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.

Sᴀᴍ Onᴇᴌᴀ · Accepted Answer · 2017-11-10 01:08:08Z

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;
    }

score 1 · Accepted Answer · 2017-09-18 14:56:41Z

1

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()

edited Sep 18, 2017 at 14:56

answered Sep 18, 2017 at 14:43

user145018

\$\begingroup\$ 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. \$\endgroup\$

ArminMeth
– ArminMeth

2017-09-18 16:11:32 +00:00
Commented Sep 18, 2017 at 16:11
\$\begingroup\$ 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') \$\endgroup\$

ArminMeth
– ArminMeth

2017-09-18 16:18:21 +00:00
Commented Sep 18, 2017 at 16:18
\$\begingroup\$ And your code works? In this order? Before - post1 - post2 - after? \$\endgroup\$

user145018
– user145018

2017-09-18 18:07:25 +00:00
Commented Sep 18, 2017 at 18:07

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