JS面试题:手写实现 Promise

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1. 实现一个 Promise

function Promise(executor) {
    let self = this;
    self.status = 'pending'; //等待态
    self.value = undefined;  //成功的返回值
    self.reason = undefined; //失败的原因

    function resolve(value){
        if(self.status === 'pending'){
            self.status = 'resolved';
            self.value = value;
        }
    }
    function reject(reason) {
        if(self.status === 'pending') {
            self.status = 'rejected';
            self.reason = reason;
        }
    }
    try{
        executor(resolve, reject);
    }catch(e){
        reject(e);// 捕获时发生异常,就直接失败
    }
}

//onFufiled 成功的回调
//onRejected 失败的回调
Promise.prototype.then = function (onFufiled, onRejected) {
    let self = this;
    if(self.status === 'resolved'){
        onFufiled(self.value);
    }
    if(self.status === 'rejected'){
        onRejected(self.reason);
    }
}

module.exports = Promise;

测试

let Promise = require('./Promise');

let promise = new Promise(function (resolve, reject) {
    resolve(100);
})

promise.then(function (data) {
    console.log('data:', data);
},function (err) {
    console.log('err:', err);
})

// data: 100

2. Promise 实例可以多次then,当成功后会将 then 中的成功方法按顺序执行,我们可以先将 then 中成功的回调和失败的回调存到数组内,当成功的时候调用成功的数组即可。

self.onResolvedCallbacks = [];  /* 存放then成功的回调*/
self.onRejectedCallbacks = [];  /* 存放then失败的回调*/
function resolve(value){
    if(self.status === 'pending'){
        self.status = 'resolved';
        self.value = value;
        self.onResolvedCallbacks.forEach(function (fn) {
            fn();
        })
    }
}
function reject(reason) {
    if(self.status === 'pending') {
        self.status = 'rejected';
        self.reason = reason;
        self.onRejectedCallbacks.forEach(function (fn) {
            fn();
        })
    }
}

if(self.status === 'pending'){
    self.onResolvedCallbacks.push(function () {
        onFufiled(self.value);
    })
    self.onRejectedCallbacks.push(function () {
        onRejected(self.reason);
    })
}

3. 实现链式调用

Promise 的一大特点,就是链式调用。而 Promise 实现链式调用就是通过 then 方法返回一个新的 Promise。

如果第一个 then 中返回了一个结果,会将 Promise 的结果继续传给下一个 then 中;如果有错误则走下一个 then 的失败。

// 添加 resolvePromise 方法 处理链式调用问题
function resolvePromise(p2, x, resolve, reject) {
    if(p2 === x){
        return reject(new TypeError('循环引用'));
    }
    if(x!==null || (typeof x === 'object' || typeof x === 'function')){
        try{
            let then = x.then;
            if(typeof then === 'function'){
                then.call(x, function (y) {
                    resolvePromise(promise2, y, resolve, reject);
                },function (err) {
                    reject(err);
                });
            }else{
                resolve(x);
            }
        }catch(e){
            reject(e);
        }
    }else{
        resolve(x);
    }
}

参考来源:手写 Promise —— xieshuning