Javascript Statements
All the Javascript code that you will write will, for the most part, be comprised of
many separate statements. A statement is setting a variable equal to a value. A statement
is also a function call, i.e. document.write(). Statements define what the script will do
and how it will be done.
Typical Ending of a Statement
In typical programming languages like C and PHP, the end of a statement is marked with a semicolon(;), but
we have seen that the semicolon is optional in Javascript.
In Javascript the end of a statement is most often marked by starting a new line.
Categories of Statements
In addition to the standard statements like changing a variable's value, assigning a new value, or calling a function,
there are groups of statements that are distinct in their purpose. We will provide a brief overview of each of these
categories in this lesson and cover them in greater detail later in the tutorial.
- Conditional Statements
- Loop Statements
- Object Manipulation Statements
- Comment Statements
- Exception Handling Statements
Conditional Statements
If you were to win a $100 million lottery then you would probably quit your job. That last statement
is a conditional if/then statement that is used a great deal in programming. If some condition (winning the lottery) is true,
then something happens (you quit your job). If the condition is false, then you probably will not take that action (quit your job).
Conditional statements are used to control your scripts so that different actions can be taken depending on the situation.
You may want to display a special image on your home page during the holidays. This condition would depend
on what day it was, and if it was a holiday, then a special holiday image would be displayed to your visitors. Without
proper knowledge of the conditional statement your scripts will not be as lively or dynamic as
they could possibly be.
Loop Statements
Have you ever had to send out marriage announcements? If not, this is how it goes. You take the invitation, place
it in the envelope, lick the envelope, seal the envelope, then send it off. Then you take the next invitation off the stack of
99 remaining invitations, place it in an envelope, lick the envelope, seal... You get the idea! It is a boring, repetitive task!
Wouldn't it be great if there was an easier way? Well in programming and in Javascript there is. The process
is called "looping" and it will turn your cute little scripts into massive work horses with the right planning.
A loop statement checks to see if some condition is true, say our condition is "Are there any invitations left?" and
if that condition is true it will execute a chunk of code. Our code in this example would be to stuff, lick, and seal the envelope.
After the code is executed the condition is checked again, if it is true then the process begins again, if it is false, then
the loop code stops execution and the script continues along.
Believe me when I say this is something you want to learn more about!
Object Manipulation Statements
These are statements that are used to take advantage of the object model to get tasks done. If you do not
know about the object model at this time, do not worry. We will be talking about it later.
Comment Statements
Comment statements are used to prevent the browser from executing certain parts of code that you
designate as non-code. Why would
you want to do this? There are many reasons. By disallowing the block of text to be read you can then place
in comments for yourself, much like HTML comments, and you can also block out segments of your code for
whatever reason you may have.
The single line comment is just two slashes (//) and the multiple line comment starts with (/*) and
ends with (*/). We will talk about comments in greater depth in a later lesson.
Exception Handling Statements
Sometimes when you are programming you do not know for sure if the file that you will be writing to,
the input stream you are reading from, or the connection you have established will be usable for
the code that you want to execute. There is a way to program safety mechanisms, so that your code handles
common problems that may arise (maybe the input stream you are reading from suddenly disappears).
The try...catch statement tries to execute a piece of code and if it fails the catch should
handle the error gracefully. This is an advanced programming subject that is interesting, but not necessary
for the majority of Javascript programmers.
Wrapup
I hope you found this overview of Javascript statements interesting. Do not despair if you
have not grasped all the details discussed above, as we will be covering them further in a later lesson.
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