What is JavaScript?

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:

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.

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.

In contrast, JavaScript descends in spirit from a line of smaller, dynamically typed languages such as HyperTalk and dBASE. These scripting languages offer programming tools to a much wider audience because of their easier syntax, specialized built-in functionality, and minimal requirements for object creation.

JavaScript compared to Java

JavaScript
Java
Object-oriented. No distinction between types of objects. Inheritance is through the prototype mechanism, and properties and methods can be added to any object dynamically.
Class-based. Objects are divided into classes and instances with all inheritance through the class hierarchy. Classes and instances cannot have properties or methods added dynamically.
Variable data types are not declared (dynamic typing, loosely typed).
Variable data types must be declared (static typing, strongly typed).
Cannot automatically write to hard disk.
Can automatically write to hard disk.

For more information on the differences between JavaScript and Java, see the chapter Details of the object model.

Hello world

To get started with writing JavaScript, open the Web Console in multi-line mode, and write your first "Hello world" JavaScript code: 


(function(){
    "use strict";
    /* Start of your code */
    function greetMe(yourName) {
        alert('Hello ' + yourName);
    }
    
    greetMe('World');
    /* End of your code */
})();

Press Cmd+Enter or Ctrl+Enter (or click the Run button) 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) as well as "a" through "z" (lowercase).

You can use most of ISO 8859-1 or Unicode letters such as å and ü in identifiers. (For more details, see this blog post.) You can also use the Unicode escape sequences as characters in identifiers.

Some examples of legal names are Number_hits, temp99, $credit, and _name.

Declaring variables

You can declare a variable in two ways:

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 global context (or the function context if the code is part of a function). The scope of x is not limited to 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

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 any 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'];

Data types

The latest ECMAScript standard defines eight data types:

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 script 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 like this:


if (condition) {
    statement_1;
} else {
    statement_2;
}

Here, the 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 can 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 ({ … }).

Best practice

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's unwise to use simple assignments in a conditional expression, because the assignment can be confused with equality when glancing over the code.

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 parameter, called number. The function consists of one statement that says to return the parameter of the function (that is, number) multiplied by itself. The statement return 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.

If you pass an object (i.e. a non-primitive value, such as Array or a user-defined object) as a parameter and the function changes the object's properties, that change is visible outside the function, as shown in the following example: 


function myFunc(theObject) {
    theObject.make = 'Toyota';
}

var mycar = {make: 'Honda', model: 'Accord', year: 1998};
var x, y;

x = mycar.make; // x gets the value "Honda"

myFunc(mycar);
y = mycar.make; // y gets the value "Toyota"
                // (the make property was changed by the function)

Reference

All information on this page was taken from MDN web docs