Basics Of JavaScript (Data Type, Variables, Function) By Arushi Gupta

There are different types of data that we can use in a JavaScript program.
​For example,

const x = 5; const y = "Hello";

Here,

• 5 is an integer data.
• "Hello" is a string data.

JavaScript Data Types

​There are eight basic data types in JavaScript. They are:

Data Types	Description	​Example
String	Represents textual data	'hello', "hello world!" etc
Number	an integer or a floating-point number	3, 3.234, 3e-2 etc.
BigInt	an integer with arbitrary precision	900719925124740999n , 1n etc.
Boolean	Any of two values: true or false	true and false
undefined	​a data type whose variable is not initialized	let a;
null	denotes a null value	let a = null;
Symbol	data type whose instances are unique and immutable	let value = Symbol('hello');
Object	key-value pairs of collection of data	let student = { };

Here, all data types except Object are primitive data types, whereas Object is non-primitive

​Note: The Object data type (non-primitive type) can store collections of data, whereas primitive data type can only store a single data.

JavaScript Variables

4 Ways to Declare a JavaScript Variable:

• Using var
• Using let
• Using const
• Using nothing
  
  

What are Variables?
​Variables are containers for storing data (storing data values).

In this example, x, y, and z, are variables, declared with the var keyword:
Example
var x = 5;
var y = 6;
var z = x + y;

In this example, x, y, and z, are variables, declared with the let keyword:
Example
let x = 5;
let y = 6;
let z = x + y;

In this example, x, y, and z, are undeclared variables:
Example
x = 5;
y = 6;
z = x + y;

From all the examples above, you can guess:

• x stores the value 5
• y stores the value 6
• z stores the value 11​
• 
  

When to Use JavaScript var?

• ​Always declare JavaScript variables with var,let, orconst.
• The var keyword is used in all JavaScript code from 1995 to 2015.
• The let and const keywords were added to JavaScript in 2015.
• If you want your code to run in older browser, you must use var.

When to Use JavaScript const?

• If you want a general rule: always declare variables with const.
• If you think the value of the variable can change, use let.

JavaScript Functions

A JavaScript function is a block of code designed to perform a particular task.A JavaScript function is executed when "something" invokes it (calls it).
​Example

function myFunction(p1, p2) {   ​return p1 * p2;   // The function returns the product of p1 and p2 ​}

JavaScript Function Syntax

​A JavaScript function is defined with the function keyword, followed by a name, followed by parentheses ().Function names can contain letters, digits, underscores, and dollar signs (same rules as variables).The parentheses may include parameter names separated by commas:(parameter1, parameter2, ...)The code to be executed, by the function, is placed inside curly brackets: {}

function name(parameter1, parameter2, parameter3) {   ​  // code to be executed ​}

​Function parameters are listed inside the parentheses () in the function definition. Function arguments are the values received by the function when it is invoked. Inside the function, the arguments (the parameters) behave as local variables. A Function is much the same as a Procedure or a Subroutine, in other programming languages.