从底层来看Promise

感悟：有的知识你从”底层“来看会觉得很清晰！

注意：这个源码并不是实际的Promise实现。我主要是想主要是想通模仿实现Promise来学习!

class Promise {
    constructor(executor) {
        this.resolvedCallbacks = [];
        this.rejectedCallbacks = [];
        this.state = "PENDING";
        this.value = "";
        //这里解释了为什么必是传入resolve,reject;
        executor(this.resolve.bind(this), this.reject.bind(this));
    }

    resolve(value) {
        if (this.state === 'PENDING') {
            // 解释了为什么执行了resolve方法会将promise
            this.state = "RESOLVED";
            // 这里解释了为什么resolve传入的值会改变promise的value属性
            this.value = value;

            this.resolvedCallbacks.map(callback => callback(value));
        }
    }
    reject(value) {
        if (this.state === 'PENDING') {
            // 解释了为什么执行了reject方法会将Promise的实例状态变为REJECTED
            this.state = "REJECTED";
            // 这里解释了为什么reject传入的值会改变promise的value属性
            this.value = value;

            this.rejectedCallbacks.map(callback => callback(value));
        }
    }


    then(onFulfilled, onRejected) {
        if (this.state === "PENDING") {
            this.resolvedCallbacks.push(onFulfilled);
            this.rejectedCallbacks.push(onRejected);
        }

        if (this.state === "RESOLVED") {
            onFulfilled(this.value);
        }
        if (this.state === 'REJECTED') {
            onRejected(this.value);
        }
    }
}