When it comes to each variable declaration (not initialization!), the following process occurs (section #10.5):
8. For every VariableDeclaration and VariableDeclarationNoIn in the code, follow these steps in sequential order according to the source text:
- Identify the Identifier in d as dn.
- Determine whether a binding for dn already exists by calling env’s HasBinding concrete method.
- If no binding exists for dn, then
- Create a mutable binding using env’s CreateMutableBinding concrete method with arguments dn and configurableBindings.
- Set the mutable binding by calling env’s SetMutableBinding concrete method with arguments dn, undefined, and strict.
Thus, whenever var x is encountered, it checks if a variable named x is already present in the environment. If so, it is disregarded; if not, the variable is declared and initialized with undefined.
Since the code runs in global scope, it verifies the existence of JSON in the global scope. Therefore, if JSON already exists, var JSON; is simply ignored.
Some reflections on testing/explaining this behavior:
I'm unsure exactly when the JavaScript execution creates the global object, but I presume it happens before evaluating other scripts. This implies that JSON exists with a value even before any variable declaration, a scenario only achievable by including two separate scripts (they can be inline).
Experiment with:
// script1.js
var foo = 'bar';
// script2.js
var foo;
if(!foo) {
foo = 'baz';
}
alert(foo);
// include script2.js after script1.js
What result do you expect? (find out here).
In a single script file, all variable declarations are lifted to the top anyway. So, if you have:
var foo = 'bar';
var foo;
if(!foo) {
foo = 'baz';
}
The script essentially runs like this:
var foo;
var foo;
foo = 'bar';
if(!foo) {
foo = 'baz';
}
You cannot verify if the second var foo; overrides the first one because it lacks a value at that point. Hence, this example doesn't effectively showcase the mentioned behavior.