This chapter introduces JavaScript and discusses some of its fundamental concepts.
This guide assumes you have the following basic background:
- A general understanding of the Internet and the World Wide Web (WWW).
- Good working knowledge of HyperText Markup Language (HTML).
- Some programming experience. If you are new to programming, try one of the tutorials linked on the main page about JavaScript.
JavaScript is a cross-platform, object-oriented scripting language used to make webpages interactive (e.g. having complex animations, clickable buttons, popup menus, etc.). There are also more advanced server side versions of JavaScript such as Node.Js which allow you to add more functionality to a website than simply downloading files (such as realtime collaboration between multiple computers). Inside a host environment (for example, a web browser), JavaScript can be connected to the objects of its environment to provide programmatic control over them.
JavaScript contains a standard library of objects, such as Array, Date, and Math, and a core set of language elements such as operators, control structures, and statements. Core JavaScript can be extended for a variety of purposes by supplementing it with additional objects; for example:
- Client-side JavaScript extends the core language by supplying objects to control a browser and its Document Object Model (DOM). For example, client-side extensions allow an application to place elements on an HTML form and respond to user events such as mouse clicks, form input, and page navigation.
- Server-side JavaScript extends the core language by supplying objects relevant to running JavaScript on a server. For example, server-side extensions allow an application to communicate with a database, provide continuity of information from one invocation to another of the application, or perform file manipulations on a server.
JavaScript and Java are similar in some ways but fundamentally different in some others. The JavaScript language resembles Java but does not have Java's static typing and strong type checking. JavaScript follows most Java expression syntax, naming conventions and basic control-flow constructs which was the reason why it was renamed from LiveScript to JavaScript.
In contrast to Java's compile-time system of classes built by declarations, JavaScript supports a runtime system based on a small number of data types representing numeric, Boolean, and string values. JavaScript has a prototype-based object model instead of the more common class-based object model. The prototype-based model provides dynamic inheritance; that is, what is inherited can vary for individual objects. JavaScript also supports functions without any special declarative requirements. Functions can be properties of objects, executing as loosely typed methods.
JavaScript is a very free-form language compared to Java. You do not have to declare all variables, classes, and methods. You do not have to be concerned with whether methods are public, private, or protected, and you do not have to implement interfaces. Variables, parameters, and function return types are not explicitly typed.
Getting started with JavaScript is easy: all you need is a modern Web browser. This guide includes some JavaScript features which are only currently available in the latest versions of Firefox, so using the most recent version of Firefox is recommended.
There are two tools built into Firefox that are useful for experimenting with JavaScript: the Web Console and Scratchpad.
The Web Console shows you information about the currently loaded Web page, and also includes a command line that you can use to execute JavaScript expressions in the current page.
To open the Web Console (Ctrl+Shift+I on Windows and Linux or Cmd-Option-K on Mac), select "Web Console" from the "Developer" menu, which is under the "Tools" menu in Firefox. It appears at the bottom of the browser window. Along the bottom of the console is a command line that you can use to enter JavaScript, and the output appears in the panel above:
The console works the exact same way as eval: the last expression entered is returned. For the sake of simplicity, it can be imagined that every time something is entered into the console, it is actual surrounded by console.log around eval like so.
function greetMe(yourName) {
alert('Hello ' + yourName);
}
console.log(eval('3 + 5'));
The Web Console is great for executing single lines of JavaScript, but although you can execute multiple lines, it's not very convenient for that, and you can't save your code samples using the Web Console. So for more complex examples Scratchpad is a better tool.
To open Scratchpad (Shift+F4), select "Scratchpad" from the "Developer" menu, which is under the menu in Firefox. It opens in a separate window and is an editor that you can use to write and execute JavaScript in the browser. You can also save scripts to disk and load them from disk.
To get started with writing JavaScript, open the Scratchpad and write your first "Hello world" JavaScript code:
(function(){
"use strict";
/* Start of your code */
function greetMe(yourName) {
alert('Hello ' + yourName);
}
greetMe('World');
/* End of your code */
})();
JavaScript borrows most of its syntax from Java, C and C++, but is also influenced by Awk, Perl and Python.
JavaScript is case-sensitive and uses the Unicode character set. For example, the word Früh (which means "early" in German) could be used as a variable name.
var Früh = "foobar";
But, the variable früh is not the same as Früh because JavaScript is case sensitive.
In JavaScript, instructions are called statements and are separated by semicolons (;).
A semicolon is not necessary after a statement if it is written on its own line. But if more than one statement on a line is desired, then they must be separated by semicolons. ECMAScript also has rules for automatic insertion of semicolons (ASI) to end statements. (For more information, see the detailed reference about JavaScript's lexical grammar.) It is considered best practice, however, to always write a semicolon after a statement, even when it is not strictly needed. This practice reduces the chances of bugs getting into the code.
The source text of JavaScript script gets scanned from left to right and is converted into a sequence of input elements which are tokens, control characters, line terminators, comments, or whitespace. Spaces, tabs, and newline characters are considered whitespace.
The syntax of comments is the same as in C++ and in many other languages:
// a one line comment
/* this is a longer,
* multi-line comment
*/
/* You can't, however, /* nest comments */ SyntaxError */
Comments behave like whitespace and are discarded during script execution.
There are three kinds of declarations in JavaScript.
var
Declares a variable, optionally initializing it to a value.
let
Declares a block-scoped, local variable, optionally initializing it to a value.
const
Declares a block-scoped, read-only named constant.