Returning a value from callback function in Node.js [duplicate]

Question

I am facing small trouble in returning a value from callback function in Node.js, I will try to explain my situation as easy as possible. Consider I have a snippet, which takes URL and hits that url and gives the output:

urllib.request(urlToCall, { wd: 'nodejs' }, function (err, data, response) {                              
    var statusCode = response.statusCode;
    finalData = getResponseJson(statusCode, data.toString());
});

I tried to wrap it inside a function and return a value like this:

function doCall(urlToCall) {
urllib.request(urlToCall, { wd: 'nodejs' }, function (err, data, response) {                              
    var statusCode = response.statusCode;
    finalData = getResponseJson(statusCode, data.toString());
    return finalData;
});
}

Because in my Node.js code, I have a lot of if-else statement where value of urlToCall will be decided, like this:

if(//somecondition) {
   urlToCall = //Url1;
} else if(//someother condition) {
   urlToCall = //Url2;
} else {
   urlToCall = //Url3;
}

The thing is all of the statements inside a urllib.request will remain same, except value of urlToCall. So definitely I need to put those common code inside a function. I tried the same but in doCall will always return me undefined. I tried like this:

response = doCall(urlToCall);
console.log(response) //Prints undefined

But if I print value inside doCall() it prints perfectly, but it will always return undefined. As per my research I came to know that we cannot return values from callback functions! (is it true)? If yes, can anyone advice me how to handle this situation, as I want to prevent duplicate code in every if-else blocks.

Kimbell

"is it true?" - yes, definitely. – Greenshank

@JanDvorak, so I don't have any option other than duplicating code? ;) – Kimbell

Would passing some of your own callbacks help? I believe so. – Greenshank

I think this link would help you understand how it works: github.com/maxogden/art-of-node#callbacks – Dilatant

@RodrigoMedeiros thanks for excellent resource. I am going through it, it helped me lot :) – Kimbell

Answer 1

Its undefined because, console.log(response) runs before doCall(urlToCall); is finished. You have to pass in a callback function aswell, that runs when your request is done.

First, your function. Pass it a callback:

function doCall(urlToCall, callback) {
    urllib.request(urlToCall, { wd: 'nodejs' }, function (err, data, response) {                              
        var statusCode = response.statusCode;
        finalData = getResponseJson(statusCode, data.toString());
        return callback(finalData);
    });
}

Now:

var urlToCall = "http://myUrlToCall";
doCall(urlToCall, function(response){
    // Here you have access to your variable
    console.log(response);
})

@Rodrigo, posted a good resource in the comments. Read about callbacks in node and how they work. Remember, it is asynchronous code.

Myer

This works fine. But i have a question. in calling the doCall function indside that. How can i return the response variable? If for example i want to get that variable outside the function? – Detestable

@Detestable same question here!! – Prima

this post was very helpful, thank you! – Dooryard

Hi, Sorry if i'm misunderstanding but how is this any different? The urllib response is now just stuck inside the scope of another function outside of it. Its not like you can now use the response object anywhere outside of doCall? – Amusement

Very helpful. Always kind of avoided creating callbacks but I now understand. Thanks – Popgun

You can now use promises too. Return a promise and call .then() on it. SO Question: #40735918. Promise explanation: developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/… – Womanizer

Answer 2

I am facing small trouble in returning a value from callback function in Node.js

This is not a "small trouble", it is actually impossible to "return" a value in the traditional sense from an asynchronous function.

Since you cannot "return the value" you must call the function that will need the value once you have it. @display_name already answered your question, but I just wanted to point out that the return in doCall is not returning the value in the traditional way. You could write doCall as follow:

function doCall(urlToCall, callback) {
    urllib.request(urlToCall, { wd: 'nodejs' }, function (err, data, response) {                              
        var statusCode = response.statusCode;
        finalData = getResponseJson(statusCode, data.toString());
        // call the function that needs the value
        callback(finalData);
        // we are done
        return;
    });
}

Line callback(finalData); is what calls the function that needs the value that you got from the async function. But be aware that the return statement is used to indicate that the function ends here, but it does not mean that the value is returned to the caller (the caller already moved on.)

Goodden

Answer 3

Example code for node.js - async function to sync function:

var deasync = require('deasync');

function syncFunc()
{
    var ret = null;
    asyncFunc(function(err, result){
        ret = {err : err, result : result}
    });

    while((ret == null))
    {
         deasync.runLoopOnce();
    }

    return (ret.err || ret.result);
}

Montford

This worked like a charm, thanks a lot for this. You could see the documentation here npmjs.com/package/deasync – Vaginectomy

Answer 4

If what you want is to get your code working without modifying too much. You can try this solution which gets rid of callbacks and keeps the same code workflow:

Given that you are using Node.js, you can use co and co-request to achieve the same goal without callback concerns.

Basically, you can do something like this:

function doCall(urlToCall) {
  return co(function *(){
    var response = yield urllib.request(urlToCall, { wd: 'nodejs' }); // This is co-request.                             
    var statusCode = response.statusCode;
    finalData = getResponseJson(statusCode, data.toString());
    return finalData;
  });
}

Then,

var response = yield doCall(urlToCall); // "yield" garuantees the callback finished.
console.log(response) // The response will not be undefined anymore.

By doing this, we wait until the callback function finishes, then get the value from it. Somehow, it solves your problem.

Enough

but this is ecma6 soultion – Dietsche

and it also impact my performance – Dietsche

Is ecma6 an issue? – Enough

No that is not an issue but the problem is many of us still not using ECMA6 in their project as node.js has some problem with ECMA compatibility. – Dietsche