Promise源碼實現

const ENUM = {
    PENDING: 'PENDING',
    FULFILLED: 'FULFILLED',
    REJECTED: 'REJECTED'
}
// 須要兼容其餘人的promise
const resolvePromise = (x, promise2, resolve, reject) => {
    if (x === promise2) {
        reject(new TypeError(`TypeError: Chaining cycle detected for promise #<Promise>`))
    }
    if ((typeof x === 'object' && x !== null) || typeof x === 'function') {
        let called;
        try {
            let then = x.then; 
            if (typeof then == 'function') {
                then.call(x, y => { 
                    if (called) return;
                    called = true;
                    resolvePromise(y, promise2, resolve, reject);
                }, r => {
                    if (called) return;
                    called = true;
                    reject(r);
                })
            } else {
                resolve(x); // {a:1}
            }
        } catch (e) {
            if (called) return;
            called = true;
            reject(e);
        }
    } else {
        resolve(x); 
    }
}
class Promise {
    constructor(executor) {
        this.status = ENUM.PENDING;
        this.value = undefined; // 實例上的屬性 每次new 都會建立一個新的
        this.reason = undefined;
        // 須要建立成功的隊列 失敗的隊列，分別存放
        this.onResolvedCallbacks = [];
        this.onRejectedCallbacks = [];
        const resolve = (value) => {
            // 若是value 是一個promise，那咱們的庫中應該也要實現一個遞歸解析
            if(value instanceof Promise){
                // 遞歸解析 
                return value.then(resolve,reject)
            }
            if (this.status === ENUM.PENDING) {
                this.status = ENUM.FULFILLED;
                this.value = value; // 先賦予的值 再循環
                this.onResolvedCallbacks.forEach(fn => fn())
            }
        }
        const reject = (reason) => {
            if (this.status === ENUM.PENDING) {
                this.status = ENUM.REJECTED;
                this.reason = reason;
                this.onRejectedCallbacks.forEach(fn => fn())
            }
        }
        try {
            executor(resolve, reject);
        } catch (e) {
            reject(e)
        }
    }
    then(onFulfilled, onRejected) {
        onFulfilled = typeof onFulfilled == 'function' ? onFulfilled : v => v;
        onRejected = typeof onRejected === 'function' ? onRejected : err => { throw err };
        // 調用then 方法 建立一個新的promise
        let promise2 = new Promise((resolve, reject) => {
            // 我須要根據 x 的情況來判斷是調用resolve仍是reject
            if (this.status == ENUM.FULFILLED) {
                // todo...
                setTimeout(() => {
                    try {
                        let x = onFulfilled(this.value);
                        // 解析promise
                        resolvePromise(x, promise2, resolve, reject)
                    } catch (e) {
                        reject(e);
                    }
                }, 0);
            }
            if (this.status == ENUM.REJECTED) {
                // todo...
                setTimeout(() => {
                    try {
                        let x = onRejected(this.reason);
                        resolvePromise(x, promise2, resolve, reject)
                    } catch (e) {
                        reject(e);
                    }
                }, 0);
            }
            if (this.status == ENUM.PENDING) {
                // 用戶尚未調用resolve或者reject方法
                this.onResolvedCallbacks.push(() => {
                    // todo...
                    setTimeout(() => {
                        try {
                            let x = onFulfilled(this.value);
                            resolvePromise(x, promise2, resolve, reject)
                        } catch (e) {
                            reject(e);
                        }
                    }, 0);
                });
                this.onRejectedCallbacks.push(() => {
                    // todo...
                    setTimeout(() => {
                        try {
                            let x = onRejected(this.reason);
                            resolvePromise(x, promise2, resolve, reject)
                        } catch (e) {
                            reject(e);
                        }
                    }, 0);

                })
            }
        });
        return promise2;
    }
    catch(errCallback){
        return this.then(null,errCallback)
    }
    static resolve(val){
        return new Promise((resolve,reject)=>{
            resolve(val);
        })
    }
    static reject(reason){
        return new Promise((resolve,reject)=>{
            reject(reason);
        })
    }
}
// 產生延遲對象

Promise.prototype.finally = function(callback) {
    return this.then((value) => {
        return Promise.resolve(callback()).then(() => value)
    }, (err) => {
        return Promise.resolve(callback()).then(() => { throw err })
    });
}
Promise.all = function(values) {
    return new Promise((resolve, reject) => {
        let resultArr = [];
        let orderIndex = 0;
        const processResultByKey = (value, index) => {
            resultArr[index] = value;
            if (++orderIndex === values.length) {
                resolve(resultArr)
            }
        }
        for (let i = 0; i < values.length; i++) {
            let value = values[i];
            if (value && typeof value.then === 'function') {
                value.then((value) => {
                    processResultByKey(value, i);
                }, reject);
            } else {
                processResultByKey(value, i);
            }
        }
    });
}
Promise.race = function(promises) {
    return new Promise((resolve, reject) => {
        // 一塊兒執行就是for循環
        for (let i = 0; i < promises.length; i++) {
            let val = promises[i];
            if (val && typeof val.then === 'function') {
                val.then(resolve, reject); // value.then(resolve,reject)
            } else { // 普通值
                resolve(val)
            }
        }
    });
}