function-callback.js

// In our example, the  first function, multiplyByTwo(), 
// accepts three parameters, loops through them, 
// multiplies them by two, and returns an array 
// containing the result.

function multiplyByTwo(a, b, c) {
    var outputArray = [];

    for (var i = 0; i < arguments.length; i++) {
        var currentElement = arguments[i];
        outputArray[i] = currentElement * 2;
    }
    return outputArray;
}

function addOne(a) {
    return a + 1;
}

// Test these functions:

var output1 = multiplyByTwo(1, 2, 3);
var output2 = addOne(100);

console.log(output1);
console.log(output2);

// Now let's say you want to have an array, myarr, 
// that contains three elements, and each of the 
// elements is to be passed through both functions.

var myArray = [];
myArray = multiplyByTwo(10, 20, 30);
// [20, 40, 60]


// loop through each element, passing it to addOne()
for (var i = 0; i < myArray.length; i++) {
    myArray[i] = addOne(myArray[i]);
}

console.log(myArray);
// output > [21, 41, 61]

// Everything works  ne, but there's room for improvement.

function multiplyByTwo2(a, b, c, callback) {
    var outputArray = [];

    for (var i = 0; i < arguments.length - 1; i++) {
        var currentElementTimes2 = arguments[i] * 2;
        var element = callback(currentElementTimes2);

        outputArray[i] = element;
    }

    return outputArray;
}

// By using the modified function, all the work is done 
// with just one function call, which passes the start 
// values and the callback function:

myArray = multiplyByTwo2(1, 2, 3, addOne);
console.log(myArray);

// Instead of defining addOne(), you can use an anonymous function, 
// therefore saving an extra global variable

multiplyByTwo(1, 2, 3, function (a) {
    return a + 1;
});


// Anonymous functions are easy to change should the need arise

var multipleBy10 = function (a) {
    return a * 10;
};

multiplyByTwo(1, 2, 3, multipleBy10);

multiplyByTwo(1, 2, 3, function (a) {
    return a * 100;
});