Your export can’t work because it is outside the function while the foodeclaration is inside. But if you put the export inside, when you use your module you can’t be sure the export was defined.

The best way to work with an ansync system is to use callback. You need to export a callback assignation method to get the callback, and call it on the async execution.

Example:

var foo, callback;
async.function(function(response) {
    foo = "foobar";

    if( typeof callback == 'function' ){
        callback(foo);
    }
});

module.exports = function(cb){
    if(typeof foo != 'undefined'){
        cb(foo); // If foo is already define, I don't wait.
    } else {
        callback = cb;
    }
}

Here async.function is just a placeholder to symbolise an async call.

In main

var fooMod = require('./foo.js');
fooMod(function(foo){
    //Here code using foo;
});

Multiple callback way

If your module need to be called more than once you need to manage an array of callback:

var foo, callbackList = [];
async.function(function(response) {
    foo = "foobar";

    // You can use all other form of array walk.
    for(var i = 0; i < callbackList.length; i++){
        callbackList[i](foo)
    }
});

module.exports = function(cb){
    if(typeof foo != 'undefined'){
        cb(foo); // If foo is already define, I don't wait.
    } else {
        callback.push(cb);
    }
}

Here async.function is just a placeholder to symbolise an async call.

In main

var fooMod = require('./foo.js');
fooMod(function(foo){
    //Here code using foo;
});

Promise way

You can also use Promise to solve that. This method support multiple call by the design of the Promise:

var foo, callback;
module.exports = new Promise(function(resolve, reject){
    async.function(function(response) {
        foo = "foobar"

        resolve(foo);
    });
});

Here async.function is just a placeholder to symbolise an async call.

In main

var fooMod = require('./foo.js').then(function(foo){
    //Here code using foo;
});

See Promise documentation