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if x=NaN, x!==x also return false

on page 72,the author said:
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If one or both values is NaN, they are not equal. The NaN value is never equal to any
other value, including itself! To check whether a value x is NaN, use x !== x. NaN is
the only value of x for which this expression will be true.
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i test it on IE,chrome,firefox,but all failed.do i misunderstand?
the following is my code:
===================
var x=new Number('a');
isNaN(x);//return true;
if (x!==x){
console.log('true');
}else{
console.log('false');
} //return false;
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