If x!=x is true, what is the possible type of x?
Assuming that x is meant to be a variable, the answer to this question is:
"number"
The only value that satisfies this requirement is NaN, which never equals itself.
As you can see, the type of NaN is "number":
typeof NaN === "number".
If x is merely a placeholder for anything, functions and object or array literals would work, too:
(function(){}) != (function(){})
({}) != ({})
[] != []
If x can be a getter, then there's all sorts of crazy options:
// Type == "string"
Object.defineProperty(window, 'x', {
get: function() { return String.fromCharCode(Math.random() * 0xFFFF | 0); }
});
// Type == "boolean"
Object.defineProperty(window, 'x', {
get: function() { return !(Math.random() * 2 | 0); }
});
// Type == "function"
Object.defineProperty(window, 'x', {
get: function() { return function() {}; }
});
// Type == "object"
Object.defineProperty(window, 'x', {
get: function() { return ({}); }
});
// Type == "object"
Object.defineProperty(window, 'x', {
get: function() { return []; }
});
Using with:
var o = {};
Object.defineProperty(o, 'x', {
get: function() { return []; } // any of the above types
});
with(o){
alert(x != x);
}
(Thanks to @Paul S.)
