
}
}
// 保证 promise 回调函数一定是在同步任务之后执行；
setTimeout(fn, 0);
}
// 完成状态 Pending 到 REJECTED 的转变，执行 rejected 队列中的回调函数
const _reject = (val) => {
const fn = () => {
if(this._status !== PENDING) {
return;
}
this._status = REJECTED;
this._value = val;
handleRejectedQueue();
}
setTimeout(fn, 0);
}

try {// 处理外部传入函数执行异常
fn(_resolve, _reject);
} catch(e) {
return _reject(e);
}
}
Promise 构造函数接收一个函数执行器作为参数，该执行器的两个参数 _resolve、_reject 均为函数类型，由 Promise 内部实现。执行器在 Promise 构造函数中被立即执行。
注意：MyPromise 使用 Timeout 实现异步，使得 MyPromise 只能添加 macrotask，实际上原生的 Promise 是 microtask
then 方法
then(successFn, failFn) {
return new MyPromise((resolve, reject) => {
// 执行成功时的回调函数
const handleSucess = (fn) => {
try {
if(typeof fn === ‘function’) {
const res = fn(this._value);
if(res instanceof MyPromise) {
res.then(resolve, reject);
} else {
resolve(res);
}
} else {
resolve(this._value)
}
} catch(e){
reject(e);
}
}
// 执行失败时的回调函数
const handleFail = (fn) => {
try {
if(typeof fn === ‘function’) {
const res = fn(this._value);
if(res instanceof MyPromise) {
res.then(resolve, reject);
} else {
resolve(res);
}
} else {
reject(this._value);
}
} catch(e) {
reject(e);
}
}
switch(this._status){
case PENDING: // 异步任务尚未完成，将回调函数推入相应队列
this.fulfilledQueue.push(() => {
handleSucess(successFn);
});
this.rejectedQueue.push(() => {
handleFail(failFn);
});
break;
case FULFILLED: // 异步任务成功完成，执行成功回调函数
handleSucess(successFn);
break;
case REJECTED: // 异步任务已失败，执行失败回调函数
handleFail(failFn);
break;
default:
console.log(‘Promise error status:’, this._status);
break;
};
});
}
then 方法是 Promise 的一个主要方法，catch 和 finally 都可以用 then 来实现。当 Promise 的状态已经流转时，回调函数会立即被执行，当 Promise 还处于 Pending 状态时，回调函数被推入相应队列中等待执行。
完整代码
class MyPromise {
constructor(fn) {
this.fulfilledQueue = [];
this.rejectedQueue = [];
this._status = PENDING;
this._value = null;

const handleFulfilledQueue = () => {
while(this.fulfilledQueue.length) {
let fulfiledFn = this.fulfilledQueue.shift();
fulfiledFn(this._value);
};
};
const handleRejectedQueue = () => {
console.log(this.rejectedQueue);
while(this.rejectedQueue.length) {
let rejectedFn = this.rejectedQueue.shift();
rejectedFn(this._value);
};
};

// 完成状态转变，执行回调队列中的回调函数
const _resolve = (val) => {
const fn = () => {
if(this._status !== PENDING) {
return;
}
if(val instanceof MyPromise) {
val.then((res) => {
this._status = FULFILLED;
this._value = res;
handleFulfilledQueue();
}, (err) => {
this._status = REJECTED;
this._value = err;
handleRejectedQueue();
});
} else {
this._status = FULFILLED;
this._value = val;
handleFulfilledQueue();
}
}
setTimeout(fn, 0);
}
// 完成状态 Pending 到 REJECTED 的转变，执行 rejected 队列中的回调函数
const _reject = (val) => {
const fn = () => {
if(this._status !== PENDING) {
return;
}
this._status = REJECTED;
this._value = val;
handleRejectedQueue();
}
setTimeout(fn, 0);
}

try {// 处理外部传入函数执行异常
fn(_resolve, _reject);
} catch(e) {

return _reject(e);
}
}

then(successFn, failFn) {
return new MyPromise((resolve, reject) => {
// 执行成功时的回调函数
const handleSucess = (fn) => {
try {
if(typeof fn === ‘function’) {
const res = fn(this._value);
if(res instanceof MyPromise) {
res.then(resolve, reject);
} else {
resolve(res);
}
} else {
resolve(this._value)
}
} catch(e){
reject(e);
}
}
// 执行失败时的回调函数
const handleFail = (fn) => {
try {
if(typeof fn === ‘function’) {
const res = fn(this._value);
if(res instanceof MyPromise) {
res.then(resolve, reject);
} else {
resolve(res);
}
} else {
reject(this._value);
}
} catch(e) {
reject(e);
}
}
switch(this._status){
case PENDING: // 异步任务尚未完成，将回调函数推入相应队列
this.fulfilledQueue.push(() => {
handleSucess(successFn);
});
this.rejectedQueue.push(() => {
handleFail(failFn);
});
break;
case FULFILLED: // 异步任务成功完成，执行成功回调函数
handleSucess(successFn);
break;
case REJECTED: // 异步任务已失败，执行失败回调函数
handleFail(failFn);
break;
default:
console.log(‘Promise error status:’, this._status);
break;
};
});
}

catch(failFn) {
return this.then(null, failFn);
}

finally(finalFn){
return this.then(finalFn, finalFn);
}

static resolve(val) {
if(val instanceof MyPromise) {
return val;
} else {
return new MyPromise((resolve, reject) =>{
resolve(val);
});
}
}

static reject(val) {
return new MyPromise((resolve, reject) => {
reject(val);
});
}

static all(promiseArr) {
return new Promise((resolve, reject) =>{
const len = promiseArr.length;
let count = 0;
let result = [];
for(let i = 0; i < len; i++) {
promiseArr[i].then((val) => {
count++;
result.push[val];
if(count === len){
resolve(result);
}
}, (err) => {
reject(err);
});
}
});
}

static race(promiseArr) {
return new Promise((resolve, reject) =>{
const len = promiseArr.length;
for(let i = 0; i < len; i++) {
promiseArr[i].then((val) => {
resolve(val);
}, (err) => {
reject(err);
});
}
});
}
}