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Practical Learning: Using Folders and Files
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From now on, you will be asked to create folders or files, to open or modify files. I assume that you know how to perform any of these operations. |
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The Script Tag
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One of the most usual ways you will use JavaScript is to display text, as if
you were using HTML. Indeed, added just a few instructions, you can use
any of the HTML tags and make them part of your script. Therefore,
everything you have learned in HTML is valid here.
To set the instructions of a script, the section that has the script must start with the <Script>
tag and end with the </Script> tag, as follows:
<Script> </Script>
Because a script is written in a particular
language
that is not HTML, and because there are various scripting
languages, to use a script, you should let the browser know what
(particular)
scripting language you are using. To let the browser know, type
the word Language, followed by the = sign, followed by the name of the script language
included in double-quotes. For example, to use JavaScript in a page, start
the section with
<Script language="JavaScript">
and end it
with the closing tag. Therefore the scripting section can be delimited
with:
<Script Language="JavaScript> </Script>
Unlike HTML, JavaScript is case sensitive. This means
that Word, WORD, and word inside of your script are not the same. To set your HTML tags apart
from the rest of the text, sometimes you can write them in
uppercase. Therefore, some of the scripts here will be included as
follows:
<SCRIPT LANGUAGE="JavaScript"> </SCRIPT>
The section between the opening <script> tag and
the closing </script> tag is called the body of the script.
Everything that is part of the body of a script belongs to that script.
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Introduction to Objects
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A JavaScript file contains code that allows a user to
interact with a browser or a web-based application. In some cases, the
user may be presented with Windows controls to fill out or retrieve values
from them. As an application developer, there are two types of values you
will deal with. Primitive types are those created from simple types. We
will study them when we get to variables. The other types of values are
referred to as class types. We will explore them more when we get to
object-oriented programming.
JavaScript uses the concept of class to
identify or manage the many of the values used in an application. Consider an
object like a house. It has such characteristics as its type (single
family, town house, condominium, etc),
the number of bedrooms, the number of bathrooms, etc. These
characteristics are used to describe a house to somebody who wants to buy
it. To get such an object, you must first define the criteria that
describe it. Here is an example:
House
[
Address
Type of House
Number of Bedrooms
Number of Bathrooms
Has Indoor Garage
The Living Room is Covered With Carpet
The Kitchen Has an Island Stove
]
This information is used to describe a
house. Based on this, the House is called a class. To actually describe a
real house, you must provide information for each of the above
characteristics. Here is an example:
House: Langston
[
Address: 6802 Leighton Ave
Type of House: Single Family
Number of Bedrooms: 4
Number of Bathrooms: 3
Has Indoor Garage: Yes
The Living Room is Covered With Carpet: Yes
The Kitchen Has an Island Stove: No
]
In this case, Langston is not a class anymore, it
is a real house and is used to explicitly describe a House. Therefore, Langston is
called an object. Based on this, a class is a technique used to provide
the criteria to define an object. An object is the result of a description
based on a class.
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Properties
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In our example of a house, we used words to describe
it. Examples are: Address, Type of House, Number of Bedrooms, Number of
Bathrooms. In computer programming, the characteristics used to describe
an object are referred to as its properties.
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Methods
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While most objects only provide characteristics to
describe them, other objects can perform actions. For example, a house can
be used to protect people when it is raining outside. In computer
programming, an action that an object can perform is referred to as
method.
Earlier, we defined a House class with its properties. Unlike
a property, a method must display parentheses on this right side to
differentiate it from a property. An example would be:
House
[
Address
TypeOfHouse
NumberOfBedrooms
NumberOfBathrooms
HasIndoorGarage
LivingRoomCoveredWithCarpet
KitchenHasIslandStove
ProtectFromOutside()
]
When an object has a method, to access that method,
type the name of the object, followed by a period, followed by the name of
the method, and followed by parentheses. For example, if you have a House
object named Langston and you want to ask it to protect its inside from
outside rain, you would type:
Langston.ProtectFromOutside()
This is also referred to as calling a method.
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Methods and their Arguments
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When asked to perform an action, a method may need one
or more values to work with. If a method needs a value, such a value is
called an argument. While a certain method may need one argument, another
method would need more than one. The number of arguments of a method
depends on its goal. The arguments of a method are provided in
parentheses.
Suppose you have a House object and you want it to
protect what is inside. There may be different reasons why the inside needs
to be protected: may be from the rain, may be from the windy dust, may be
at night time from too much light that prevents from sleeping, etc. Based on
this, you may have to provide additional information to indicate why or
how the inside should be protected. For this reason, when such a method is
called, this additional information must be provided, in the parentheses
of the method. Here is an example:
House
[
Address
TypeOfHouse
NumberOfBedrooms
NumberOfBathrooms
HasIndoorGarage
LivingRoomCoveredWithCarpet
KitchenHasIslandStove
ProtectFromOutside(Reason)
]
On this site, we will use the word syntax to indicate
the name of the object, its method, and its argument. This will allow use
to get acquainted with what is required to use an object and especially
its method. For example, to represent a method that takes one argument,
such as the above, can be said to use a syntax as the following:
House.ProtectFromOutside(Reason)
This syntax indicates that the ProtectFromOutside()
method of the House object takes one argument. In future lessons, we will
get more information about these issues.
As mentioned above, a method can be created to take
more than one argument. In this case, the arguments are separated with
commas. Here is an example:
House
[
Address
TypeOfHouse
NumberOfBedrooms
NumberOfBathrooms
HasIndoorGarage
LivingRoomCoveredWithCarpet
KitchenHasIslandStove
ProtectFromOutside(Reason, WhenToProtect)
]
The arguments are used to assist the object with
performing the intended action.
Once a method has been created, it can be
used. Using a method is referred to as calling it. If a method
takes one argument, when calling it, you must provide a value for the
argument, otherwise the method would not work.
To call a method that takes an argument, type the name of the method followed by the opening parenthesis “(“, followed by the value that will be the argument, followed by a closing parenthesis “)â€. The argument you pass can be a regular constant value or it can be the name of another object. If the method is taking more than one argument, to call it, type the values for the arguments, in the exact order indicated, separated from each other by a comma. |
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The Document Class
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Introduction
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JavaScript uses an object called document. This object manages many
of the instructions that JavaScript can handle for HTML. One of the
methods of that object is to display something on the screen. A string is
a piece of text that the browser is asked to use "as is".
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The write Method
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The method used to display a string is called write. The syntax of the write
method of the
Document object is:
document.write(String)
The String to display can be provided in
double-quote or as a variable as we will learn soon. Here is an example of
displaying somebody's name using the document.write function:
To display various lines of text, you can use as many document.write
lines as you need.
A file can have many different script sections and you can include as many lines of code as necessary in a file. To
have different script sections, start each with the <Script> tag and
close it with the </Script> closing tag. Here is an example:
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Practical Learning: Introduction to JavaScript
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- Change the contents of the file as follow:
<Script Language="JavaScript"> document.write("Leaving Sydney") document.write("When we decided to leave, we knew we were ") document.write("making a hard decision. We had spent so much ") document.write("time this had become our new home. A few weeks ") document.write("or months before, we wanted to make Sydney ") document.write("our newly found settlement, a permanent place ") document.write("we could proudly call ours. It appeared that, ") document.write("unpredictably, fate had decided otherwise.") </Script> - Save the file and preview it in the browser:
- Notice that the whole page displays as one paragraph
- After previewing the page, return to your text editor and close it
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