Simplest way to pass value back from anonymous callback?

Question

I'm still trying to understand javascript scoping. What is the simplest way to return a value from an anonymous callback function? For instance,

var test = 'outside';
callsFunction(function() { test = 'inside'; });
console.log(test);

where callsFunction calls the anonymous function. I would like this to print 'inside'.

Answer 1

Did you call callsFunction?

Answer 2

I'm a little bit confused, but I believe this is what you're after

function callsFunction( fn )
{
  fn();
}

var test = 'outside';
callsFunction(function() { test = 'inside'; });
console.log(test);

Note that any function can be invoked via the usage of (). However, there are special methods for invoking functions from different contexts, such as Function.call() and Function.apply()

Answer 3

At the risk of pointing out the obvious, the simplest way to have something done after a callback is to simply include it in the callback (and this is, in fact, the way to go):

var test = 'outside';
callsFunction(function() { 
    test = 'inside'; 
    console.log(test);
});

A real example: let's say you want to update a div with the contents of a file from an Ajax call.

Ajax('file.html', function (data) {
    // Update the div here
    document.getElementById('d').innerHTML = data;
});

// Not here

Answer 4

Maybe what you are wanting to research is closures in JavaScript?

Answer 5

You didn't define callsFunction in your question, but if you make it a self-invoking anonymous function, you can return the new 'test' value:

var test = 'outside';
(function() { test = 'inside'; })();

console.log(test); // returns 'inside'

If you do define callsFunction:

var test = 'outside';
var callsFunction = function() { 
    test = 'inside';
    return test;
};

console.log(callsFunction()); // returns 'inside'

You can get more complicated with closures.