Asynchronous JavaScript to fetch remote JSON data with promises and generators

Question

I am trying to figure out how to do async calls using promises and generators (without 3rd party libraries). Most of the examples I found on the Internet are either incomplete or confusing (at least to me), so I decided to try and write the code myself.

I would really appreciate it if someone could review the code and provide comments and suggestions for improving it.

function getJSON(url) {
 return new Promise((resolve, reject) => {
 const https = require("https");
 try {
 https.get(url, (response) => {
 let data = "";
 response.on("data", (part) => {
 data += part;
 });
 response.on("end", () => {
 try {
 resolve(JSON.parse(data));
 }
 catch(e) {
 reject("Error parsing data");
 }
 });
 }).on("error", () => {
 reject("Error downloading data");
 });
 }
 catch(e) {
 reject("Error: " + e.message);
 }
 });
}
function request(generator) {
 var iterator = generator();
 function process(result) {
 if (result.done) {
 return;
 }
 const value = result.value;
 if (value instanceof Promise) {
 value.then(
 (data) => {
 console.log(data);
 process(iterator.next());
 }
 )
 .catch(
 err => {
 iterator.throw(err);
 }
 );
 }
 }
 try {
 process(iterator.next());
 } 
 catch(e) {
 iterator.throw(e);
 }
}
function *DataGenerator() {
 try {
 yield getJSON("https://jsonplaceholder.typicode.com/posts/1");
 yield getJSON("https://jsonplaceholder.typicode.com/posts/2");
 // yield getJSON("https://jsonplaceholder.typicode.com/"); // Parsing error
 // yield getJSON(""); // Domain name error
 }
 catch (e) {
 console.log(e);
 }
}
// Run
console.log("Before");
request(DataGenerator);
console.log("After");

This is also available as a repo.

Answer 1

The code looks pretty good - like a nice application of generators and iterators for promises.

The only thing I could think of that could be updated is the iterator variable in the request() function could be declared with the const statement, since it doesn't appear to be re-assigned (unless I am missing something).

var iterator = generator();

Then in that nested function process(), did you ever see the first conditional (i.e. result.done) have a truthy value? I tried running it a few times but never saw that occur...

function process(result) {
 if (result.done) {
 return;
 }

Answer 2

Why do not use async / await functions? Its just syntactic sugar for promises / generators. Check this vanilla JS snippet, is live:

var url1 = 'https://jsonplaceholder.typicode.com/posts/2'
var url2 = 'https://jsonplaceholder.typicode.com/posts/3'
async function mimicSyncInside () {
 console.log('Before')
 console.log(await fetch(url1).then(resp => resp.json()))
 console.log(await fetch(url2).then(resp => resp.json()))
 console.log('After')
}
mimicSyncInside()

• ArminMeth: There are many reasons. One of them is to better understand async/await (take a look behind the scenes sort of thing). Also, it is not either/or type of thing (eg. if you want to parallelize async/await tasks you need to use Promise.all) so depending on situation one might actually prefer using promises and generators over async/await.
• ArminMeth: BTW, to mimic the same behavior, the aync/await code should be console.log('Before') then the async function with await tasks and then console.log('After')
• user145018: And your code works? In this order? Before - post1 - post2 - after?