Mrthdrb

What's happening here? I get a different result if I declare a variable after console.log in the inner function

I understand that var has a functional scope and inner function can access the variable from their parent

function outer() 
 var a = 2;

 function inner() 
 a++;
 console.log(a) //log NaN
 var a = 8
 
 inner()

outer()

function outer() 
 var a = 2;

 function inner() 
 a++;
 console.log(a) //log 3
 var b = 8
 
 inner()

outer()

The log returns NaN in the first example and log 3 in the second example

This is due to hoisting

The declaration of a in the inner function is hoisted to the top of the function, overriding the outer function's a, so a is undefined

undefined++ returns NaN, hence your result.

Your code is equivalent to:

function outer() 
 var a=2;

 function inner() 
 var a;
 a++;
 console.log(a); //log NaN
 a = 8;
 

 inner();


outer();

Rewriting your code in this way makes it easy to see what's going on.

Because var is hoisted through the function, you're essentially running undefined++ which is NaN. If you remove var a = 8 in inner, the code works as expected:

function outer() 
 var a = 2;

 function inner() 
 a++;
 console.log(a);
 
 inner();

outer();

Javascript has the mechanism called hoisting .With the help of this mechanism javascript will automatically take all the variable definitions and kept it in the top of the function (whether it may be outer function or inner function). So in your case it will take only the variable definition and hoist it in the top . After hoisting you code will be like this

function outer() 
 var a = 2;

 function inner() 
 var a;
 a++;
 console.log(a) //log NaN
 a = 8
 
 inner()

outer()

you are trying to increment a undefined variable . This is the reason you are getting NaN error.
So if you initialize the variable after declaration(hoisting) then you will get the result.

They can’t access the inner function value so we have to defined globally. After globally you can use A value anywhere in the code

var a=0;
function outer()
a=2;
function inner()
a=a+1;
console.log(a)
 a = 8

inner()

outer()