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Why do variable in an inner function return nan when there is the same variable name at the inner function declared after log

6

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

javascript function

share|improve this question

edited 17 mins ago

Nick Parsons

10.3k2926

asked 25 mins ago

ClaudeClaude

426

add a comment | 

6

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

javascript function

share|improve this question

edited 17 mins ago

Nick Parsons

10.3k2926

asked 25 mins ago

ClaudeClaude

426

add a comment | 

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

javascript function

share|improve this question

edited 17 mins ago

Nick Parsons

10.3k2926

asked 25 mins ago

ClaudeClaude

426

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()

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 NaN
 var a = 8
 
 inner()

outer()

function outer() 
 var a = 2;

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

outer()

function outer() 
 var a = 2;

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

outer()

share|improve this question

edited 17 mins ago

Nick Parsons

10.3k2926

asked 25 mins ago

ClaudeClaude

426

share|improve this question

edited 17 mins ago

Nick Parsons

10.3k2926

asked 25 mins ago

ClaudeClaude

426

edited 17 mins ago

Nick Parsons

10.3k2926

edited 17 mins ago

Nick Parsons

10.3k2926

asked 25 mins ago

ClaudeClaude

426

asked 25 mins ago

ClaudeClaude

426

3 Answers
3

active

oldest

votes

8

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.

share|improve this answer

edited 7 mins ago

Shidersz

9,3112933

answered 20 mins ago

jrojro

552111

add a comment | 

1

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();

share|improve this answer

answered 17 mins ago

Jack BashfordJack Bashford

13.8k31848

add a comment | 

-1

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

inner()

outer()

share|improve this answer

answered 17 mins ago

Darshit ShahDarshit Shah

53

• 
  
  
  

  
  3
  

  
  
  
  
  
  

  
  
  How does this piece of code explains the issue? Can you provide an explanation of the code you have posted?
  
  – Shidersz
  10 mins ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  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
  
  – Darshit Shah
  8 mins ago
  

  
  
  
  

add a comment | 

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8

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.

share|improve this answer

edited 7 mins ago

Shidersz

9,3112933

answered 20 mins ago

jrojro

552111

add a comment | 

8

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.

share|improve this answer

edited 7 mins ago

Shidersz

9,3112933

answered 20 mins ago

jrojro

552111

add a comment | 

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.

share|improve this answer

edited 7 mins ago

Shidersz

9,3112933

answered 20 mins ago

jrojro

552111

function outer() 
 var a=2;

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

 inner();


outer();

share|improve this answer

edited 7 mins ago

Shidersz

9,3112933

answered 20 mins ago

jrojro

552111

edited 7 mins ago

Shidersz

9,3112933

edited 7 mins ago

Shidersz

9,3112933

answered 20 mins ago

jrojro

552111

answered 20 mins ago

jrojro

552111

1

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();

share|improve this answer

answered 17 mins ago

Jack BashfordJack Bashford

13.8k31848

add a comment | 

1

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();

share|improve this answer

answered 17 mins ago

Jack BashfordJack Bashford

13.8k31848

add a comment | 

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();

share|improve this answer

answered 17 mins ago

Jack BashfordJack Bashford

13.8k31848

function outer() 
 var a = 2;

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

outer();

function outer() 
 var a = 2;

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

outer();

function outer() 
 var a = 2;

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

outer();

share|improve this answer

answered 17 mins ago

Jack BashfordJack Bashford

13.8k31848

answered 17 mins ago

Jack BashfordJack Bashford

13.8k31848

answered 17 mins ago

Jack BashfordJack Bashford

13.8k31848

-1

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

inner()

outer()

share|improve this answer

answered 17 mins ago

Darshit ShahDarshit Shah

53

• 
  
  
  

  
  3
  

  
  
  
  
  
  

  
  
  How does this piece of code explains the issue? Can you provide an explanation of the code you have posted?
  
  – Shidersz
  10 mins ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  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
  
  – Darshit Shah
  8 mins ago
  

  
  
  
  

add a comment | 

-1

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

inner()

outer()

share|improve this answer

answered 17 mins ago

Darshit ShahDarshit Shah

53

• 
  
  
  

  
  3
  

  
  
  
  
  
  

  
  
  How does this piece of code explains the issue? Can you provide an explanation of the code you have posted?
  
  – Shidersz
  10 mins ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  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
  
  – Darshit Shah
  8 mins ago
  

  
  
  
  

add a comment | 

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

inner()

outer()

share|improve this answer

answered 17 mins ago

Darshit ShahDarshit Shah

53

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

inner()

outer()

share|improve this answer

answered 17 mins ago

Darshit ShahDarshit Shah

53

answered 17 mins ago

Darshit ShahDarshit Shah

53

answered 17 mins ago

Darshit ShahDarshit Shah

53