Lets begin with some highlevel JavaScript basics :)…

Java script simple statements could be ended without semi colon

Statements:

Both the below statements are valid

var a= 6;
var a=5

you could use ‘;’ semicolon  to write multiple statement in single line

var1 =10;var 2=23;var 5= 9

Java script is case sensitive, meaning ‘Time’ and ‘time’ are two different elements.

Comments: 

//single line comment
/* multi line statement
can be made like this */

html comment closing ‘–> ‘ is not detected by Javascript

Data Types:

Primitive:

JS following primitive data types:

  • Numbers – 1,2,3 ( Java script does not have separate representation for floating point and whole numbers, all the numeric are represented as floating points (Eg: 2.0)
  • Strings – “name”
  • Boolean – true or false

also supports ‘null‘ and ‘undefined‘

Composite 

JS also so support data type called ‘object‘

Declaring variables:

Variables are elements that store data and helps in re-usability

In JS , variables needs to be declared before it could be used. In JS , variable declaration could be made mainly using three keywords and are :

  1. var
  2. let
  3. const

Note: JS is dynamically typed (untyped) , meaning data type is detected dynamically. For instance, if for below variables , we store username = 1 and password = “we”; then the data type of username will be taken as Number and password as string.

Using Var

var username
var password

Multiple variable declaration in a single line is also permitted

var username,password,url

Note:

  • Variables declared with var are function scoped.
  • Could be modified
  • Could be re-declared ( It will replace the previous variable)
  • Does support hoisting (meaning that , if the variable is called before declaring , then JS will automatically declare and initialize it with value ‘undefined‘)
console.log(username)
var username = "user"

// This prints "undefined" because java script changes this code internally as 

var username = undefined
console.log(username)
username = "user"

//This is called hoisting

Using let:

let username;

Note:

  • Variables declared with let are block scoped.
  • Could be modified
  • Could not be re-declared ( It will replace the previous variable)
  • Does not support hoisting. Variable should be declared before it could be used

Using const:

const username;

Note:

  • Variables declared with const are block scoped.
  • Could not be modified
  • Could not be re-declared ( It will replace the previous variable)
  • Does not support hoisting. Variable should be declared before it could be used

To know more read : ( Read : https://medium.com/@toddhd/javascript-understanding-the-difference-between-var-let-and-const-d0390da913 )

Variable Scope:

JavaScript only two scope :

  • Global – Available anywhere in the code
  • Local – Available only to the local function in which its defined

Variable naming rules:

Operators in JavaScript:

  • Arithmetic Operators

+ , – , * , / ,% , ++, —

  • Comparison Operators

==, !=, > , <, >=, <=,

  • Logical (or Relational) Operators

&& (AND) , || (OR), !

  • Assignment Operators

=, += (add and assign), -=, *= ,/=, %=

  • Conditional (or ternary) Operators

?:

eg: a==b? true:false

  • Bitwise Operators

Note:

Javascript executes statements from left to right so in Javascript

  1. “a” +9 = “a9“
  2. “a” + 1 + 5 = “a15”
  3. 1+5+”a” = “6a“

That is , it considers the element neighboring to a string as string. Here in case 2 , a is string so 1 was considered as string and it gave “a1” string and as “a1” is string 5 was also considered as string which gave the final output a15.

In case 3, 1 was integer so 5 was also considered as integer and the output was 6. Now the operator statement becomes 6+”a”, now 6 is integer but “a” is string, so 6 is also considered as string, resulting in 6a.