How to Promisify this function - nodejs [duplicate]

Question

This question already has an answer here:

• How do I convert an existing callback API to promises? 20 answers

I have an ajax call which needs to return a promise. The function is as follows

client.tickets.create(ticket,  function(err, req, result) {
  if (err) {    
    logger.error(err);

    return false;
  }

  return JSON.stringify(result);
});

I have to wait for this function to execute before I can perform the next action. How can I promisify this function ?

I tried the following and it gave me an error saying Cannot call method then of undefined:

return client.tickets.create(ticket).then(function(result){
    return JSON.stringify(result);
},function(err){
    logger.error(err);
    return false;
});

Answer 1

You have the error because create() is not a Promise. Promisifying an async function is quite easy (nodejs has a built-in Promise support nowadays):

function createTicket(ticket) {
    // 1 - Create a new Promise
    return new Promise(function (resolve, reject) {
        // 2 - Copy-paste your code inside this function
        client.tickets.create(ticket, function (err, req, result) {
            // 3 - in your async function's callback
            // replace return by reject (for the errors) and resolve (for the results)
            if (err) {
                reject(err);
            } else {
                resolve(JSON.stringify(result));
            }
        });
    });
}

// 4 - consume your promise with then() (resolved promise) and catch (rejected promise)
createTicket(ticket).then(function (result) {
    // deal with result here
}).catch(function (err) {
    // deal with error here
});

Answer 2

rather than manually wrapping async code into promises, I would advice using libraries like Bluebird to do that for you:

var Bluebird = require('bluebird');

//either 
client.tickets = Bluebird.promisifyAll(client.tickets);
//or 
client.tickets.createAsync = Bluebird.promisify(client.tickets.create);
...
    return client.tickets.createAsync(ticket)
      .then(JSON.stringify)
      .catch(err => {
        logger.error(error); 
        return false
      });

Answer 3

function createTicket(){
var deferred = Q.defer()
client.tickets.create(ticket,  function(err, req, result) {
     if (err) { 
                logger.error(err);
                return deferred.reject(err)
            }
   return deferred.resolve(result)
});
}

createTicket().then(function(){
    //success here
}, function(){
    //failure here
});

Using Q api you can achive promise.