Any kind of variable, function, array or object that you use while programming in JS falls into one of the following categories.
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
Not an Number. It is a numerical value that represents something that is not...quite a number.
//NaN 0/0 123 + 0/0
let gameOver = false; //0 let isStillAlive = true; //1 const riverFull = true; //1
'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'.
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 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