gpio.js

gpio.read(pin, function(err, value) {
    if (err) throw err;
    console.log(value);
});

function getX (callback) {
    gpio.read(pin, function (err, value) {
        if (err) throw err;
        callback(value);
    });
}

// Say you wanted to log X or print it to the screen:
getX(function (x) {
    console.log(x);
});
printX(function (screen, x) {
    // I just made up some screen function, this isn't a real thing.
    screen.write(x);
});


// Now lets do the same thing with promises
// https://www.promisejs.org/
// https://functionwhatwhat.com/introduction-to-promises-in-javascript/
function getX_withPromises () {
    var defer = q.defer();
    
    gpio.read(pin, function (err, value) {
        if (err) {
            defer.reject(err);
        }
        defer.resolve(value);
    });
    
    return defer.promise;
}

// Without "handling" the error
getX_withPromises().then(function (x) {
    console.log(x);
});
getX_withPromises().then(function (screen, x) {
    screen.write(x);
});

// Adding error handling
function handleError (err) {
    throw err;
}
// Here you see that you can also just pass a function name and it will be handled.
// The "then" function takes two parameters: successfunction, errorFunction
// reject will pass to the errorFunction and resolve passes to successFunction
getX_withPromises().then(function (x) {
    console.log(x);
}, handleError);
getX_withPromises().then(function (screen, x) {
    screen.write(x);
}, handleError);