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Some javascript code


#1

Welcome to the course for Javascript. Although it is aimed at complete beginners to the subject I do assume that you have some knowledge of HTML and a little CSS. You don’t have to be an expert, though. Just the basics will do. You don’t need to buy any software for this Javascript course - you probably already have everything you need to get started.

A normalJavascript variable like

var age = 18 holds a single piece

of data. An array is a way to hold

more than one piece of data

under the same name. There’s a

few ways to set up an array but

the simplest looks like this:

var my_array = [10, “ten”, true];

This sets up an array to hold

three values all under the single

variable name we called

my_array. Notice that you can

store numbers, strings of text,

and Boolean values in an array.

Each value needs to be separated

by a comma. The values need to

go between two square brackets

and after an equal sign.

To get at the values in your array

you use something called the

index value. This is a number

between square brackets. The

number corresponds to a value’s

position in the array (position

numbers start at 0):

my_array[0]

my_array[1]

my_array[2]

So my_array[0] will hold the value

10, my_array[1] will hold the text

"ten", and my_array[2] will hold

the Boolean value true. You could

then write the values to a web

page:

document.write( my_array[0] +

"
");

document.write( my_array[1] +

"
");

document.write( my_array[2] +

"
");

Test it out with the following

script:

Run the script in a browser and

you should see the number 10

displayed. Now change the 0 in

between the square brackets of

my_array[0] to a 1. Save the

change and refresh. The text

"ten" will be written out. Change

the 1 into a 2 and you’ll see

"true" displayed.

Now change the 2 into a 3. This

time the word “undefined” will

display. The reason is because

you only set up your array to

hold 3 items. When you typed a 3

instead of a 2 you were trying to

access item number 4 (0 to 3,

remember). But you haven’t

defined a fourth item. Even

though you haven’t got a fourth

item, Javascript doesn’t throw

you up an error, it doesn’t refuse

to work. Instead, it adds a new

position to the end of your array.

You can even store something in

this new position. Change your

script to this (the new parts are

in bold):

var my_array = [10, “ten”, true];

document.write( my_array[3] +

"
");

my_array[3] =“new item”;

document.write( my_array[3] );

Javascript first prints out

"undefined" then “new item”.

Notice the way we’re assigning

this new value:

my_array[3] =“new item”;

On the left of the equal sign, we

have the name of the array then

a pair of square brackets. Inside

of the square brackets is the

array position number we want

to access. After the equal sign

you type the value you want to

assign. So it’s just like normal

variable assignment except you

add the square brackets and the

array position after the variable

name.

Another way to set up an array is

with the new keyword:

var my_array = new Array( );

This time after the equal sign we

have the word new then a space.

The word Array comes next

(uppercase A and never

lowercase). Immediately after the

word Array you need a pair of

round brackets. If you like you

can add how many positions the

array is going to hold:

var my_array = new Array(3);

This says “set up an array with 3

positions”. You don’t have to add

any array positions, though. It’s

up to you.

However, when you’re assigning

values to each position you do

need to add the index positions.

You do it like this:

var my_array = new Array( );

my_array[0] = 10;

my_array[1] = “ten”;

my_array[2] = true;

So the first line sets up an array

called my_array. The next three

lines assign values to each

position in the new array. The

method above is the one we’ll

use to set up an array from now

on.

Arrays and Loops

Arrays are quite often used with

loops. That’s because the index

position of arrays are numbers

that you swap for the loop

variable. As an example, study the

following code (of course, you

can try it out as well):

var counter = 0;

var lottery_numbers = new

Array();

while ( counter < 49 ) {

lottery_numbers[counter] =

counter + 1;

document.write

( lottery_numbers[counter] +

"
" );

counter++;

}

What the script does is to assign

values to an array called

lottery_numbers. We’ve used a

while loop. The loop goes round

and round executing this line:

lottery_numbers[counter] =

counter + 1;

Instead of saying

lottery_numbers[0],

lottery_numbers[1],

lottery_numbers[2], etc, we’ve

replaced the index number with

the variable name counter. We

can do this because counter

changes each time round the

loop. Javascript knows what’s in

the counter variable and uses

this as the index. After the equal

sign, we’re assigning whatever is

in the counter variable (plus 1) to

each position in the array. (Can

you see why we’d need to add

1?)

There’s a lot more you can do

with arrays, and you’ll meet them

again in a later section. For now,

let’s move on. Next up is

functions.