Javascript async example doesn't work

Question

i'm experimenting with es6 promises and chaining them and don't understand why my example doesn't work.

i want to chain printInterval() and setInterval() multiple times and expect _interval to decrease like this:

• waited 3000ms to display this message
• setting interval to 2000
• waited 2000ms to display this message
• setting interval to 1000
• waited 1000ms to display this message
• setting interval to 500
• waited 500ms to display this message

but i get the following:

• waited 3000ms to display this message
• setting interval to 2000
• setting interval to 1000
• setting interval to 500
• waited 500ms to display this message
• waited 500ms to display this message
• waited 500ms to display this message

.

function printInterval() {
    return new Promise(function(resolve, reject){
        setTimeout(function () {
            console.log('waited ' + _interval + 'ms to display this message')
            resolve(_interval);
        }, _interval)
    })
}

function setInterval(interval){
    return new Promise(function(resolve, reject) {
        setTimeout(function () {
            console.log('setting interval to ', interval)
            _interval = interval;
            resolve(_interval);
        }, 0);
    })
}

var _interval = 3000;

printInterval()
.then(function(){setInterval(2000)})
.then(function(){printInterval()})
.then(function(){setInterval(1000)})
.then(function(){printInterval()})
.then(function(){setInterval(500)})
.then(function(){printInterval()});

thank you!

Answer 1

You should return those functions not just call them:

printInterval()
.then(function(){return setInterval(2000)})
.then(function(){return printInterval()})
.then(function(){return setInterval(1000)})
.then(function(){return printInterval()})
.then(function(){return setInterval(500)})
.then(function(){return printInterval()});