Javascript

Local variables are deleted when the function is completed

Global variables are deleted when you close the browser window or tab

var a = 10; // global variable

function f()
{
	var b = 100; // local variable b
	console.log(a); // access global variable a
}

f();

// console.log(b); // cannot access the local variable defined in f()

All global variables belong to the window object

Do NOT create global variables unless you intend to, your global variables or functions can overwrite window variables or functions

a = 10; // undeclared variable is referenced, a new variable gets created in the global object

// a and window.a are the same object
console.log(a) // 10
console.log(window.a) // 10, global object window

var c = 1;

console.log(c); // 1
console.log(window.c); // 1

function f()
{
	b = 100;
}

f();

console.log(b); // 100
console.log(window.b); // 100, b is not removed after calling function f

for(var b in window) { 
  	if(window.hasOwnProperty(b)) console.log(b); 
}

function f(a)
{
	a++;
	console.log(a);
}

// console.log(a); // not defined

{
	var a = 100; // global scope
}

console.log(a);
console.log(window.a);

{
	let b = 100; // block scope
}

// console.log(b); // b is undefined

var a = 10;

{
	var a = 100; // overwrite a outside block
	console.log(a); // 100
}

console.log(a); // 100

var b = 10;

{
	let b = 100; // screen b outside block
	console.log(b); // 100
}

console.log(b); // 10

let b = 10; // b is not a attribute of window object

console.log(b); // 10
console.log(window.b); // undefined

var i = 5;
for (var i = 0; i < 10; i++) {
  	// some statements
}
		
console.log(i); // Here i is 10

let j = 5;
for (let j = 0; j < 10; j++) {
  	// some statements
}
		
console.log(j); // Here j is 5

Const behaves like let, except they cannot be reassigned

Const defines a constant reference to a value, cannot change constant primitive values, but we can change the properties of constant objects

const PI = 3.141592653589793;
//PI = 3.14;      // This will give an error
//PI = PI + 10;   // This will also give an error

// block scope
var a = 10;

{
	const a = 100;
}

console.log(a); // 10

const person = {name: 'Lin', age: 39};

//person_1 = {}; // not allowed

person.name = 'Yanhua';

console.log(person); // can change the properties

const cars = ['Buick', 'Acura', 'Lincoln', 'Honda'];

// cars = []; // not allowed

cars[3] = 'Toyota';
cars.push('Audi');

console.log(cars); // constant array can change

All functions have access to the scope "above" them

function add() {
  var counter = 0;
  function plus() {counter += 1;}
  plus();    
  return counter; 
}