JavaScript (js) is a light-weight object-oriented programming language which is used by several websites for scripting the webpages. It was introduced in the year 1995 for adding programs to the webpages in the Netscape Navigator browser. In addition to web browsers, databases such as CouchDB and MongoDB uses JavaScript as their scripting and query language.

>Features of JavaScript

There are the following features of JavaScript:

1. All popular web browsers support JavaScript as they provide built-in execution environments.
2. JavaScript follows the syntax and structure of the C programming language. Thus, it is a structured programming language.
3. JavaScript is a weakly typed language, where certain types are implicitly cast (depending on the operation).
4. JavaScript is an object-oriented programming language that uses prototypes rather than using classes for inheritance.
5. It is a light-weighted and interpreted language.
6. It is a case-sensitive language.
7. JavaScript is supportable in several operating systems including, Windows, macOS, etc.
8. It provides good control to the users over the web browsers.

Disadvantages of JavaScript:

1. This may be difficult to develop large applications, although you’ll also use the TypeScript overlay.
2. The main problem or disadvantage in JavaScript is that the code is always visible to everyone anyone can view JavaScript code.
3. If the error occurs in JavaScript, it can stop rendering the whole website. Browsers are extremely tolerant of JavaScript errors.
4. This continuous conversion takes longer than the conversion of a number to an integer. This increases the time needed to run the script and reduces its speed.

JavaScript provides different data types to hold different types of values. There are two types of data types in JavaScript.

1. Primitive data type
2. Non-primitive (reference) data type

There are five types of primitive data types in JavaScript. They are 

• Number : represents numeric values e.g. 100, 500, 3and 50 etc.
• String : represents a sequence of characters e.g. "ekgurukul.com", "knowledgeuniverseonline.com", "A Universal Portal for Education".
• Boolean: represents boolean value either false or true (0 or 1).
• Null: represents null i.e. no value at all.
• Undefined: it represents an undefined value.

Non-primitive data types in JavaScript. They are 

• Object: represents an instance through which we can access members.
• Array: represents a group of similar values.
• RegExp: It represents regular expressions.

Global variables are available throughout the length of the code so that it has no scope. The var keyword is used to declare a local variable or object. If the var keyword is omitted, a global variable is declared.

Example:

// Declare a global:

myGlobalVarIs= "This is a Global Varaible ”;

Timers are used to execute a piece of code at a set time or repeat the code in a given interval. This is done by using the functions setTimeout, setInterval, and clearInterval.

The setTimeout(function, delay) function is used to start a timer that calls a particular function after the mentioned delay. The setInterval(function, delay) function repeatedly executes the given function in the mentioned delay and only halts when canceled. The clearInterval(id) function instructs the timer to stop.

Timers are operated within a single thread, and thus events might queue up, waiting to be executed.

==” checks only for equality in value, whereas “===” is a stricter equality test and returns false if either the value or the type of the two variables are different.

The NULL value is used to represent no value or no object. It implies no object or null string, no valid boolean value, no number, and no array object.

Undeclared variables are those that do not exist in a program and are not declared. If the program tries to read the value of an undeclared variable, then a runtime error is encountered.

Undefined variables are those that are declared in the program but have not been given any value. If the program tries to read the value of an undefined variable, an undefined value is returned.

Cookies are the small test files stored in a computer, and they get created when the user visits the websites to store information that they need. 

A cookie is a piece of data that is stored on your computer to be accessed by your browser. If you saved your user ID and password so that you do not have to type it each and every time you try to login? If yes, then you are using cookies.

Cookies are saved as key/value pairs.

Javascript Set Cookie : You can create cookies using document. cookie property like this. For example

document.cookie = "cookiename=cookievalue; expires= Thu, 21 Aug 2014 20:00:00 UTC"