promise 實現（es6 完整源碼）

概覽

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 內部維護着三種狀態 pending、fulfilled 和 rejected，狀態只能從 pending 轉變到 fulfilled，或從 pending 轉變到 rejected 且該轉變不可逆。

Promise 主要提供了三個實例方法 then，catch，finally，和4個靜態方法 resolve，reject，all，race。所有方法都都返回一個Promise對象。

構造函數

    constructor(fn) {
        this.fulfilledQueue = [];
        this.rejectedQueue = [];
        this._status = PENDING;
        this._value  = null;

        // 執行成功隊列中的回調函數
        const handleFulfilledQueue = () => {
            while(this.fulfilledQueue.length) {
                let fulfiledFn = this.fulfilledQueue.shift();
                fulfiledFn(this._value);
            };
        };
        
        // 執行失敗隊列中的回調函數
        const handleRejectedQueue = () => {
            while(this.rejectedQueue.length) {
                let rejectedFn = this.rejectedQueue.shift();
                rejectedFn(this._value);
            };
        };

        // 完成狀態轉變，執行回調隊列中的回調函數
        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();
                }
            }
            // 保證promise 回調函數一定是在同步任務之後執行；
            setTimeout(fn, 0);
        }
        // 完成狀態Pending到REJECTED的轉變，執行rejected隊列中的回調函數
        const _reject = (val) => {
            const fn = () => {
                if(this._status !== PENDING) {
                    return;
                }
                this._status = REJECTED;
                this._value = val;
                handleRejectedQueue();
            }
            setTimeout(fn, 0);
        }
        
        try {  // 處理外部傳入函數執行異常
            fn(_resolve, _reject);            
        } catch(e) {
            return _reject(e);
        }
    }

Promise 構造函數接收一個函數執行器作爲參數，該執行器的兩個參數 _resolve、_reject均爲函數類型，由 Promise 內部實現。執行器在 Promise 構造函數中被立即執行。

注意： MyPromise 使用 Timeout 實現異步，使得 MyPromise 只能添加 macrotask，實際上原生的Promise 是 microtask

then 方法

    then(successFn, failFn) {
        return new MyPromise((resolve, reject) => {
            // 執行成功時的回調函數
            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);
                }
            }
            // 執行失敗時的回調函數
            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:       // 異步任務尚未完成，將回調函數推入相應隊列
                    this.fulfilledQueue.push(() => {
                        handleSucess(successFn);
                    });
                    this.rejectedQueue.push(() => {
                        handleFail(failFn);
                    });
                    break;
                case FULFILLED:     // 異步任務成功完成，執行成功回調函數
                    handleSucess(successFn);
                    break;
                case REJECTED:      // 異步任務已失敗，執行失敗回調函數
                    handleFail(failFn);
                    break;
                default:
                    console.log('Promise error status:', this._status);
                    break;
            };
        });
    }

then 方法是 Promise 的一個主要方法，catch 和 finally 都可以用 then 來實現。當 Promise 的狀態已經流轉時，回調函數會立即被執行，當 Promise 還處於 Pending 狀態時，回調函數被推入相應隊列中等待執行。

完整代碼

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);
            };
        };

        // 完成狀態轉變，執行回調隊列中的回調函數
        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);
        }
        // 完成狀態Pending到REJECTED的轉變，執行rejected隊列中的回調函數
        const _reject = (val) => {
            const fn = () => {
                if(this._status !== PENDING) {
                    return;
                }
                this._status = REJECTED;
                this._value = val;
                handleRejectedQueue();
            }
            setTimeout(fn, 0);
        }
        
        try { // 處理外部傳入函數執行異常
            fn(_resolve, _reject);            
        } catch(e) {
            
            return _reject(e);
        }
    }

    then(successFn, failFn) {
        return new MyPromise((resolve, reject) => {
            // 執行成功時的回調函數
            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);
                }
            }
            // 執行失敗時的回調函數
            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:       // 異步任務尚未完成，將回調函數推入相應隊列
                    this.fulfilledQueue.push(() => {
                        handleSucess(successFn);
                    });
                    this.rejectedQueue.push(() => {
                        handleFail(failFn);
                    });
                    break;
                case FULFILLED:     // 異步任務成功完成，執行成功回調函數
                    handleSucess(successFn);
                    break;
                case REJECTED:      // 異步任務已失敗，執行失敗回調函數
                    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);
                });
            }
        });
    }
}