var test;

// Synchronous
test = 'before';

// Asynchronous
var myFunction = function (cb) {
    // Let's pretend this is network or filesystem access...
    setTimeout(function () {
        cb('after')
    }, 1000);
}
myFunction(function(result) {
   test = result;
});

// This executes out of order, because setTimeout is async
setTimeout(function () {
    console.log(test);
}, 2000);
console.log(test);

// Output:
// > before
// > after