a callback example

callback.js

function firstFunc(somevalue, callback) {
    // pass the values, error as null (no possibility of error here) and the value is the new string
    // this is the normal node convention
    callback(null, somevalue += ' ok');
}

// call the function with the string and an anon function that get's the values from the callback
firstFunc('this should be', function (error, callbackvalue) {
    if (!error) {
        // if the callback didn't pass an error, then things should be ok
        console.log(callbackvalue);
    }
});