Javascript

COMPUTER SCIENCE
TOPIC: JAVASCRIPT
BY: Priyanka pradhan
MJPRU
Priyanka Pradhan

JavaScript
O JavaScript is the programming language
of HTML and the Web.
Priyanka Pradhan

Why Study JavaScript?
O JavaScript is one of the 3 languages all web
developers must learn:
1. HTML to define the content of web pages
2. CSS to specify the layout of web pages
3. JavaScript to program the behavior of web
pages
Priyanka Pradhan

O JavaScript accepts both double and single
quotes
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The <script> Tag
O In HTML, JavaScript code must be
inserted
between <script> and </script> tags.
O Example:
<script>
document.getElementById("demo").innerHT
ML = "My First JavaScript";
</script>
Priyanka Pradhan

<!DOCTYPE html>
<html>
<body>
<h2>JavaScript in Body</h2>
<p id="demo"></p>
<script>
document.getElementById("demo").innerHTML = "My First JavaScript";
</script>
</body>
</html>
Priyanka Pradhan

JavaScript in Body
My First JavaScript
Old JavaScript examples may use a type
attribute: <script type="text/javascript">.
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JavaScript Functions and
Events
O A JavaScript function is a block of JavaScript
code, that can be executed when "called" for.
O For example, a function can be called when
an event occurs, like when the user clicks a
button.
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JavaScript in <head> or
<body>
O You can place any number of scripts in an
HTML document.
O Scripts can be placed in the <body>, or in
the <head> section of an HTML page, or
in both.
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JavaScript in <head>
O a JavaScript function is placed in
the <head> section of an HTML page.
O The function is invoked (called) when a
button is clicked
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<!DOCTYPE html>
<html><head>
<script>
function myFunction() {
document.getElementById("demo").innerHTML = "Paragraph changed.";
}
</script>
</head>
<body>
<h1>A Web Page</h1>
<p id="demo">A Paragraph</p>
<button type="button" onclick="myFunction()">Try it</button>
</body>
</html>
Priyanka Pradhan

JavaScript in Head
A Paragraph.
Try it
JavaScript in Head
Paragraph changed.
Priyanka Pradhan

JavaScript in <body>
O a JavaScript function is placed in
the <body> section of an HTML page.
O The function is invoked (called) when a
button is clicked
Priyanka Pradhan

O <!DOCTYPE html>
<html>
<body>
<h1>A Web Page</h1>
<p id="demo">A Paragraph</p>
<script>
document.getElementById("demo").innerHTML = "Paragraph changed.";
}
</script>
</body>
</html>
Priyanka Pradhan

External JavaScript
O Scripts can also be placed in external
files:
O External file: myScript.js
O function myFunction() {
document.getElementById("demo").inner
HTML = "Paragraph changed.";
}
Priyanka Pradhan

O External scripts are practical when the
same code is used in many different web
pages.
O JavaScript files have the file extension .js.
O To use an external script, put the name of
the script file in the src (source) attribute
of a <script> tag
Priyanka Pradhan

<!DOCTYPE html>
<html>
<body>
<h2>External JavaScript</h2>
<p id="demo">A Paragraph.</p>
<p>(myFunction is stored in an external file called "myScript.js")</p>
<script src="myScript.js"></script>
</body>
</html>
Priyanka Pradhan

O You can place an external script reference
in <head> or <body> as you like.
O The script will behave as if it was located
exactly where the <script> tag is located.
O External scripts cannot
contain <script> tags.
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O External JavaScript Advantages
O Placing scripts in external files has some
advantages:
O It separates HTML and code
O It makes HTML and JavaScript easier to read and
maintain
O Cached JavaScript files can speed up page loads
O To add several script files to one page - use
several script tags
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O <script src="myScript1.js"></script>
<script src="myScript2.js"></script>
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External References
O External scripts can be referenced with a full
URL or with a path relative to the current web
page.
O This example uses a full URL to link to a
script:
Example
<script src="https://www.w3schools.com/js/myS
cript1.js"></script>Priyanka Pradhan

O This example uses a script located in a
specified folder on the current web site:
Example
<script src="/js/myScript1.js"></script>
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O This example links to a script located in
the same folder as the current page:
Example
<script src="myScript1.js"></script>
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O JavaScript Display Possibilities
O JavaScript can "display" data in different
ways:
O Writing into an HTML element,
using innerHTML.
O Writing into the HTML output
using document.write().
O Writing into an alert box, using window.alert().
O Writing into the browser console,
using console.log().Priyanka Pradhan

