Is it possible to access a function local variable from outside of the function?

Question

I heard today that:

it is possible to access a local variable of a function since everything in JavaScript is global.

As far as I know, you can't access a local variable from outside of the scope of the variable.

For example:

function f()
{
    var myvar = "something";
}

myvar = "c"; // i'm not accessing myvar in f();

I also heard that it's possible to use for(var i in window) to access myvar. I want to confirm it is not possible since I'm not the author of the language.

Updated:

I asked him a code snippet, and here's what I have received.

var person = {
    whoIs : function()
    {
        var name = "name";
        return name;
    }
};


var str = "TEST:\n";

for(var n in person)
{
    str += n;
    str += " = [" + person[n] + "]\n";
}

// perform regular exp. to get the value of name variable.


alert(str);

It's not accessing the variable.. it's simply printing how the function looks like...

"it is possible to access a local variable of a function since everything in javascript is global" this is definitely wrong. — Behrang Saeedzadeh
– Behrang Saeedzadeh, Commented Mar 29, 2012 at 3:29
To expand on that a bit, JavaScript has some very odd scoping (if you're not familiar with it), but "everything in JavaScript is a global" is just plain wrong. Like, run away from that developer wrong. — Corbin
– Corbin, Commented Mar 29, 2012 at 3:32
ask that guy "when was the last time you used JavaScript?" — Joseph
– Joseph, Commented Mar 29, 2012 at 3:42

Yves M. · Accepted Answer · 2023-03-25 11:40:58Z

That developer was wrong. Those two myvar are different. The outside one is equivalent to window.myvar, but the inside one is only inside the f.

Edit: a very simple example: http://jsfiddle.net/mRkX3/

Edit 2:

A quote from the ECMAScript standard:

If the variable statement occurs inside a FunctionDeclaration, the variables are defined with function-local scope in that function, as described in section 10.1.3. Otherwise, they are defined with global scope (that is, they are created as members of the global object, as described in section 10.1.3) using property attributes { DontDelete }. Variables are created when the execution scope is entered. A Block does not define a new execution scope. Only Program and FunctionDeclaration produce a new scope. Variables are initialised to undefined when created. A variable with an Initialiser is assigned the value of its AssignmentExpression when the VariableStatement is executed, not when the variable is created.

Found through http://www.adequatelygood.com/2010/2/JavaScript-Scoping-and-Hoisting though that article is referencing a deadlink (live link: http://www.ecma-international.org/publications/files/ECMA-ST/Ecma-262.pdf).

crinklywrappr · Accepted Answer · 2012-03-29 03:29:45Z

5

Sounds like a communication breakdown. I'm betting he was trying to make the point that you can define a global variable from inside a function by attaching it to the window object. Try:

function f()
{
    window.myvar = "something";
}

myvar = "c"; // overwriting "something"

answered Mar 29, 2012 at 3:29

crinklywrappr

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Comments

Derek 朕會功夫 · Accepted Answer · 2012-03-29 03:56:35Z

3

In your example:

function f(){
    var myvar = "something";
}

myvar = "c";    //nope, they are different.

myvar = "c" is referring to window.myvar, which is not the same as the myvar inside f(). You can not access the variables inside the function unless it is declared outside it.

var myvar;

function f(){
    myvar = "something";
}

myvar = "c";    //now it changes "something" to "c"!

answered Mar 29, 2012 at 3:56

Derek 朕會功夫

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Comments

Epuri · Accepted Answer · 2012-03-29 03:27:51Z

2

Yes, you are right about it. Variables declared within function scope will not be accessible out side the function. If function returns myvar then it will be possible access its value but not the variable directly.

answered Mar 29, 2012 at 3:27

Epuri

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Comments

helly0d · Accepted Answer · 2012-03-29 03:28:58Z

2

in your case myVar is in the scope of the function and if it is defined there with var it will never be accessible to the outside, but if it is not defined than you are setting myVar to the global object ( window ) and it could lead to lots of trouble.

function(f) {
myVar = "hello"
}

In this case you can find myVar, because it is not defined inside the function.

answered Mar 29, 2012 at 3:28

helly0d

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user188654 · Accepted Answer · 2012-03-29 03:33:24Z

2

Judging by the example he used (for(var i in window)) he is clearly counting on your function variables to be declared as global (without var proceeding them).

Since in that case all of the defined variables are bound for the window object it would be easy to access them by simply traversing the window object. However, if you use var to declare local function variables they will be available only to the function itself (window object won't contain them).

answered Mar 29, 2012 at 3:33

user188654

Comments

Bhavya Anand · Accepted Answer · 2013-09-09 17:11:27Z

-1

function f()
{
   var myvar = "Moon";
}

f();

myvar = "Sunlight";

answered Sep 9, 2013 at 17:11

Bhavya Anand

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