Simze Simze - 6 months ago 20
Javascript Question

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.

Answer

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.