JavaScript is a cross-platform, object-oriented scripting language.
It is a small and lightweight language.
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.
What you should already know
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 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 incode
idual 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.
To get started with writing JavaScript, open the Scratchpad and write your first "Hello world" JavaScript code:
function greetMe(yourName) { alert("Hello " + yourName); }
greetMe("World");
Select the code in the pad and hit Ctrl+R to watch it unfold in your browser!
You use variables as symbolic names for values in your application. The names of variables,
called identifiers, conform to certain rules.
A JavaScript identifier must start with a letter, underscore (_), or dollar sign ($);
subsequent characters can also be digits (0-9). Because JavaScript is case sensitive,
letters include the characters "A" through "Z"
(uppercase) and the characters "a" through "z" (lowercase).
You can use ISO 8859-1 or Unicode letters such as å and ü in identifiers.
You can also use the Unicode escape sequences
as characters in identifiers.
Some examples of legal names are Number_hits, temp99, and _name.
You can declare a variable in three ways:
With the keyword var. For example,
var x = 42.
This syntax can be used to declare both local and global variables.
By simply assigning it a value. For example,
x = 42.
This always declares a global variable. It generates a strict JavaScript warning. You shouldn't use this variant.
With the keyword let. For example,
let y = 13.
This syntax can be used to declare a block scope local variable. See Variable scope below.
A function definition (also called a function declaration, or function statement)
consists of the function keyword, followed by:
- The name of the function.
- A list of arguments to the function, enclosed in parentheses and separated by commas.
- The JavaScript statements that define the function, enclosed in curly brackets, { }.
For example, the following code defines a simple function named square:
function square(number) { return number * number; }
The function square takes one argument, called number. The function
consists of one statement that says to return the
argument of the function (that is, number) multiplied by itself.
The return statement specifies the value returned by the function.
return number * number;
Primitive parameters (such as a number) are passed to functions
by value; the value is passed to the function,
but if the function changes the value of the parameter,
this change is not reflected globally or in the calling function.
A while statement executes its statements as long as a specified
condition evaluates to true. A while statement looks as follows:
while (condition) statement
If the condition becomes false, statement within the loop stops executing
and control passes to the statement following the loop.
The condition test occurs before statement in the loop is executed. If the condition returns
true, statement is executed and the condition is tested again. If the condition returns false,
execution stops and control is passed to the statement following while.
To execute multiple statements, use a block statement ({ ... }) to group those statements.
Example:
The following while loop iterates as long as n is less than three:
var n = 0; var x = 0; while (n < 3) { n++; x += n; }
With each iteration, the loop increments n and adds that value to x. Therefore, x and n take on the following values:
- After the first pass: n = 1 and x = 1
- After the second pass: n = 2 and x = 3
- After the third pass: n = 3 and x = 6
After completing the third pass, the condition n < 3 is no longer true, so the loop terminates.
You can create a read-only, named constant with the const keyword.
The syntax of a constant identifier is the
same as for a variable identifier: it must start with a letter, underscore or
dollar sign and can contain alphabetic, numeric, or underscore characters.
const PI = 3.14;
A constant cannot change value through assignment or be re-declared while
the script is running. It has to be initialized to a value.
The scope rules for constants are the same as those for let
block scope variables. If the const keyword is omitted,
the identifier is assumed to represent a variable.
You cannot declare a constant with the same name as a function or variable in the same scope. For example:
// THIS WILL CAUSE AN ERROR function f() {}; const f = 5; // THIS WILL
CAUSE AN ERROR ALSO function f() { const g = 5; var g; //statements}
However, object attributes are not protected, so the following statement is executed without problems.
const MY_OBJECT = {"key": "value"}; MY_OBJECT.key = "otherValue";