Global JSON variable override

Question

I had to use json2.js in my project as browser(IE8) JSON object was not available for parsing strings to JSON.

I ran through json2.js and am having a doubt with the variable declaration.

A JSON global variable is declared in json2.js like

var JSON;
if(!JSON){
    JSON={};
}

What is the effect of declaration var JSON; on the global JSON object. I hope the declaration should override the global JSON object in any browser (IE8/IE7). But for my surprise it is not overriding when a global object is available. Only a variable definition / initiation overrides a global variable? Kindly clarify.

But my intranet web-page (with JSF & Richfaces) is always loaded in IE7 mode.That is a different story. — Ahamed Mustafa M
– Ahamed Mustafa M, Commented May 2, 2012 at 18:36

Felix Kling · Accepted Answer · 2012-05-02 18:53:53Z

For each variable declaration (not initialization!). The following happens (section #10.5):

8. For each VariableDeclaration and VariableDeclarationNoIn d in code, in source text order do

Let dn be the Identifier in d.

Let varAlreadyDeclared be the result of calling env’s HasBinding concrete method passing dn as the argument.

If varAlreadyDeclared is false, then

Call env’s CreateMutableBinding concrete method passing dn and configurableBindings as the arguments.

Call env’s SetMutableBinding concrete method passing dn, undefined, and strict as the arguments.

So you see, whenever var x is encountered, it is tested whether a variable with name x already exists in the environment. If yes, it is just ignored, but if not, then the variable is declared and initialized with undefined.

Since the code is run in global scope it tests whether JSON exists in global scope. So if JSON already exists, var JSON; is just ignored.

Just some thoughts regarding testing/explaining this behaviour:

I don't know at which point in the JavaScript execution the global object is created, but I assume before all other scripts are evaluated. That means, JSON exists and has a value before any variable declaration, something you can only simulate if you include two scripts (can also be inline I guess, they are evaluated after another).

Try:

// script1.js
var foo = 'bar';

// script2.js
var foo;
if(!foo) {
    foo = 'baz';
}
alert(foo);

// include script2.js after script1.js

What's the result? (cheaters look here).

Whenever you are in a single script file, all variable declarations are hoisted to the top anyways. So if you have

var foo = 'bar';
var foo;
if(!foo) {
    foo = 'baz';
}

the script is actually executed as:

var foo;
var foo;
foo = 'bar';
if(!foo) {
    foo = 'baz';
}

You could not actually test whether the second var foo; overwrites the first one, since at this point it has no value yet. So this is not a good example to demonstrate the behaviour quoted above.

Denys Séguret · Accepted Answer · 2012-05-02 18:33:46Z

3

The var keyword ensures there is a variable in the enclosing function or (like here) in window but it doesn't initialize one.

In fact, the order doesn't really impact.

Look at this code as an example :

a = 3;
var a;
alert(a);

It alerts "3".

So the var declaration in the code you show simply ensures there won't be errors in the test, and doesn't remove the existing value.

It's good to keep this in mind because a common error in javascript is to have more than one var declaration in a function and being surprised by the value set before even the var declaration. Run this for example :

if (true) {
    a = 3;
}
// lot of code, at a different level
if (true) {
    var a;
    if (433/43==3) a=true;
    if (a) alert('should not happen');
};

Yes, the var is attached to the function, not the block...

edited May 2, 2012 at 18:33

answered May 2, 2012 at 18:28

Denys Séguret

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1 Comment

Felix Kling Over a year ago

It's a bit different here though. In your example, the variable statement is hoisted to the top. So even if you write it like that, the actual execution order is var a; a = 3; (or maybe it is var a; a = 3; var a; (not hoisted) which would make sense as well and then it would fit). In this case, JSON is a predefined symbol in the environment, which already has a value (I assume) before the variable declaration is evaluated. I don't know, I just wanted to point out the difference.

jbabey · Accepted Answer · 2012-05-02 18:27:55Z

0

It may not be an option for you, but it is also possible to set IE8 to compatibility mode to get the native JSON object back:

// in your preinit page event
Response.AddHeader("X-UA-Compatible", "IE=8");

The other option would be to create the JSON variable only if it does not exist:

var JSON = JSON || {};

answered May 2, 2012 at 18:27

jbabey

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Comments

Adam Jenkins · Accepted Answer · 2012-05-02 18:26:46Z

-1

If you are in the global scope when executing that block of code, then var JSON will overwrite the global JSON object to undefined.

If you are in any other scope (like inside a function), then there will be no effect.

answered May 2, 2012 at 18:26

Adam Jenkins

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2 Comments

Felix Kling Over a year ago

Normally the script is executed in global scope. So why is it there?

Felix Kling Over a year ago

What I mean is: It is executed in global scope and it does not overwrite the global JSON object if it exists. Why should it? That would be a complete waste.

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