CODEWITHSUNDEEP
node.jspromisenode-modules
Dev Aggarwal
Dev Aggarwal

Reputation: 783

Export a dynamic variable

I'm trying to export a variable in node.js like this:

let news = [];

const fetchNews = new Promise ((resolve, reject) => {

  let query = 'SELECT id, name FROM news';

  mysql.query(query, [], (error, results) => {

    if (error)
      reject({error: `DB Error: ${error.code} (${error.sqlState})`})

    results = JSON.parse(JSON.stringify(results));

    news = results;

    resolve(results);

  });

});

if(!news.length)
  fetchNews
    .then(results => {news = results})
    .catch(err => {console.log('Unable to fetch news', err)});

exports.news = news;

When I use this code in some other module like this:

const news = require('./news.js').news;

console.log(news);
//returns [];

Can somebody point out my mistake in first code?

Upvotes: 1

Views: 1277

Answers (1)

Akrion
Akrion

Reputation: 18525

There are a couple of things that seem odd in the way you are doing this:

You have an async operation but you want just the value without actually awaiting on the operation to complete. Try something like this:

module.exports = new Promise ((resolve, reject) => {
  mysql.query('SELECT id, name FROM news', (error, results) => {
    if (error)
      reject({error: `DB Error: ${error.code} (${error.sqlState})`})

    resolve(JSON.parse(JSON.stringify(results)));
  });
});

Then to get the news:

var getNewsAsync = require('./news')
getNewsAsync.then(news => console.log(news))

It would be cleaner/shorter if you actually utilize async/await with the mysql lib.

Update:

With Node 8 and above you should be able to promisify the mySQL lib methods. Although there might be better npm options out there to get this to work. Here is an untested version:

const mysql = require('mysql');
const util = require('util');
const conn = mysql.createConnection({yourHOST/USER/PW/DB});
const query = util.promisify(conn.query).bind(conn);

module.exports = async () => {
  try {return await query('SELECT id, name FROM news')} finally {conn.end()}
}

To get the news:

var getNewsAsync = require('./news')
console.log(await getNewsAsync())

Upvotes: 6

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