手写源码-链式 promise

实现方式

const promise1 = () => {
    return new Promise(resolve => {
        setTimeout(() => {
            console.log('promise1');
            resolve();
        }, 1000);
    });
};

const promise2 = () => {
    return new Promise(resolve => {
        setTimeout(() => {
            console.log('promise2');
            resolve();
        }, 1000);
    });
};

const promise3 = () => {
    return new Promise(resolve => {
        setTimeout(() => {
            console.log('promise3');
            resolve();
        }, 1000);
    });
};

const promiseQueue = arr => {
    arr.reduce(
        (accPromise, curPromise) => accPromise.then(() => curPromise()),
        Promise.resolve()
    );
};

等价于以下代码

promise1().then(() => {
    return promise2();
}).then(() => {
    return promise3();
});

亦或以下代码

new Promise(resolve => {
    setTimeout(() => {
        console.log('promise1');
        resolve();
    }, 1000);
})
    .then(() => {
        return new Promise(resolve => {
            setTimeout(() => {
                console.log('promise2');
                resolve();
            }, 1000);
        });
    })
    .then(() => {
        return new Promise(resolve => {
            setTimeout(() => {
                console.log('promise3');
                resolve();
            }, 1000);
        });
    });