What would be a proper way of returning this promise?

Question

I'm new to coding and Promises are somewhat of an abstract concept for me. I know that in order to return a Promise you must use .then and .catch , but I'm unsure how exactly they should be used in the following code.

/**
 * Make an HTTPS request to the MTA GTFS API for a given feed.
 * @param  {String} baseUrl    - base URL for MTA GTFS API
 * @param  {String} feedId     - identifier for which realtime feed
 * @param  {String} apiKey     - key for MTA GTFS API
 * @return {<Object>}          - Promise of parsed feed.
 */

function makeRequest(baseUrl, feedId, apiKey) {
  const feedUrl = baseUrl + feedId;
  
  return new Promise((resolve, reject) => {
    const req = https.request(feedUrl,
      { headers: { 'x-api-key': apiKey } },
      (res) => {
        if (res.statusCode < 200 || res.statusCode >= 300) {
          return reject(new Error('statusCode=' + res.statusCode));
        }
        var data;
        data = [];
        res.on('data', (chunk) => {
          return data.push(chunk);
        });
        return res.on('end', function() {
          var msg;
          data = Buffer.concat(data);
          try {
            msg = nstrDecoder.decode(data);
          } catch (err) {
            try {
              console.log(err.message);
              msg = nsssDecoder.decode(data);
            } catch (err) {
              console.log(err.message);
              msg = "";
            }
          }
          resolve(msg);
        });
      }
    );
    req.on('error', (e) => {
      reject(e.message);
    });
    req.end();
  });

}

return console.log(makeRequest(baseUrl, feedId, apiKey));

After running this code I get a message saying the Promise is pending. Thoughts??

Answer 1

When calling a function that returns a promise, you're going to get the promise in a pending state being it hasn't resolve or rejected yet. You have two options.

Option one. node 10+ you can use async await.

async function main(){
const res = await makeRequest(baseUrl, feedId, apiKey)
return res
}

Pre node 10

makeRequest(baseUrl, feedId, apiKey)
  .then(data => {
    console.log(data)
  })
  .catch(err => {
    console.log(err)
  })

Essentially if you have to wait for the data, you have to have the code that relies on that data after the await, or in the.then block.

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/async_function

Answer 2

Your makeRequest() function returns a promise. To use that promise, you use .then() or await to get the resolved value from the promise.

Using .then() :

makeRequest(baseUrl, feedId, apiKey).then(result => {
    console.log(result);
}).catch(err => {
    console.log(err);
});

or using await :

async function someFunction() {
    try {
        let result = await makeRequest(baseUrl, feedId, apiKey);
        console.log(result);
    } catch(e) {
        console.log(err);
    }
}

FYI, a lot of what makeRequest() is doing can be done simpler using an http request library that already supports promises. There is a list of competent libraries to choose from here .

My favorite is got() , but you can review the list to decide which one you like. The main advantages are that they already collect the whole response for you, already support promises, support a wide variety of authentication schemes with built-in logic and can often decode or parse the response for you - all things which https.request() does not know how to do by itself.