Variables

A variable is a container for a value. You can:

• Create a variable with a descriptive name.
• Store or update information stored in a variable.
• Reference or “get” information stored in a variable.

let name	declaration. Value: 'undefined'. when using let, a variable with the same name can only be declared once (this is a good thing!)
name = 'Max'	attribution
let name = 'Max'	initializing

• var - declares/initilizes the variable - pre-ES6
• let - is used in web development, an alternative to var, but stricter (prefered)
• const - is used for constants, the content can’t change (must be assigned a value when declared). A common practice when naming constants is to use all uppercase letters, with words separated by an underscore. const FAV_PET = "Dogs";
• When JavaScript variables are declared, they have an initial value of undefined. If you do a mathematical operation on an undefined variable your result will be NaN which means "Not a Number". If you concatenate a string with an undefined variable, you will get a literal string of "undefined".
• Create multiple Variables at once: let a = 2, b = 3, c = 4;

Hoisting

Before any JavaScript code is executed, all variables declared with var are "hoisted"

let and const

Variables declared with let and const eliminate this specific issue of hoisting because they're scoped to the block, not to the function.

let and const also have some other interesting properties.

• Variables declared with let can be reassigned, but can't be redeclared in the same scope.
• Variables declared with const must be assigned an initial value, but can't be redeclared in the same scope, and can't be reassigned.

Scope

• Scope determines the accessibility (visibility) of these variables.
• JavaScript has 'function scope': Each function creates its own new scope.
• there are two types of scope: Local & Global Scope

Local Scope

• Variables declared inside a function are not accessible (visible) from outside the function. They become local to the function.
• Local variables have function scope: They can only be accessed from within the function.
• Since local variables are only recognized inside their functions, variables with the same name can be used in different functions.
• Local variables are created when a function starts, and deleted when the function is completed.
• block scope. When a variable is defined inside a block, it is only accessible to the code within the curly braces {}.

Global Scope

• A variable declared outside a function becomes global.
• A global variable has global scope: All scripts and functions on a web page can access it
• it’s best practice to not define variables in the global scope -> having too many global variables can cause problems -> Scope pollution
• Global namespace is the space in our code that contains globally scoped information.