Creating A Function In Javascript

The basic structure of a function looks like this:

function functionName() {
// the JavaScript you want to run
}

The keyword function lets the JavaScript interpreter know you’re creating a function—
it’s similar to how you use if to begin an if/else statement or var to create a variable.

function printToday()
{
var today = new Date();
document.write(today.toDateString());
}

The function’s name is printToday. It has just two lines of JavaScript code that retrieve
the current date, convert the date to a format we can understand

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Javascript Do While Loops

There’s another, less common type of loop, known as a do/while loop. This type of
loop works nearly identically to a while loop. Its basic structure looks like this:

do {
// javascript to repeat
} while (condition) ;

do {
var luckyNumber = prompt('What is your lucky number?','');
luckyNumber = parseInt(luckyNumber, 10);
} while (isNaN(luckyNumber));

Save this file and preview it in a web browser. Try typing text and other nonnumeric
symbols in the prompt dialog box. That annoying dialog box continues
to appear until you actually type a number.

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Javascript For Loops

JavaScript offers another type of loop, called a for loop, that’s a little more compact
(and a little more confusing). For loops are usually used for repeating a series of
steps a certain number of times, so they often involve some kind of counter variable,
a conditional test, and a way of changing the counter variable. In many cases,
a for loop can achieve the same thing as a while loop, with fewer lines of code. For
example.

for (var num=1; num<=100; num++) {
document.write('Number ' + num + '<br>');
}


var days = ['Monday', 'Tuesday', 'Wednesday', 'Thursday','Friday', 'Saturday', 'Sunday'];
for (var i=0; i<days.length; i++) {
document.write(days[i] + ', ');
}

Since for loops provide an easy way to repeat a series of steps a set number of times,
they work really well for working through the elements of an array. The while loop
in Figure 3-5, which writes each item in an array to the page, can be rewritten using
a for loop,

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Javascript While Loops

A while loop repeats a chunk of code as long as a particular condition is true; in other
words, while the condition is true. The basic structure of a while loop is this:

while (condition) {
// javascript to repeat
}

The first line introduces the while statement. As with a conditional statement, you
place a condition between the set of parentheses that follow the keyword while.

Say you want to print the numbers 1 to 5 on a page. One possible way to do that is
like this:

document.write('Number 1 <br>');
document.write('Number 2 <br>');
document.write('Number 3 <br>');
document.write('Number 4 <br>');
document.write('Number 5 <br>');

Notice that each line of code is nearly identical—only the number changes from line
to line. In this situation, a loop provides a more efficient way to achieve the same goal:

var num = 1;
while (num <= 5) {
document.write('Number ' + num + '<br>');
num += 1;
}

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Javascript Array

You can access the contents of a simple variable just by using the variable’s name.
For example, alert(lastName) opens an alert box with the value stored in the variable
lastName. However, because an array can hold more than one value, you can’t just
use its name alone to access the items it contains. A unique number, called an index,
indicates the position of each item in an array. To access a particular item in an array,
you use that item’s index number. For example, say you’ve created an array with
abbreviations for the days of the week, and want to open an alert box that displayed
the first item.

var days = ['Mon', 'Tues', 'Wed', 'Thurs', 'Fri', 'Sat', 'Sun'];
alert(days[0]);

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Javascript Object Type

Objects are
created by using the new operator followed by the name of the object type to create. Developers
create their own objects by creating instances of the Object type and adding properties and/or
methods to it, as shown here:

var o = new Object();

Each Object instance has the following properties and methods:
constructor — The function that was used to create the object. In the previous example,
the constructor is the Object() function.

hasOwnProperty(propertyName) — Indicates if the given property exists on the object
instance (not on the prototype). The property name must be specifi ed as a string (for
example, o.hasOwnProperty(“name”)).

isPrototypeOf(object) — Determines if the object is a prototype of another object.

propertyIsEnumerable(propertyName) — Indicates if the given property can be
enumerated using the for-in statement (discussed later in this chapter). As with
hasOwnProperty(), the property name must be a string.

toLocaleString() — Returns a string representation of the object that is appropriate for
the locale of execution environment.

toString() — Returns a string representation of the object.

valueOf() — Returns a string, number, or Boolean equivalent of the object. It often
returns the same value as toString().

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