Introduction

Why Study JavaScript?

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

JavaScript Can Change HTML Content

One of many JavaScript HTML methods is getElementById().

The example below "finds" an HTML element (with id="demo"), and changes the element content (innerHTML) to "Hello JavaScript":

Example

document.getElementById("demo").innerHTML = "Hello JavaScript";

JavaScript Can Change HTML Attribute Values

In this example JavaScript changes the value of the src (source) attribute of an <img> tag:

JavaScript Can Change HTML Styles (CSS)

Changing the style of an HTML element, is a variant of changing an HTML attribute:

Example

document.getElementById("demo").style.fontSize = "35px";

JavaScript Can Hide HTML Elements

Hiding HTML elements can be done by changing the display style:

Example

document.getElementById("demo").style.display = "none";

JavaScript Can Show HTML Elements

Showing hidden HTML elements can also be done by changing the display style:

Example

document.getElementById("demo").style.display = "block";

The <script> Tag

In HTML, JavaScript code is inserted between <script> and </script> tags.

Example

<script>

document.getElementById("demo").innerHTML = "My First JavaScript";

</script>

Old JavaScript examples may use a type attribute: <script type="text/javascript">.

The type attribute is not required. JavaScript is the default scripting language in HTML.

JavaScript Functions and Events

A JavaScript function is a block of JavaScript code, that can be executed when "called" for.

For example, a function can be called when an event occurs, like when the user clicks a button.

JavaScript Functions and Events

A JavaScript function is a block of JavaScript code, that can be executed when "called" for.

For example, a function can be called when an event occurs, like when the user clicks a button.

Example

<!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>

JavaScript in <body>

In this example, a JavaScript function is placed in the <body> section of an HTML page.

The function is invoked (called) when a button is clicked:

Example

<!DOCTYPE html>

<html>

<body>

<h1>A Web Page</h1>

<p id="demo">A Paragraph</p>

<button type="button" onclick="myFunction()">Try it</button>

<script>

function myFunction() {

document.getElementById("demo").innerHTML = "Paragraph changed.";

}

</script>

</body>

</html>

External JavaScript

Scripts can also be placed in external files:

External file: myScript.js

function myFunction() {

document.getElementById("demo").innerHTML = "Paragraph changed.";

}

External scripts are practical when the same code is used in many different web pages.

JavaScript files have the file extension .js.

To use an external script, put the name of the script file in the src (source) attribute of a <script> tag:

Example

<script src="myScript.js"></script>

You can place an external script reference in <head> or <body> as you like.

The script will behave as if it was located exactly where the <script> tag is located.

External scripts cannot contain <script> tags.

External JavaScript Advantages

Placing scripts in external files has some advantages:

  • It separates HTML and code

  • It makes HTML and JavaScript easier to read and maintain

  • Cached JavaScript files can speed up page loads

To add several script files to one page - use several script tags:

Example

<script src="myScript1.js"></script>

<script src="myScript2.js"></script>


External References

External scripts can be referenced with a full URL or with a path relative to the current web page.

This example uses a full URL to link to a script:

Example

<script src="https://www.w3schools.com/js/myScript1.js"></script>

This example uses a script located in a specified folder on the current web site:

Example

<script src="/js/myScript1.js"></script>

This example links to a script located in the same folder as the current page:

Example

<script src="myScript1.js"></script>