VARIABLES AND PRIMITIVE TYPES

Variables and Primitive Data Types

Variables :

Variable means anything that can vary. JavaScript includes variables which hold the data value and it can be changed anytime.

JavaScript uses reserved keyword var to declare a variable.

Example :

var x; // declaring a variable as x.

If you console log variable x, it will give output as undefined because no value is assigned to a variable x.

var x;
console.log(x); // Prints: undefined

You can assign a value to a variable using equal to (=) operator when you declare it or before using it.

Example :

var x = 5; // assigning value to a variable
console.log(x); // Prints: 5

If a variable is neither declared nor defined then we try to reference such variable, the result will be not defined.

Example :

var x = 5;
console.log(age); // ReferenceError: age is not defined

Rules for naming variables :

Variable name can have digits, letters, $ symbol, underscore.
Must start with letter or $ symbol or underscore.
Variable name should not be a keyword.
Variable name can not start with a number.

There are other variable keywords introduced in ES6. They are

let - let keyword to define a variable with restricted scope.
const - const keyword to define a variable that cannot be reassigned.

Primitive Data Types :

Primitive data types are stored as simple data types.

Primitive data types are immutable.

There are 6 primitive data types:

string
number
boolean
null
undefined
symbol()

string :

The sequence of characters delimited by either single or double-quotes.

Example :

var car = "Audi";
console.log(typeof car); // Prints: string

number :

Any integer or floating-point numeric value.

Example :

var model = 2018;
console.log(typeof model); // Prints: number

boolean :

Which tells true or false

Example :

var available = true;
console.log(typeof available); // Prints: boolean

null :

This is special primitive-type that has only one value i.e., null.

Example :

var owner = null;
console.log(typeof owner);
 // Prints: object

In above example, as you can see it printed as object instead of null because it is a bug in javascript. It’s there since begining of javascript.

undefined :

A primitive type that had only value, undefined. The undefined is the value assigned to a variable that wasn’t initialized.

Example :

var condition = undefined;
console.log(typeof condition); // Prints: undefined

symbol( ) :

A unique and unaltered value.

Example :

var vol = Symbol();
console.log(typeof vol); // Prints: symbol

Coercion

The way to convert from one data type to another(such as string to number, object to boolean, and so on) is called coercion.

In above example, JavaScript has coerced the 9 from a number into a string and then concatenated the two values together, resulting in a string of 59.

String Concatenation

The same + operator you use for adding two numbers can be used to concatenate two strings.

Example :

var msg1 = "There are ";
var numStudents = 25;
var msg2 = " students in class";

console.log(msg1 + numStudents + msg2); //Output:  "There are 25 students"

Referential Data Types (non-primitive)

Functions
Arrays
Objects