JavaScript Conditional Statements

Conditional statements in JavaScript are used to perform different actions based on different conditions.

Conditional Statements

Very often when you write code, you want to perform different actions for different decisions. You can use conditional statements in your code to do this.

In JavaScript we have three conditional statements:

• if statement - use this statement if you want to execute a set of code when a condition is true
• if...else statement - use this statement if you want to select one of two sets of lines to execute
• switch statement - use this statement if you want to select one of many sets of lines to execute

If and If...else Statement

You should use the if statement if you want to execute some code if a condition is true.

Syntax

if (condition)
{
code to be executed if condition is true
}

Example

//If the time on your browser is less than 10,
//you will get a "Good morning" greeting.

<script type="text/javascript">
var d=new Date()
var time=d.getHours()

if (time<10) 
{
document.write("<b>Good morning</b>")
}
</script>

Notice that there is no ..else.. in this syntax. You just tell the code to execute some code if the condition is true.

If you want to execute some code if a condition is true and another code if a condition is false, use the if....else statement.

Syntax

if (condition)
{
code to be executed if condition is true
}
else
{
code to be executed if condition is false
}

Example

//If the time on your browser is less than 10,
//you will get a "Good morning" greeting.
//Otherwise you will get a "Good day" greeting.

<script type="text/javascript">
var d = new Date()
var time = d.getHours()

if (time < 10) 
{
document.write("Good morning!")
}
else
{
document.write("Good day!")
}
</script>

Switch Statement

You should use the Switch statement if you want to select one of many blocks of code to be executed.

Syntax

switch (expression)
{
case label1:
  code to be executed if expression = label1
  break    
case label2:
  code to be executed if expression = label2
  break
default:
  code to be executed
  if expression is different 
  from both label1 and label2
}

This is how it works: First we have a single expression (most often a variable), that is evaluated once. The value of the expression is then compared with the values for each case in the structure. If there is a match, the block of code associated with that case is executed. Use break to prevent the code from running into the next case automatically.

Example

//You will receive a different greeting based
//on what day it is. Note that Sunday=0,
//Monday=1, Tuesday=2, etc.

<script type="text/javascript">
var d=new Date()
theDay=d.getDay()
switch (theDay)
{
case 5:
  document.write("Finally Friday")
  break
case 6:
  document.write("Super Saturday")
  break
case 0:
  document.write("Sleepy Sunday")
  break
default:
  document.write("I'm looking forward to this weekend!")
}
</script>

Conditional Operator

JavaScript also contains a conditional operator that assigns a value to a variable based on some condition.

Syntax

variablename=(condition)?value1:value2 

Example

greeting=(visitor=="PRES")?"Dear President ":"Dear "

If the variable visitor is equal to PRES, then put the string "Dear President " in the variable named greeting. If the variable visitor is not equal to PRES, then put the string "Dear " into the variable named greeting.

JavaScript Looping

 Looping statements in JavaScript are used to execute the same block of code a specified number of times.

Looping

Very often when you write code, you want the same block of code to run a number of times. You can use looping statements in your code to do this.

In JavaScript we have the following looping statements:

• while - loops through a block of code while a condition is true
• do...while - loops through a block of code once, and then repeats the loop while a condition is true
• for - run statements a specified number of times

while

The while statement will execute a block of code while a condition is true.. 

while (condition)
{
    code to be executed
}

do...while

The do...while statement will execute a block of code once, and then it will repeat the loop while a condition is true

do
{
    code to be executed
}
while (condition)

for

The for statement will execute a block of code a specified number of times

for (initialization; condition; increment)
{
    code to be executed
}

JavaScript String Object

 The String object is used to work with text.

The Most Common Methods

An Array object is used to store a set of values in a single variable name.

The Array object

An Array object is used to store a set of values in a single variable name. Each value is an element of the array and has an associated index number. 

You can refer to a particular element in the array by using the name of the array and the index number. The index number starts at zero. 

You create an instance of the Array object with the "new" keyword.

var family_names=new Array(5)

The expected number of elements goes inside the parentheses, in this case 5.

You assign data to each of the elements in the array like this:

family_names[0]="Tove"
family_names[1]="Jani"
family_names[2]="Ståle"
family_names[3]="Hege"
family_names[4]="Kai"

And the data can be retrieved from any element by using the index of the particular array element you want. Like this:

mother=family_names[0]
father=family_names[1] 

The Most Common Methods