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.
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.
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.
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 */ }
All the documentation in this page is taken from
MDN