JavaScript: Tutorial - A Guide to JavaScript - Page 09

Functions

A function is declared using the function keyword, followed by a set of arguments and then the body of the function.

function square(n) {
	return n * n;
}
function showMessage(message) {
	alert(message);
}
showMessage("Hello, World!"); // shows "Hello, World!"
var num = square(12);		 // num is 144
alert(num);
showMessage(square(9));		 // shows "81"

Functions as Values

Functions can be used any place any other value can be used because function names are variables. This means it is possible to pass a function into another function as an argument and also to return a function as the result of another function.

function callSomeFunction(someFunction, someArgument){
	return someFunction(someArgument);
}
function square(num){
	return num * num;
}
var result = callSomeFunction(square, 8);
alert(result); // 64

function createComparisonFunction(propertyName){
	return function(object1, object2){
		var value1 = object1[propertyName];
		var value2 = object2[propertyName];
		if (value1 < value2){
			return -1;
		}
		else if (value1 > value2){
			return 1;
		}
		else{
			return 0;
		}
	};
}

Recursion

Recursion happens when a function calls itself. arguments.callee is a pointer to the function executing.

function f(num){
	if (num <= 1){
		return 1;
	} else {
		return num * arguments.callee(num-1); // arguments.callee is the f function.
	}										 // return num * f(num-1); is equivalent.
}

The arguments object acts like an array. One can specify as many arguments as one wishes. Access each argument using bracket notation. As below:

var globalVar = 0;						// globalVar is a global variable
function sum() {
	var retval = 0;					 // retval is a private variable
	for(var i = 0; i < arguments.length; i++) {
		retval += arguments[i];
	}
	return retval;
}
var sum1 = sum(1, 3, 5, 7, 10, 11, 15); // sum1 == 52
alert(sum1);
var sum2 = sum(10, 12, 15);			 // sum2 == 37
alert(sum2);

Reference Types

The Object Type

(Learn about JavaScript Classes)

There are two ways to create an object. First, use the new operator with the Object constructor.

var animal = new Object();
animal.type = "dog";
animal["name"] = "Napolean";
alert(animal.name);

Objects created with the Object constructor, as above, are always passed by reference instead by value.

Second, use object literal notation.

var animal = {
	type: "dog",
	name: "Napolean"
};
alert(animal.name);

Or:

var animal = {}
animal.type = "dog";
animal.name = "Napolean";
alert(animal.name);

There are several different ways to access Object properties.

var animal = {
	type: "dog",
	name: "Napolean"
};
alert(animal["name"]);			// shows "Napolean"
alert(animal.name);			 // shows "Napolean"
var propertyName = "name";
alert(animal[propertyName]);	// shows "Napolean"

Be careful when working with the Object type. It is assigned by reference only. In otherwords, the variable is actually a pointer to the object.

var person = new Object();
person.name = "Joe Bleaux";
person.age = 93;

var oldman = person;
alert(oldman.name);		 // shows "Joe Bleaux"

person.name = "Jane Doe";
alert(oldman.name);		 // shows "Jane Doe"

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