What is the goal of self.self == self check in JavaScript?

Question

backbone.js starts with:

//Establish the root object, `window` (`self`) in the browser, or `global` on the server.  
//We use `self` instead of `window` for `WebWorker` support.  
var root = (typeof self == 'object' && self.self == self && self) ||  
           (typeof global == 'object' && global.global == global && global);  

What is self.self == self for? When can it be false?
Same about global.global == global.

Answer 1

function Mistake(x);
    self = this;
    x.on("event", function() {
        console.log(self);
    });
}
new Mistake(…);

Did you spot it? Now we've got a global self that is not the self that backbone expects. So it checks whether self actually is the global object, which is likely to be the case when self is an object and the object has the "global variable" self as a property that points to the object itself.

Same for global.