when we are using javascript variable declaration, Javascript has to use define data type with a variable value. Data Type is a common thing in a programming language.
Javascript have two type data.

1. Primitive Type

2. Non-Primitive Type or reference Type

3. Primitive Type
   -> number
   -> string
   -> Big Int
   -> undefined
   -> null
   -> boolean

4. Non-primitive /reference /Object
   -> function
   -> object
   -> Array
   Reference: https://js.zonayed.me/basic/post-3