Scope

Scope answers the question: "Where are my variables and functions available to me?".

Global Variables

Available anywhere. They are created not inside a function, module, if statement etc.

<script> const first = 'Soumya'; </script> //here first is a global variable.

In the browser, the global scope is called window. Methods like setTimeout, setInterval are available on the window object.

const first = 'wes';
let second = 'bos';
var third = 100;

console.log(window.first); // undefined
console.log(window.second); // undefined
console.log(window.third); // 100

The above occurs because var variables are attached to the window object and are globally scoped, whereas const and let variables are globally scoped but not attached to the window object.

Anything except let and const declared things which are globally declared are attached to window object.

function sayHi() {
	console.log('HI');
}

sayHi(); // HI
window.sayHi(); // HI

Using global scoped variables etc. are not advised in general.

More examples:

const age = 100;
function go() {
	const cool = true;
}
go();

console.log(age); // 100
console.log(cool); // Uncaught ReferenceError: cool is not defined

Above, cool is function-scoped. Function scoping means anything declared inside of a function is only accessible inside the function and not outside (unless we return it from the function and store the returned value in a variable when the function is run)

Scope Lookup: If variables are not found inside a function, it goes a level higher to look for the variable in that scope and if its not available in that scope as well, it goes another level higher.

const age = 100;
function go() {
	const cool = true;
	console.log(age); 
	console.log(cool); 
}
go();

// console output:
// 100 - this is printed as it doesn't find variable age in go function scope it goes a level higher to find it
// true

const age = 100;
function go() {
	const cool = true;
	const age = 200; // global variable age has been shadowed by this variable - shadow variable
	console.log(age); 
	console.log(cool); 
	}
go();

// console output:
// 200 - this is printed because it finds variable age in go function scope only
// true

Block Scope:

if(1 === 1){
	const cool = true; // or let cool = true;
}

console.log(cool);

//Output: Uncaught ReferenceError: cool is not defined

if(1 === 1){
	var cool = true;
}

console.log(cool);

//Output: true - var varibales leak outside the block

A set of curly brackets are referred to as a block.

To access a variable outside a block, we use:

let cool;
if(1 === 1){
	cool = true;
}

console.log(cool);

//Output: true

Above the cool variable is declared in the higher scope, here global scope.

var variables are function scoped. Let's see here:

function isCool(name) {
  if (name === 'wes') {
    var cool = true;
  }
  console.log(cool);
  return cool;
}

isCool('wes');

//Output: true

Above, as the variable cool was declared using var keyword, it was not limited within the if block, rather it was available inside the entire isCool function.

let and const variables are block scoped.

Example:

const dog = "snickers";

function logDog() {
  console.log(dog);
}

function go() {
  const dog = "sunny";
  logDog();
}

go();

// Output: snickers

Above happens as JavaScript uses Lexical Scoping or Static Scoping. It means, variable lookup or scope lookup happens where the functions are defined not where they are run.

In the above example, logDog function is defined above where there is no dog variable inside its definition, so it goes and looks in upper/higher scope to find value as 'snickers'. It doesn't care where it is run, in this case inside go function.

const dog = "snickers";

function logDog(dog) {
  console.log(dog);
}

function go() {
  const dog = "sunny";
  logDog("Rufus");
}

go();

//Output : Rufus

The above is the output because here the function logDog creates a local variable for parameter dog which has value 'Rufus' passed to it. So, 'Rufus' is printed.

Best practices for variables:

Try not to create global variables.

Function Scoping:

Functions are scoped the exact same way as variables.

function sayHi(name) {
 function yell() {
  console.log(name.toUpperCase());
	}
}
yell();

// Output: Uncaught ReferenceError: yell is not defined

Just like variables, functions are scoped to the parent function. So yell function above is only accessible inside sayHi function, and not outside it.