Does Javascript / jQuery have system vars?

Question

I noticed strange issue.

Look at this jQuery:

$(function(){
    status = 1;
    status1 = 2;

    $('body').append(status+' - '+status1);
});

Fiddle

As you can see output is:

- 2

So status is system var for JavaScript or jQuery?

And is there any other vars like this?

Answer 1

JavaScript has global variables and in browsers, global variables are properties of the global object which is window.

Now, window itself has a couple of predefined properties and some of them are read-only, like window.status ^[MDN] (this can also differ from browser to browser!). Creating a global variable with such a name will therefore fail (the variable already exists, but you cannot assign a new value to it).

You can find a list of predefined properties in the MDN documentation.

---

This one of the reasons why you should avoid global variables. If you use local variables (by declaring variables with var and if necessary, put all your code in a function), you don't have this problem:

(function() {
    var status = 'foo';
    // ....
}());