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 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.
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.
When you declare a variable outside of any function, it
is called a global variable, because it is available to
any other code in the current document. When you declare
a variable within a function, it is called a local
variable, because it is available only within that
function.
JavaScript before ECMAScript 2015 does not have block
statement scope; rather, a variable declared within a
block is local to the function (or global scope) that
the block resides within. For example the following code
will log 5, because the scope of x is the function (or
global context) within which x is declared, not the
block, which in this case is an if statement.
if (true) { var x = 5; } console.log(x); // 5
This behavior changes, when using the let declaration
introduced in ECMAScript 2015.
if (true) { let y = 5; } console.log(y); //
ReferenceError: y is not defined
Global variables are in fact properties of the global
object. In web pages the global object is window, so you
can set and access global variables using the
window.variable syntax.
Consequently, you can access global variables declared
in one window or frame from another window or frame by
specifying the window or frame name. For example, if a
variable called phoneNumber is declared in a document,
you can refer to this variable from an iframe as
parent.phoneNumber.
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";
The latest ECMAScript standard defines seven data types:
-
Six data types that are primitives:
- Boolean. true and false.
-
null. A special keyword denoting a null
value. Because JavaScript is case-sensitive,
null is not the same as Null, NULL, or any
other variant.
-
undefined. A top-level property whose value
is undefined.
- Number. 42 or 3.14159.
- String. "Howdy"
-
Symbol (new in ECMAScript 2015). A data type
whose instances are unique and immutable.
- and Object
Although these data types are a relatively small amount,
they enable you to perform useful functions with your
applications. Objects and functions are the other
fundamental elements in the language. You can think of
objects as named containers for values, and functions as
procedures that your application can perform.
Use the if statement to execute a statement if a logical
condition is true. Use the optional else clause to
execute a statement if the condition is false. An if
statement looks as follows:
if (condition) { statement_1; } else { statement_2; }
condition can be any expression that evaluates to true
or false. See Boolean for an explanation of what
evaluates to true and false. If condition evaluates to
true, statement_1 is executed; otherwise, statement_2 is
executed. statement_1 and statement_2 can be any
statement, including further nested if statements.
You may also compound the statements using else if to
have multiple conditions tested in sequence, as follows:
if (condition_1) { statement_1; } else if (condition_2)
{ statement_2; } else if (condition_n) { statement_n; }
else { statement_last; }
In the case of multiple conditions only the first
logical condition which evaluates to true will be
executed. To execute multiple statements, group them
within a block statement ({ ... }) . In general, it's
good practice to always use block statements, especially
when nesting if statements:
if (condition) { statement_1_runs_if_condition_is_true;
statement_2_runs_if_condition_is_true; } else {
statement_3_runs_if_condition_is_false;
statement_4_runs_if_condition_is_false; }
It is advisable to not use simple assignments in a
conditional expression, because the assignment can be
confused with equality when glancing over the code. For
example, do not use the following code:
if (x = y) { /* statements here */ }
If you need to use an assignment in a conditional
expression, a common practice is to put additional
parentheses around the assignment. For example:
if ((x = y)) { /* statements here */ }
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.
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.