手写Promise

简单版：

function MyPromise(exec) {
    const that = this;
    that.state = 'PENDING';
    that.value = null;
    that.resolvedCallbacks = [];
    that.rejectedCallbacks = [];
    function resolve(v) {
        if(that.state === 'PENDING') {
            that.state = 'RESOLVED';
            that.value = v;
            that.resolvedCallbacks.map(cb => cb(that.v));
        }
    }
    function reject(r) {
        if(that.state === 'PENDING') {
            that.state = 'REJECTED';
            that.value = r;
            that.rejectedCallbacks.map(cb => cb(that.v));
        }
    }
    try {
        exec(resolve, reject)
    } catch (e) {
        reject(e);
    }
}
MyPromise.prototype.then = function(onfulfilled, onrejected) {
    const that = this;
    onfulfilled = typeof onfulfilled === 'function' ? onfulfilled : v => v;
    onrejected = typeof onrejected === 'function' ? onrejected : r => {throw r};
    if(that.state === 'PENDING') {
        that.resolvedCallbacks.push(onfulfilled);
        that.rejectedCallbacks.push(onrejected);
    }
    if(that.state === 'RESOLVED') {
        onfulfilled(that.value);
    }
    if(that.state === 'REJECTED') {
        onrejected(that.value);
    }
}

完整版：

class MyPromise {
    constructor (execute) {
        if(typeof execute !== 'function') throw new Error('type error');
        const that = this;
        that.status = 'PENDING';
        that.value = null;
        that.resolvedCallbacks = [];
        that.rejectedCallbacks = [];
        try{
            execute(that._resolve.bind(that), that._reject.bind(that));
        } catch (e) {
            that._reject(e);
        }
    }
    _resolve (val) {
        const that = this;
        const run = () => {
            if (that.status !== 'PENDING') return;
            const runFulfilled = (value) => {
                let cb;
                while (cb = that.resolvedCallbacks.shift()) {
                    cb(value);
                }
            }
            const runRejected = (error) => {
                let cb;
                while (cb = that.rejectedCallbacks.shift()) {
                    cb(error);
                }
            }
            if (val instanceof MyPromise) {
                val.then(value => {
                    that.value = value;
                    that.status = 'RESOLVED';
                    runFulfilled(value);
                }, err => {
                    that.value = err;
                    that.status = 'REJECTED';
                    runRejected(err);
                })
            } else {
                that.value = val;
                that.status = 'RESOLVED';
                runFulfilled(val);
            }
        }
        setTimeout(run, 0)
    }
    _reject (err) { 
        const that = this;
        if (that.status !== 'PENDING') return;
        const run = () => {
            that.status = 'REJECTED';
            that.value = err;
            let cb;
            while (cb = that.rejectedCallbacks.shift()) {
                cb(err);
            }
        }
        setTimeout(run, 0)
    }
    then (onFulfilled, onRejected) {
        const that = this;
        return new MyPromise((onFulfilledNext, onRejectedNext) => {
        let fulfilled = value => {
            try {
                if (typeof onFulfilled !== 'function') {
                    onFulfilledNext(value);
                } else {
                    let res =  onFulfilled(value);
                    if (res instanceof MyPromise) {
                        res.then(onFulfilledNext, onRejectedNext);
                    } else {
                        onFulfilledNext(res);
                    }
                }
            } catch (err) {
                onRejectedNext(err);
            }
        }
        let rejected = error => {
            try {
                if (typeof onRejected !== 'function') {
                    onRejectedNext(error);
                } else {
                    let res = onRejected(error);
                    if (res instanceof MyPromise) {
                        res.then(onFulfilledNext, onRejectedNext);
                    } else {
                        onFulfilledNext(res);
                    }
                }
            } catch (err) {
                onRejectedNext(err);
            }
        }
        switch(that.status) {
            case 'PENDING':{
                that.resolvedCallbacks.push(onFulfilled);
                that.rejectedCallbacks.push(onRejected);
            };break;
            case 'RESOLVED':onFulfilled(that.value);break;
            case 'REJECTED':onRejected(that.value);break;
        }
      })
    }
    _catch (onRejected) {
      return this.then(undefined, onRejected)
    }
    static resolve (value) {
      if (value instanceof MyPromise) return value
      return new MyPromise(resolve => resolve(value))
    }
    static reject (value) {
      return new MyPromise((resolve ,reject) => reject(value))
    }
    static all (list) {
      return new MyPromise((resolve, reject) => {
        let values = []
        let count = 0
        for (let [i, p] of list.entries()) {
          this.resolve(p).then(res => {
            values[i] = res
            count++
            if (count === list.length) resolve(values)
          }, err => {
            reject(err)
          })
        }
      })
    }
    static race (list) {
      return new MyPromise((resolve, reject) => {
        for (let p of list) {
          this.resolve(p).then(res => {
            resolve(res)
          }, err => {
            reject(err)
          })
        }
      })
    }
    finally (cb) {
      return this.then(
        value  => MyPromise.resolve(cb()).then(() => value),
        reason => MyPromise.resolve(cb()).then(() => { throw reason })
      );
    }
}