Usage of callback functions and asynchronous requests

Question

Usage of callback functions and asynchronous requests

I am working with a node.js application that uses Facebook APIs. In one part of my code I need to return the access_token from a common function. That is, a lot of other functions need to call this function to retrieve the Facebook access token.

Below is my code:

function getAccesstoken(code) {
    var options = {
        host: 'graph.facebook.com',        
        path: '/oauth/access_token?client_id=xxxx&redirect_uri=xxxxxx&client_secret=xxxxx&code='+code.toString()
    };

    var acc_token = ''

    https.get(options, function(resp) {
        resp.on('data', function(d) {                                    
            acc_token = acc_token+d.toString()
        });    

        resp.on('end', function() {
            var expiry_index = acc_token.indexOf('&expires=')

            acc_token = acc_token.substring(0, expiry_index)            
        });
    });        

    return acc_token.toString()
}

Since https.get call is asynchronous, function always returns an empty string. What should be the best way to do this?

javascript
facebook
node.js

Answer 1

You've already answered the question in your title - use a callback.

Define another variable for getAccesstoken that should be a function that gets called when acc_token is filled, and pass acc_token as an argument for that function to use.

It's the same way that https.get is working - the second argument is a function that's called when the request is completed, and gets passed the result of the request.

Answer 2

well supply a callback function to your getAccesToken function and call the function after the response has ended and pass acc_token to that function..

 function getAccessToken(code,cb){
    .....
    resp.on('end',function(){
       .....
       cb(null,acc_token)
    })
 }

then call your function like

getAccesToken("whatever",function(err,token){
  console.log("got token", token)
})