Any kind of variable, function, array or object that you use while programming in JS falls into one of the following categories.

Numbers

Operations that can be performed on numbers:

//Addition
12+45

//Subtraction
12-45

//Multiplication
12*45

//Division - returns the quotient
45/12

//Modulo - returns the remainder
45%12

NaN

Not an Number. It is a numerical value that represents something that is not...quite a number.

//NaN
0/0
123 + 0/0

Boolean

let gameOver = false;  //0

let isStillAlive = true;  //1

const riverFull = true;  //1

Null

'null' is used to indicate the intentional absence of a value

To check of a value is 'null', use the '===' strict equality operator. This operator not only checks value but also type of values.

value === null

'null' has a 'falsey' value. This means that when used in conditionals it will be coerced into 'false'.

The typeof null return 'object'.

null vs. undefined

If you declare a variable without initializing it, the variable will evaluate to 'undefined'.

'null' represents a missing object.

null === undefined; //false

Variables

Variables can be thought of a labelled container for a value. The label specifies the name of your variable while the container holds the data. This container later on can be used or changed.

let livesLeft = 9;

Variable declared with 'const' cannot be changed at any time during your program.

const pi = 3.14;

Variables can change Type

let livesLeft = 9;  //its a number
let livesLeft = 'Cat';  //its now a string
let livesLeft = true;  //its now a boolean