JavaScript is a cross-platform, object-oriented scripting language used to make webpages interactive (e.g. having complex animations, clickable buttons, popup menus, etc.). There are also more advanced server side versions of JavaScript such as Node.Js which allow you to add more functionality to a website than simply downloading files (such as realtime collaboration between multiple computers). 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.
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.
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.
Java is a class-based programming language designed for fast execution and type safety. Type safety means, for instance, that you can't cast a Java integer into an object reference or access private memory by corrupting Java bytecodes. Java's class-based model means that programs consist exclusively of classes and their methods. Java's class inheritance and strong typing generally require tightly coupled object hierarchies. These requirements make Java programming more complex than JavaScript programming.
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 declare a variable in two ways:
- With the keyword
var. For example,var x = 42. This syntax can be used to declare both local and global variables, depending on the execution context. - With the keyword
constorlet. 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 global context (or the function if the following codes are part of the function), not the immediate if statement block.
if (true) {
var x = 5;
}
console.log(x); // x is 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 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.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 must 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 TOO
function f() {
const g = 5;
var g;
//statements
}
However, the properties of objects assigned to constants are not protected, so the following statement is executed without problems.
const MY_OBJECT = {'key': 'value'};
MY_OBJECT.key = 'otherValue';
Also, the contents of an array are not protected, so the following statement is executed without problems.
const MY_ARRAY = ['HTML','CSS'];
MY_ARRAY.push('JAVASCRIPT');
console.log(MY_ARRAY); //logs ['HTML','CSS','JAVASCRIPT'];
The latest ECMAScript standard defines eight data types:
- Seven data types that are primitives:
- Boolean.
trueandfalse. - null. A special keyword denoting a null value. Because JavaScript is case-sensitive,
nullis not the same asNull,NULL, or any other variant. - undefined. A top-level property whose value is not defined.
- Number. An integer or floating point number. For example:
42or3.14159. - BigInt. An integer with arbitrary precision. For example:
9007199254740992n. - String. A sequence of characters that represent a text value. For example: "Howdy"
- Symbol (new in ECMAScript 2015). A data type whose instances are unique and immutable.
- and Object
Although these data types are relatively few, 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.
The most basic statement is a block statement that is used to group statements. The block is delimited by a pair of curly brackets:
{
statement_1;
statement_2;
.
.
.
statement_n;
}
All the documentations on this page were taken from MDN. You can learn more about javascript here.