Variables are named memory locations that can hold different types of values. Whereas data type is a classification of data which tells the compiler or interpreter how the programmer intends to use the data. In javascript, we have 3 types of datatype –
- Primitive
- Non-Primitive
- Special

Primitive
A primitive type is the basic type of data type by which we can create Non Primitive type variables. It’s defined as immutable values.
Boolean | true or false |
String | text |
Number | 007 or 0.07 |
Non-Primitive
Non-primitive types are the combination of primitive types of variables.
Array | [1, 2, 3] |
Date | Mon Jan 02 2017 11:55:56 GMT+0530 (IST) |
Function | function () {} |
Object | { text: “text” } |
RegExp | /text/ |
Special
Special types are some special value. Like –
Boolean | undefined |
Null | null |
How to use the variable inside javascript?
In Javascript we use the keyword variable_name = value; this formula for creating a variable. Ex-
var isTure = true; // for boolean
var location = “India”; // for string
var age = 23; // for number
var isUndefined; // for undefined
var isNull = null; // for null
var obj = { isTrue: true }; // for object
var arr = [1, 2, 3]; // for array
var date = new Date(); // for date
var reg = /test/; // for regex
var fun = function () {}; // for function
Be careful when you are using null and undefined values. So the difference between null and undefined is –
If we have a property without value or definition, is undefined. And Null is a special value meaning “no value”. Null is an object type of value whereas undefined is the undefined type itself.
JavaScript Identifiers
A variable should be identified with unique names. Unique names are called identifiers. It is a good practice to create a more descriptive variable (name, age). Some basic rules to create variables –
- Variable can contain letters, digits, underscores, and dollar signs.
- Variable can Start with letter or $ and _.
- Variable should be case sensitive.
- Variable cannot use reserved words.
For more details – w3school