var timer = (ms) => {
    return new Promise((resolve, reject) => {
        setTimeout(() => {
            resolve();
        }, ms)
    });
}

var promise1 = timer(4000);
timer(4000).then(() => {
    console.log('Finish timer')
    promise1.then(() => {
        console.log('Finish promise') // log out with the same time as above log
    })
})