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up vote 12 down vote favorite
1

This question already has an answer here:

I am new to JavaScript and I was doing some practices on local and global variable scopes, following is my code(fiddle):

var myname = "initial"
function c(){
    alert(myname);
    var myname = "changed";
    alert(myname);
}
c();

when first alert is called, it is showing myname as undefined. so my confusion is why I am not able to access global instance of myname and if I don't define myname within the function then it will work fine.

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marked as duplicate by Qantas 94 Heavy, nwellnhof, Viruss mca, Liam, Code Lღver Nov 1 '13 at 13:34

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.
    
if you do alert(window.myname); then you will get values –  Just code Nov 1 '13 at 5:31
    
@dholakiyaankit I've tried using window.myname still it is saying undefined... –  Bharat Soni Nov 1 '13 at 5:37
    
@BharatSoni window.myname works in my browser. –  EmptyArsenal Nov 1 '13 at 5:42
    
It should work bharat –  Just code Nov 1 '13 at 5:44
    
@dholakiyaankit I've tried in fiddle.. didn't work –  Bharat Soni Nov 1 '13 at 5:44

2 Answers 2

up vote 9 down vote

In Javascript, the variables are automatically moved to the top of the function. So, the interpreter would make it look more like this:

var myname = "initial"
function c(){
    var myname;
    // alerts undefined
    alert(myname);
    myname = "changed";
    // alerts changed
    alert(myname);
}
c();

This is called 'hoisting'.

Since the scope for any variable is the function it's declared in and hoisting, it's standard practice to list all variables at the top of a function to avoid this confusion.

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that I've understood, so there is now way to access a global variable with same name as local variable.. ?? –  Bharat Soni Nov 1 '13 at 5:34
1  
You should be able to access it by doing window.myname. See here: stackoverflow.com/questions/15826751/… But global variables are not usually considered a good practice, and in most cases it'd be better to define a global object and access it through that. i.e. var me = { myname: "initial" } then call me.myname in the function. –  EmptyArsenal Nov 1 '13 at 5:38
up vote 2 down vote

It is not replace the global variable. What is happening is called "variable hoisting". That is, myname var myname; gets inserted at the top of the function. Always initialize your variables before you use them

Try this 

var myname = "initial"
function c(){
    alert(myname);
    myname = "changed";
    alert(myname);
}
c();

Fiddle is

http://jsfiddle.net/xjmBf/2/

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sir, I am not just trying to change the value.. what you are doing is changing the value of global variable itself.. I want to understand the concept "WHY" it is happening... –  Bharat Soni Nov 1 '13 at 5:32
    
This fixes the problem of alerting undefined, but it eliminates any local variables as it's only using the global variable myname. –  EmptyArsenal Nov 1 '13 at 5:32
    
It is not replace the global variable. What is happening is called "variable hoisting". That is, myname var myname; gets inserted at the top of the function. Always initialize your variables before you use them you can refer this stackoverflow.com/questions/11938961/… –  Sridhar R Nov 1 '13 at 5:35

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