How to return a value from within closure to external scope?

Question

I came across the need to return a value from one function to another and for a while scratched my head in confusion when this simple task didn't work. I then realised I was operating within a closure function (or anonymous function?) and can't find documentation on how to release a variable out of such a function's scope.

For example, this doesn't work:

function aFunc()
{
   var result;
   object.event = function(){
      result = true;
   }
   return result;
}

Nor does returning from inside the closure. Do I need to do both? I tried using a global variable within the largest scope possible (outside of all functions) and this didn't work either. What am I missing?

I'm not sure whether I'm using the term closure correctly, I'm referring to the anonymous function.

Thanks.

Answer 1

You need to call the closure:

function aFunc() {
   var result;
   object.event = function() {
      result = true;
   };
   object.event();
   return result;
}

Or, if it runs elsewhere, this is a timing issue. You can have something like a promise:

function aFunc() {
   var promise = { hasRun: false, result: null };
   object.event = function() {
      result.hasRun = true;
      result.result = true; // or something else...
   };
   return promise;
}

// check if it has run and get the result:

if (promise.hasRun) {
  // access promise.result
}

But, a simple callback should suffice. Since you mentioned the XMLHttpRequest object, you should be able to attach a callback to its onreadystatechange event, or pass a callback to aFunc:

function aFunc(callback) {
   object.event = function() {
      var result = true;
      callback(result);
   };
}

Then your callback gets called with the result at the time it is available.

Answer 2

You could pass an object to the closure, whose attribute 'result' it would set when the function is called.

function aFunc (foo) {
    object.event = function() {
        foo.result = true;
    }
}

The problem is that the value of result is returned by aFunc, rather than a reference to it, which means that any subsequent change of result that happens inside the closure would not affect the result that was returned earlier.