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.