Javascript has five primitive data types: 1. number 2. string 3. boolean 4. undefined 5. null

Anything that doesn’t belong to any of these five primitive types is considered an object.

First 3 data types has a corresponding object constructor. For example

var word = "something";

And then as an object:

 var word = new String("something");

For object constructor notice the new keyword. It creates object reference.

Another thing to notice that

var greeting = "something";
var word = new String("something");
greeting == word ----> True as their value is same
greeting === word -----> False because there value same but type is different .

As for var keyword being same for all the cases,remember that Javascript is dynamically typed language. That means it resolves data type checking during runtime rather than compile time(like Java, C++ ).

This makes javascript extremely powerful. Though this unique feature has drawback too. Please co through this wikipedia for details: https://en.wikipedia.org/wiki/Type_system#Static_and_dynamic_type_checking_in_practice

Hope this helps.