Scopes

• JavaScript has 3 types of scope: Block, Function, Global

Block Scope

• Variables declared using let, const inside a { } block cannot be accessed outside of the block. Because it has block scope.
• Variables declared using var inside a { } block can be accessed outside of the block. Because it has function scope

if(true) {
    let i = 10;
    const msg = "hi"
    var a = 5;
    console.log(i);
    console.log(msg);
    console.log(a);
}
console.log(i); //error
console.log(msg); //eror
console.log(a);

Function Scope

• Each function creates a new scope.
• Variables declared using let, const, var within a function, becomes local variable and have function scope.
• They can only be accessed from within the function and not outside the function.
• Variables with the same name can be used in different functions.
• Parameters work as local variables inside functions.

function display() {
    let i = 10;
    const msg = "hi"
    var a = 5;
    console.log(i);
    console.log(msg);
    console.log(a);
}
display();
console.log(i); //error
console.log(msg); //eror
console.log(a); //error

Global Scope

• A variable declared outside a function, becomes global variable and have global scope.
• All scripts and functions on a web page can access it.
• If you assign a value to a variable that has not been declared, it will automatically become a GLOBAL variable.
• Global variables defined with the var keyword belong/attach to the window object:
• Global variables defined with the let, const keyword doesn't belong/attach to the window object
• Function is also a global and is added to the window object.

let i = 10;
const msg = "hi"
var a = 5;
console.log(i);
console.log(msg);
console.log(a);

Avoid var

• Avoid var and instead use let or const
• Don't create global variable using var keyword, because it gets attached to window object. If a third party library also has a variable with the same name, that variable can override your variable. So you should avoid adding stuff to the window object. Otherwise you're going to create all sorts of issues in your program