Returning a value from callback function in Node.js

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.

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](

[To see links please register here]

) in the comments. Read about _callbacks_ in node and how they work. Remember, it is asynchronous code.

> 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.)

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][1] and [co-request][2] 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.
[1]:

[To see links please register here]

[2]:

[To see links please register here]

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);
}