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JavaScript_HTML - tìm hiểu và trao đỏi các mã java để làm trang web bạn sinh động hơn, các thủ thuật code html css
Javascript If - Esle if Statement - điều kiện nếu - hoặc là
As your Javascript programs get more sophisticated you will need to make use of conditional statements that allow your program to make decisions. Nearly all other programming languages use conditionals and Javascript is no exception.
The If Statement is a way to make decisions based on a variable or some other type of data. For example you might have a variable that stores the date. With this tiny bit of information you could easily program a small script to print out "Today is my Birthday!" whenever the day and month were equal to your birthday.
This lesson will teach you the basics of using an If Statement in Javascript.
Javascript If Statement Syntax
There are two major parts to an if statement: the conditional statement and the code to be executed.
The conditional statement is a statement that will evaluate to either True or False. The most common type of a conditional statement used is checking to see if something equals a value. An example would be checking if the date equaled your birthday.
The code to be executed contains the Javascript code that will be executed only if the If Statement's conditional statement was true. In this simple If Statement example we print out a message if the variable we are checking is equal to 7.
Javascript Code:
| Code: : |
<script type="text/javascript">
<!--
var myNum = 7;
if(myNum == 7){
document.write("Lucky 7!");
}
//-->
</script> |
Display:
This simple example created myNum and set it to 7. We then checked to see if myNum was equal to 7 "myNum == 7" in the If Statement's conditional statement which evaluated to True.
Because the conditional statement was True the block of code associated with our If Statement "document.write..." was executed, as you can see in the
Javascript If Statement: Else
We already taught you how to execute code if a given condition is True, but what if you want to execute another piece of code if something is False? The answer is to use an extension to the If Statement; the Else clause.
The Else clause is executed when the conditional statement is False. Let's take our example from above, add an Else clause and change the value of myNum so that our conditional statement is False.
Javascript Code:
| Code: : |
<script type="text/javascript">
<!--
var myNum = 10;
if(myNum == 7){
document.write("Lucky 7!");
}else{
document.write("You're not very lucky today...");
}
//-->
</script> |
Display:
| Quote: : |
| You're not very lucky today... |
==============
Javascript Else If Statement
In the previous lesson you learned how to create a basic If Statement in Javascript, which is good enough for most programming situations. However, sometimes it is helpful to have the ability to check for more than one condition in a single If Statement block.
The Else If statement is an extension to the If Statement that allows you to create as many checks (conditional statements) as you want.
Javascript Else If Example
Imagine that you want to have a small "student" script that will print out a customized message depending who is accessing the web page. If you had more than two custom messages you could use the Else If extension to solve this programming problem.
Javascript Code:
<script type="text/javascript">
<!--
var visitor = "principal";
if(visitor == "teacher"){
document.write("My dog ate my homework...");
}else if(visitor == "principal"){
document.write("What stink bombs?");
} else {
document.write("How do you do?");
}
//-->
</script>
Display:
| Quote: : |
| What stink bombs? |
There are two important things to note about the Else If extension:
1. You must have a normal If Statement before you can use the Else If statement. This is because the Else If statement is an addon to the If Statement.
2. YOu can have multiple Else If add-ons. In our example we only used one Else If extension, but you can add as many as you require.
