手撸Promise 离大厂又远一步-优快云博客

本文链接：https://blog.youkuaiyun.com/Vue1024/article/details/131994478

该文章介绍了一个自定义的Promise实现，包括PENDING、FULFILLED和REJECTED三种状态转换，构造函数接受一个执行器函数，then方法用于链式调用，静态方法resolve和reject用于创建Promise实例，还有静态的all和race方法处理多个Promise并行执行。

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class Promise {
    static PENDING = 'pending';
    static FULFILLED = 'fulfilled';
    static REJECTED = 'rejected';
    constructor(fn) {
        this.status = Promise.PENDING
        this.value = null;
        this.callbacks = []
        try {
            fn(this.resolve.bind(this), this.reject.bind(this))
        } catch (error) {
            this.reject(error);
        }
    }
    resolve(value) {
        if (this.status !== Promise.PENDING) return false
        this.status = Promise.FULFILLED
        this.value = value;
        setTimeout(() => {
            this.callbacks.map(callback => callback.onFulfilled(value))

        });
    }
    reject(reason) {
        if (this.status !== Promise.PENDING) return false
        this.status = Promise.REJECTED
        this.value = reason;
        setTimeout(() => {
            this.callbacks.map(callback => callback.onRejected(reason))
        });
    }
    then(onFulfilled, onRejected) {
        onFulfilled = onFulfilled instanceof Function ? onFulfilled : () => this.value
        onRejected = onRejected instanceof Function ? onRejected : () => this.value
        let promise = new Promise((resolve, reject) => {
            if (this.status === Promise.FULFILLED) {
                setTimeout(() => {
                    this.parse(promise, onFulfilled(this.value), resolve, reject)
                });
            }
            if (this.status === Promise.REJECTED) {
                setTimeout(() => {
                    this.parse(promise, onRejected(this.value), resolve, reject)
                });
            }
            if (this.status === Promise.PENDING) {
                this.callbacks.push({
                    onFulfilled: value => {
                        this.parse(promise, onFulfilled(value), resolve, reject)
                    },
                    onRejected: value => {
                        this.parse(promise, onRejected(value), resolve, reject)
                    }
                })
            }
        })
        return promise
    }
    parse(promise, result, resolve, reject) {
        if (promise == result) {
            throw new TypeError('Chaining cycle detected')
        }
        try {
            if (result instanceof Promise) {
                result.then(resolve, reject)
            } else {
                resolve(result)
            }
        } catch (error) {
            reject(error)
        }
    }
    static resolve(value) {
        return new Promise((resolve, reject) => {
            if (value instanceof Promise) {
                value.then(resolve, reject)
            } else {
                resolve(value)
            }
        })
    }
    static reject(value) {
        return new Promise((resolve, reject) => {
            if (value instanceof Promise) {
                value.then(resolve, reject)
            } else {
                reject(value)
            }
        })
    }
    static all(promises) {
        let arr = []
        return new Promise((resolve, reject) => {
            promises.forEach((el, index) => {
                el.then(
                    res => {
                        arr[index] = res
                        if (arr.length === promises.length) {
                            resolve(arr)
                        }
                    }, err => {
                        reject(err)
                    })
            })
        })
    }
    static race(promises) {
        return new Promise((resolve, reject) => {
            promises.map(el => {
                el.then(resolve, reject)
            })
        })
    }
}