Javascript Promises

While explaining Promises to a friend I remembered reading a blog post couple of years back that explained the concept very well and made it all clear to me. However, I never bookmarked and searching for it, I can't find the specific post. Then I thought why not recreate some of the samples that the blog author had used to explain the concepts.

Here's what I came up with:

// Function to convert setTimeout into a Promise based
// abstraction 
function delaySeconds(seconds) {
   return new Promise((resolve, reject) => {
      setTimeout(resolve, seconds*1000);
   });
}

/*
 * A simple function that converts clunky setTimeout
 * callback based interface to a promise based
 * abstraction.
 */
function delayAndReturn(seconds, retval) {
   return new Promise((resolve, reject) => {
      setTimeout(() => {
        resolve(retval);
      }, seconds*1000);
   });
}

// example of how to use the above abstraction
console.log("Calling delaySeconds with 10 seconds delay");
delaySeconds(10).then(() => {
    console.log("delaySeconds completed waiting 10 seconds");
});

// Function returns a Promise object that is rejected.
function rejectPromise() {
   return new Promise((resolve, reject) => {
      reject();
   });
}

console.log("Calling delayAndReturn with 2 seconds timeout");
/*
 * This sample demonstrates:
 *  - how to chain promises together
 *  - returning values from an 'outer' Promise to an 'inner'
 *    one
 *  - return values can be any concrete datatype, including
 *    another Promise instance.
 *  - returning and handling errors/rejections
 */
delayAndReturn(2, 10).then(ret => {
    console.log("delayAndReturn - retval: ", ret);
    console.log("delayAndReturn, waiting 3 seconds");
    return delaySeconds(3);
}).then(() => {
    console.log("delayAndReturn -- second promise completion handler");
    return 20;
}).then((ret) => {
    console.log('delayAndReturn - final return value: ', ret, ', calling rejectPromise()..');
    return rejectPromise();
}).then(() => {
    console.log('empty Promise completion handler');
    console.log('First delaySeconds promise should complete in ~5 seconds');
}).catch(err => {
    console.log('delayAndReturn promise chain err: ', err);
});

It's not nearly as complete as what's in the original post, but guess it ought to be enough to grasp the core concepts.