手写代码 -- Promise

这里只实现了一个简单的Promise，更加完善的Promise实现请参照 Promises/A+ 标准

　　　　class _Promise {
            constructor(fn) {
                this.state = "pendding";
                this.value = undefined;
                this.resolvedCallback = [];
                this.rejectedCallback = [];
                fn(this.resolve.bind(this), this.reject.bind(this))
            }

            resolve(value) {
                if (this.state == "pendding") {
                    this.state = "resolved";
                    this.value = value;
                    this.resolvedCallback.forEach(fn => fn(this.value))
                }
            }

            reject(value) {
                if (this.state == "pendding") {
                    this.state = "rejected";
                    this.value = value;
                    this.rejectedCallback.forEach(fn => fn(this.value))
                }
            }

            then(resolve = () => {}, reject = () => {}) {
                if (this.state == "pendding") {
                    this.resolvedCallback.push(resolve)
                    this.rejectedCallback.push(reject)
                }
                if (this.state == "resolved") {
                    resolve(this.value)
                }
                if (this.state == "rejected") {
                    reject(this.value)
                }
            }
        }

简单的测试一下

        new _Promise((resolve, reject) => {
            setTimeout(()=>{
                resolve("hello")
            }, 2000)
        }).then(res => {
            console.log(res)
        }, err => {
            console.log("err", err)
        })