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This lecture introduce JavaScript Statements. In JavaScript we have the following conditional statements, that is If statement, if...else statement, if...else if...else statement, and switch statement. In this lecture, we will learn this four statements.
Nội dung trích xuất từ tài liệu:
Web engineering: Lecture 17, 18 - Majid Mumtaz
Web Engineering
Lecture 17-18
MAJID MUMTAZ
Department of Computer Science, CIIT Wah
1
JavaScript Statements
• In JavaScript we have the following conditional
statements:
– if statement - use this statement to execute some
code only if a specified condition is true
– if...else statement - use this statement to execute
some code if the condition is true and another code if
the condition is false
– if...else if....else statement - use this statement to
select one of many blocks of code to be executed
– switch statement - use this statement to select one of
many blocks of code to be executed
2
JavaScript Statements
• If Statement
if (condition)
{
code to be executed if condition is true
}
• Example
if (time JavaScript Statements
• If...else Statement: Use the if....else statement to execute some code if a
condition is true and another code if the condition is not true.
– if (condition)
{
code to be executed if condition is true
}
else
{
code to be executed if condition is not true
}
Example:
if (time JavaScript Statements
• If...else if...else Statement: Use the if....else if...else
statement to select one of several blocks of code to be
executed.
– if (condition1)
{
code to be executed if condition1 is true
}
else if (condition2)
{
code to be executed if condition2 is true
}
else
{
code to be executed if neither condition1 nor condition2 is true
}
5
JavaScript Statements
• The JavaScript Switch Statement :Use the switch statement to select one
of many blocks of code to be executed.
Syntax:
switch(n)
{
case 1:
execute code block 1
break;
case 2:
execute code block 2
break;
default:
code to be executed if n is different from case 1 and 2
}
• Home work:
– Display today's weekday-name. Note that Sunday=0, Monday=1, Tuesday=2,
etc:
6
JavaScript Loops
• Loops can execute a block of code a number of
times.
• Different Kinds of Loops: JavaScript supports
different kinds of loops:
– for - loops through a block of code a number of times
– for/in - loops through the properties of an object
– while - loops through a block of code while a specified
condition is true
– do/while - also loops through a block of code while a
specified condition is true
7
The For Loop
• The for loop has the following syntax:
for (initialization; condition; Increment/decrement)
{
the code block to be executed
}
8
The For/In Loop
• The JavaScript for/in statement loops through the properties of an
object:
Syntax
for(var property_name in object){
statements;
}
Example:
var txt='';
var person={Fname:“Ali“, Lname:“khan', age:25};
for (var x in person)
{
txt=txt + person[x];
}
Document.write(” ” + txt); // Alikhan25
9
JavaScript While Loop
• Loops can execute a block of code as long as a specified
condition is true.
• Syntax
while (condition)
{
code block to be executed
}
Example:
var i =0;
while (i The Do/While Loop
• The do/while loop is a variant of the while loop. This loop will
execute the code block once, before checking if the condition is
true, then it will repeat the loop as long as the condition is true.
• Syntax
do
{
code block to be executed
}
while (condition);
Example
do
{
document.write ('The number is ' + i + '“);
i++;
}
while (i Home work
• Create a for loop that displays numbers as: 10
9 8 7 6 5 4 3 2 1. Put the numbers in HTML
table cells.
• Repeat same task using while loop
• JavaScript dialog Boxes
– Alert box
– Confirm box
– Prompt box
12
JavaScript Break and Continue
• The break statement 'jumps out' of a loop.
• The continue statement 'jumps over' one iteration in the
loop. E.g.
Examples:
1. for (i=0;i JavaScript Errors
• The try statement lets you test a block of code
for errors.
• The catch statement lets you handle the error.
• The throw statement lets you create custom
errors.
14
JavaScript Errors
• JavaScript try and catch:
var txt='';
– Syntax
function message()
{
try
try {
adddlert('Welcome guest!');
{ }
catch(err)
{
//Run some code here txt='There was an error on this page.\n\n';
txt+='Error description: ' + err.message +
} '\n\n';
txt+='Click OK t ...
