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JavaScript Asynchronous Code in Express Asynchronous Code in Express Using Callbacks

karan Badhwar
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karan Badhwar
Web Development Techdegree Graduate 18,135 Points
Posted by karan Badhwar
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karan Badhwar
Web Development Techdegree Graduate 18,135 Points

accessing res inside Callback

Hi, I am wondering how are we able to use res.render inside a callback 

function getUsers(cb){
  fs.readFile('data.json', 'utf8', (err, data) => {
    if (err) return cb(err);
    const users = JSON.parse(data);
    return cb(null, users);
  });
}

app.get('/', (req,res) => {
  getUsers((err,users) =>{
    if(err){
      res.render('error', {error:err})
    } else{
      res.render('index', {title: "users", users: users.users});
    }
  })
});

Because when we pass the callback, Callstack gets loaded up with getUsers(cb) Function, and we are no longer inside the route,then how the callback is using the res.render Method? And how do i know which values, methods are accessible to callbacks when being passed?

1 Answer

Steven Parker
Steven Parker
231,236 Points
Steven Parker
Steven Parker
231,236 Points

Since res is one of the arguments passed to the callback ( " (req,res) => ... " ), then the code inside the callback has access to res and all of its methods and values.

I apologize if you only wanted an answer from Caleb. I'll assume that's your intention if you tag someone specific within 24 hours of a posting in future.

karan Badhwar
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.a{fill-rule:evenodd;}techdegree seal-36
karan Badhwar
Web Development Techdegree Graduate 18,135 Points
karan Badhwar
.a{fill-rule:evenodd;}techdegree seal-36
karan Badhwar
Web Development Techdegree Graduate 18,135 Points

Hey Steven Parker, thankyou sir for the response, no actually a lot of my questions are left unanswered and the specific ones which I feel the topic is important, I only tag in those one's as so that I can get at least an answer for those, as I search a lot and was stuck for a very long time, and sometimes if I need to complete the task to move forward that's why I post and tag and plus leave a message in the Slack, coz once I get stuck I cannot move forward until I understand sometimes that takes around 3-4 days of mine as well. I apologize if you felt that way sir.

And sir how would you able to answer my this question ? here

karan Badhwar
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.a{fill-rule:evenodd;}techdegree seal-36
karan Badhwar
Web Development Techdegree Graduate 18,135 Points
karan Badhwar
.a{fill-rule:evenodd;}techdegree seal-36
karan Badhwar
Web Development Techdegree Graduate 18,135 Points

I did select it as Best Answer and Sorry I guess I mistyped I meant would you be able to assist me in my other question sir please this one here

Caleb Kemp
Caleb Kemp
12,755 Points
Caleb Kemp
Caleb Kemp
12,755 Points

Glad to see your questions got answered :smile: