According to http://www.ecma-international.org/ecma-262/6.0/index.html#sec-undefined, JSGlobalObject got properties such as "undefined" which are non-writable and non-configurable.
So the following code should throw a type error:
```
Object.defineProperty(this, "undefined", {
    configurable: true,
    get: function() {
        return false;
    }
});
```
And it does in Chrome, but currently does not in Safari.