Javascript: variable scope in callback

Question

I'm trying to re-use variable inside a function that calls a callback, but it does not work the way I think it should;

another()(); //=> logs "somevalue"
callingfn(); //=> logs " someval is not defined"

function a(fn){
  var someval = "some-value";
  return fn();
} 

function callingfn(){
 return a.call(this, function(){
   console.log(someval)
  })
}

function another(){
  var sv = "somevalue";
  return function(){
    console.log(sv);
  }
}

I'm not able to understand if this is closure-related problem, but at first I expected that someval in callingfn would have been defined.

Where am I wrong?

someval is local to the function a() and will be accessible only inside a() and any closures inside a() (there are none in the code you posted). — techfoobar
– techfoobar, Commented Apr 9, 2015 at 13:55
Uhm, right. I was wondering if there was a way to make the callback a closure in a() — steo
– steo, Commented Apr 9, 2015 at 13:59
Thats not possible, for it to be a callback it has to be defined inside a() — techfoobar
– techfoobar, Commented Apr 9, 2015 at 15:41

serenesat · Accepted Answer · 2015-04-09 14:09:18Z

6

function fn() is different from a() though it receives fn as parameter.

You could possibly send someval as parameter.

another()(); //=> logs "somevalue"
callingfn(); //=> logs " someval is not defined"

function a(fn){
  var someval = "some-value";
  return fn(someval);
} 

function callingfn(){
 return a.call(this, function(someval){
   console.log(someval)
  })
}

function another(){
  var sv = "somevalue";
  return function(){
    console.log(sv);
  }
}

Or simply declare the var someval as global scope, currently it is inside a function which makes it local.

Hope this helps.

edited Apr 9, 2015 at 14:09

serenesat

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answered Apr 9, 2015 at 13:55

Vinod Kumar

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Comments

Amit K. · Accepted Answer · 2015-04-09 13:55:45Z

1

Try This:

another()(); //=> logs "somevalue"
callingfn(); //=> logs " someval is not defined"
var someval;
var sv;

function a(fn){
  someval = "some-value";
  return fn();
} 

function callingfn(){
 return a.call(this, function(){
   console.log(someval)
  })
}

function another(){
 sv = "somevalue";
  return function(){
    console.log(sv);
  }
}

answered Apr 9, 2015 at 13:55

Amit K.

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Comments

KJ Price · Accepted Answer · 2015-04-09 13:56:34Z

1

Define someval outside the scope of the functions:

var someval; // <- outside of the scope of any one function
another()(); //=> logs "somevalue"
callingfn(); //=> logs " someval is not defined"

function a(fn){
  someval = "some-value"; // <-remove "var" to access the variable outside the scope
  return fn();
} 

function callingfn(){
 return a.call(this, function(){
   console.log(someval)
  })
}

function another(){
  var sv = "somevalue";
  return function(){
    console.log(sv);
  }
}

answered Apr 9, 2015 at 13:56

KJ Price

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