SOURCE

function output(i) {
    let index = i
    return new Promise((resolve, reject) => {
        setTimeout(function () {
            console.log(new Date, index);
            resolve()
        }, 1000);
    })
}
let p = Promise.resolve()
// for (let i = 0; i < 5; i++) {
//    p.then(() => { return this.output(i) }).then(()=>{
//          return this.output(i)
//     })
//     //this.ouput(i)
// }

p.then(() => { return this.output(1) }).then(()=>{
        return this.output(2)
})