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.