JS
Documentation
Introduction
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:
What you should already know This guide assumes you have the following basic background:
JavaScript and Java
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.
Hello World
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!
Variables
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.
Declaring variables
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.
Variable scope
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
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.
Constants
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";
Data types The latest ECMAScript standard defines seven data types:
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.
if...else statement
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 */ }
while statement
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 completing the third pass, the condition n < 3 is no longer true, so the loop terminates.
Function declarations
A function definition (also called a function declaration, or function statement) consists of
the function keyword, followed by:
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.