SOURCE

const arr = [1, 2, 3];
arr.reduce((p,x) =>{
    return p.then(() => {
        return new Promise(r => {
            setTimeout(() => r(console.log(x)), 1000)
        })
    })
}, Promise.resolve());


function red() {
    console.log('red');
}

function green() {
    console.log('green');
}

function yellow() {
    console.log('yellow');
}

const light = function(tiemer, cb){
    return new Promise(resolve => {
        setTimeout(() => {
            cb();
            resolve();
        }, tiemer);
    })
}

const step = function() {
    Promise.resolve().then(() => {
        return light(3000, red);
    }).then(() => {
        return light(2000, green);
    }).then(() => {
        return light(1000, yellow);
    }).then(() => {
        return step();
    })
}
 // step();

 const time = (timer) => {
     return new Promise(resolve => {
         setTimeout(() => {
             resolve()
         }, timer)
     })
 }

 const ajax1 = () => time(2000).then(() => {
     console.log(1);
     return 1;
 })
 const ajax2 = () => time(1000).then(() => {
     console.log(2);
     return 2;
 })
 const ajax3 = () => time(1000).then(() => {
     console.log(3);
     return 3;
 })

function mergePromise(ajaxArray) {
    const data = [];
    let promise = Promise.resolve();
    ajaxArray.forEach(ajax => {
        promise = promise.then(ajax).then(res => {
            data.push(res);
            return data;
        })
    })

    return promise;
}

mergePromise([ajax1, ajax2, ajax3]).then(data => {
    console.log('done');
    console.log(data);
})