Why do i get undefined, even when using a promise?

Question

I am trying to use a promise to call a function getTweets. Not using an AJAX call, but a simple promise 'call' from 1 javascript file to another. The function works, but i keep getting 'undefined'. I have read dozens of questions here on stackoverflow and have spent days to understand promises, but still can't solve it.

var Twit = require('twit') // Imports the Twitter library
require('dotenv').config() // to get the environment vars into the app
// This is the function:
function getTweets (screen_name) {
  let T = new Twit({ /* <twitter key and token here> */ });
  T.get('statuses/user_timeline', { screen_name: screen_name, count: 3}, function (err, data, response) {
    let myTweets = [];
    for (let i in data) {
      let text = data[i].text; 
      myTweets.push(text); 
    }
    return myTweets;
  })
}
 module.exports.getTweets = getTweets;

And this is the promise that tries to get the tweets:

 var promise = tweets.getTweets('dizid');
 promise.then(
     console.log(myTweets), 
     console.log(err))
 // gives error: promise.then(
 //                      ^
 // TypeError: Cannot read property 'then' of undefined

Any help greatly appreciated.

Answer 1

Your problem is that you never return anything from your getTweets() function even though it needs to return a promise. The function calls T.get() and pass it a callback function. You return from this callback function but this doesn't do anything, it doesn't mean that this value gets returned from getTweets().

This is a pretty common mistake when it comes to working with asynchronous calls. What needs to be done is to make getTweets() return a promise that gets resolved when it should.
When working with asynchronous calls that don't implement the promise interface, you need to wrap this call with a new promise. Your getTweets() function should then look like this:

function getTweets (screen_name) {
  let T = new Twit({ /* <twitter key and token here> */ });
  return new Promise(function(resolve, reject) {
    T.get('statuses/user_timeline', { screen_name: screen_name, count: 3}, function (err, data, response) {
      if (err) {
        reject(err); // Reject the promise since there was an error
      } else {
        let myTweets = [];
        for (let i in data) {
          let text = data[i].text; 
          myTweets.push(text); 
        }
        resolve(myTweets); // Resolve the promise with the result
      }
    });
  });
}

However, it seems the Twit API does support the promise interface, so instead of providing a callback function you can just use the promise created by T.get(). HMR's answer explains how to do this.

Another mistake you've made is with this code:

promise.then(
  console.log(myTweets), 
  console.log(err))

The way you've written it, it reads "Run console.log(myTweets) and console.log(err), then invoke promise.then() with the result of the former as the first argument and the result of the latter as the second argument.
then() takes callback functions (which get invoked depending on the resolving/rejection of the promise) as arguments, so the code should look like this:

promise.then(
  function(myTweets) { 
    console.log(myTweets);
  },
  function(err) { 
    console.log(err);
  });

---

Async/await

If you're interested in taking things further, the modern approach to working with asynchronous code is async/await, which is syntactic sugar for promises that lets you write asynchronous code more similar to regular synchronous code.

A function marked as async will implicitly return a promise, but you write it as if you return a regular value. Using the await keyword inside an async function will implicitly wait for a promise to resolve and unwrap the resolved value. The main practical benefits of this is that you can use asynchronous calls in loops and handle errors with regular try-catch blocks. Your getTweets() function would look like this using async/await:

async function getTweets(screen_name) {
  let T = new Twit({ /* <twitter key and token here> */ });
  const data = await T.get('statuses/user_timeline', { screen_name: screen_name, count: 3});
  // Let's also use map() instead of a for loop
  let myTweets = data.map(function(item) { return item.text; });
  return myTweets;
}

Answer 2

Since get seems to return a promise you don't need to use a callback. Get Tweets can look something like this:

//  in getTweets
return T.get(
  'statuses/user_timeline', 
  { screen_name: screen_name, count: 3}
).then(
  function (data) {
    console.log("data:",JSON.stringify(data,undefined,2));
    return data.map(item=>item.text);
  }
)
// exports the function getTweets so that other modules can use it
module.exports.getTweets = getTweets;

If that didn't work please let us know what the output of the program is (update question).

You can call getTweets like so:

tweets.getTweets('dizid')
.then(
  myTweets=>
    console.log(myTweets),
  err=>
    console.log(err)
)

Answer 3

I think you forget add function like

promise.then(function(res){
    //code
}

Answer 4

Your .then() should include a call back function.

promise.then( res => {
    console.log(res);
});

edit: I'm using an ES6 syntax for arrow functions, in case you're new to that.