Why variable is undefined as I declared globally?

Question

I am just showing the value of a in alert but I am getting a is undefined why? I will explain the problem:

First I call function with false parameter, it show alert with a = 1;. But when I pass true as parameter, it first show alert 2 (as expected it is local) but when again it show 2? Third it say a is undefined?

function ab(p){
    a = 1;
    if(p){
        var a = 2
        alert(a)
    }
    alert(a)
}

ab(false);
alert(a);

Unexpected result when ab(true)?

`a` is undefined because it is never declared in the global scope. — Derek 朕會功夫, Sep 06 '14 at 08:34
Read this also: http://docstore.mik.ua/orelly/webprog/jscript/ch04_03.htm **and** http://www.adequatelygood.com/JavaScript-Scoping-and-Hoisting.html — Grijesh Chauhan, Sep 06 '14 at 15:59

score 4 · Accepted Answer · answered Sep 06 '14 at 07:35

4

That is what called as variable hoisting. and actually the variable that you are thinking as a global one will be hoisted inside of that function and it would turn as a local one.

Compiler would consider your code like this,

function ab(p){
  var a;  //will be hoisted here.
  a=1;
  if(p){
     a=2;
     alert(a);
  }
  alert(a);
 }

answered Sep 06 '14 at 07:35

Balachandran

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mean in if condition make a (global variable) to local variable – Pallavi Sharma Sep 06 '14 at 07:38
No.. Please read about variable hoisting. That would help you in this case. – Balachandran Sep 06 '14 at 07:39
where is the concept of varible hosting and function hosting ? – Pallavi Sharma Sep 06 '14 at 07:39
3

@PallaviSharma Read this https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/var – Balachandran Sep 06 '14 at 07:41

Milind Anantwar · Answer 2 · 2014-09-06T07:39:34.060

2

That is not global. you have defined the variable in if condition. its context will remain inside if only. use :

function ab(p){
       a=1;
       if(p){
           a=2
           alert(a)
       }
       alert(a)
   }

ab(false);
 alert(a);

Working Demo

edited Sep 06 '14 at 07:39

answered Sep 06 '14 at 07:33

Milind Anantwar

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I did not get ..whenever I make variable without using var it become global – Pallavi Sharma Sep 06 '14 at 07:35
1

You can even skip first declaration `var a=''` – javiyu Sep 06 '14 at 07:35
@PallaviSharma: yes it will. see this answer by T.J. Crowder -http://stackoverflow.com/a/5786899/1719752 – Milind Anantwar Sep 06 '14 at 07:39

score 1 · Answer 3 · answered Sep 06 '14 at 07:40

You are using var inside your function, so JavaScript see your code as below (there is no matter where you use var, inside if or in loops or ...):

function ab(p) {
    var a=1;
    if (p) {
       a=2;
       alert(a);
    }

    alert(a);
}

ab(false);
alert(a);

Why variable is undefined as I declared globally?

3 Answers3