Datatypes

• Numbers, eg. 1, 2.5 etc.
• Strings e.g. "This is a string".
• Boolean (true or false) type
• Object: It is the most important data-type of JavaScript (composite data type).

JavaScript also defines two more data types, null and undefined, each of which defines only a single value. JavaScript does not make a distinction between integer values and floating-point values. All numbers in JavaScript are represented as floating-point values (no distinction between integer and floating point values).

Variables

• Using var
• Using let
• Using const
• Do not declare

You need not declare a variable in JavaScript to assign values. Assume variables as containers for storing data.

Examples

var mynum = 10;
let mynum = 10;
mynum = 10;
All the above three methods do the same thing; assigning a value of 10 to the variable "mynum".

Some more examples
var myname = "Ram";
var price = 500.50;
x = 5; y = 10;
var total = x + y;
let name = "Ram", hobby = "bird watching", height = 6;

Scope of Variables

Scope refers to the visibility of variables. The scope of a variable is the region of your program in which it is defined. JavaScript variables have two scopes.
Global Variables: Global variables can be defined anywhere in a JavaScript program.
Local Variables: Local variables are defined inside a function and its scope limited to that function.
Like other programming languages, reserved keywords can not be used as a variable name.

Keywords in JavaScript