Returning a value from callback function in Node js

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

User · Answer

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.

User · 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

User · Answer

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

User · Answer

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

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