JavaScript is a programming language.
JavaScript programs run inside the browser, tell the browser what to do

Internal (embedded) JavaScript

One or more lines of JavaScript inside a script element

Internal JavaScript runs when the web browser (which reads the source code from top to bottom) gets to the script tag

Use your browser’s “View Source” command to see (and copy) the JavaScript source code

Source code looks something like this:

JavaScript Internal Hello World

Inline JavaScript

One or more lines of JavaScript inside a tag
This example demonstrates the alert statement

Hello World

Source code looks something like this:

JavaScript Inline Hello World

External JavaScript files

An external JavaScript file is a text file that has the .js file extension, containing JavaScript code

External files are useful because multiple pages can share a single JavaScript file

Sometimes known as global code, shared code, etc.

Display demo_version

Display greeting

The above examples demonstrate the concept of variables

External style sheet located here:

Source code (on this page, to attach the external JavaScript file) looks something like this:

JavaScript External file

JavaScript functions

To keep the browser from executing a script when the page loads, you can put your script into a function

A function contains code that will be executed by an event or by a call to the function

Functions look like this:

function functionName (arguments) {

// JavaScript statements go here


You may call a function from anywhere within a page, or from an external JavaScript file

Functions can be defined both in the and in the section of a document. However, to assure that a function is read/loaded by the browser before it is called, it could be wise to put functions in the section.

Function can use arguments: one or more values that the function needs to complete its tasks, which might change depending on which element is calling the function and for what purpose

— Arguments are the part of the function that go inside the parenthesis ()
— Arguments are optional
— If a function does not use arguments, use the parentheses with nothing inside

Functions are useful for writing re-usable code

See JavaScript Functions at