callback.js

// define function with two args and callback
function foo(a, b, cb) {
    console.log(a + ' ' + b);

    // check type of cb
    if (typeof(cb) === "function") {
        cb();
    }
    else {
        console.log('error ' +  cb + ' not a function');
    }
};

// execute function
foo('cat', 'dog', function() {
    console.log('hello');
})

callback2.js

function foo(firstName, secondName, cb) {
    var fullName = firstName + ' ' + secondName;

    if (typeof(cb) === "function") {
        cb(fullName);
    }
    else {
        console.log('error ' +  cb + ' not a function');
    }
}

foo('Alex', 'Born', function(takenFromCb) {
    console.log(takenFromCb.toUpperCase());
})
// >> ALEX BORN