Using innerHTML
O To access an HTML element, JavaScript
can use
the document.getElementById(id) method.
O The id attribute defines the HTML
element. The innerHTML property defines
the HTML content:
Priyanka Pradhan

O <!DOCTYPE html>
<html>
<body>
<h1>My First Web Page</h1>
<p>My First Paragraph</p>
<p id="demo"></p>
<script>
document.getElementById("demo").innerHTML = 5 + 6;
</script>
</body>
</html>
Priyanka Pradhan

My First Web Page
My First Paragraph.
11
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Using document.write()
O For testing purposes, it is convenient to
use document.write():
Priyanka Pradhan

O <!DOCTYPE html>
<html>
<body>
<p>My first paragraph.</p>
<script>
document.write(5 + 6);
</script>
</body>
</html>
Priyanka Pradhan

O The document.write() method should only
be used for testing.
My First Web Page 11
My first paragraph.
Try it
Priyanka Pradhan

Using window.alert()
O You can use an alert box to display data
Priyanka Pradhan

O <!DOCTYPE html>
<html>
<body>
<p>My first paragraph.</p>
<script>
window.alert(5 + 6);
</script>
</body>
</html>
Priyanka Pradhan

Using console.log()
O For debugging purposes, you can use
the console.log() method to display data.
Priyanka Pradhan

O <!DOCTYPE html>
<html>
<body>
<script>
console.log(5 + 6);
</script>
</body>
</html>
Priyanka Pradhan

JavaScript Statements
<!DOCTYPE html>
<html>
<body>
<h2>JavaScript Statements</h2>
<p>A <b>JavaScript program</b> is a list of <b>statements</b> to be executed by a computer.</p>
<p id="demo"></p>
<script>
var x, y, z; // Statement 1
x = 5; // Statement 2
y = 6; // Statement 3
z = x + y; // Statement 4
document.getElementById("demo").innerHTML =
"The value of z is " + z + ".";
</script>
</body>
</html>
Priyanka Pradhan

A JavaScript program is a list
of statements to be executed by a
computer.
The value of z is 11.
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O JavaScript Programs
O A computer program is a list of "instructions" to
be "executed" by a computer.
O In a programming language, these programming
instructions are called statements.
O A JavaScript program is a list of
programming statements.
O In HTML, JavaScript programs are executed by the
web browser.
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Semicolons ;
O Semicolons separate JavaScript statements.
O Add a semicolon at the end of each
executable statement:
O var a, b, c; // Declare 3 variables
a = 5; // Assign the value 5 to a
b = 6; // Assign the value 6 to b
c = a + b; // Assign the sum of a and b to c
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O When separated by semicolons, multiple
statements on one line are allowed:
O a = 5; b = 6; c = a + b;
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O JavaScript statements can be grouped
together in code blocks, inside curly
brackets {...}.
O The purpose of code blocks is to define
statements to be executed together.
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<!DOCTYPE html>
<html>
<body>
<h2>JavaScript Statements</h2>
<p>JavaScript code blocks are written between { and }</p>
<button type="button" onclick="myFunction()">Click Me!</button>
<p id="demo1"></p>
<p id="demo2"></p>
<script>
document.getElementById("demo1").innerHTML = "Hello Dolly!";
document.getElementById("demo2").innerHTML = "How are you?";
}
</script>
</body>
</html>Priyanka Pradhan

JavaScript Statements
JavaScript code blocks are written between {
and }
Click Me!
Hello Dolly!
How are you?
Priyanka Pradhan

JavaScript Keywords
Keyword Description
break Terminates a switch or a loop
continue Jumps out of a loop and starts at the top
debugger Stops the execution of JavaScript, and
calls (if available) the debugging function
do ... while Executes a block of statements, and
repeats the block, while a condition is true
for Marks a block of statements to be
executed, as long as a condition is true
function Declares a function
if ... else Marks a block of statements to be
executed, depending on a condition
return Exits a function
switch Marks a block of statements to be
executed, depending on different cases
try ... catch Implements error handling to a block of
statements
var Declares a variable
JavaScript keywords are reserved words.
Reserved words cannot be used as names for variables.
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Javascript

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