what is the reserved keyword for NaN in javascript

问题:

if i want to test the result of an expression and the function would return NaN how would i check that?
examples:
$('amount').value.toInt()!='NaN'
^ does not work and i assume that the returned value is not a string,
$('amount').value.toInt()!=NaN
^ doesnt seem to work either and this one seems obvious

so how do i check wether the returned value is not a number?

回答1:

The NaN value is defined to be unequal to everything, including itself. Test if a value is NaN with the isNaN() function, appropriately enough. (ECMAScript 6 adds a Number.isNan() function with different semantics for non-number arguments, but it's not supported in all browsers yet as of 2015).

There are two built-in properties available with a NaN value: the global NaN property (i.e. window.NaN in browsers), and Number.NaN. It is not a language keyword. In older browsers, the NaN property could be overwritten, with potentially confusing results, but with the ECMAScript 5 standard it was made non-writable.

• As @some pointed out in the comments, there is also the global function isFinite() which may be useful.

回答2:

the best way to check the result of numeric operation against NaN is to aproach this way , example:

var x  = 0;
var y  = 0;
var result = x/y;  // we know that result will be NaN value
// to test if result holds a NaN value we should use the following code :

if(result !=result){
console.log('this is an NaN value');
} 

and it's done.

the trick is that NaN can't be compared to any other value even with it self(NaN !=NaN is always true so we can take advantage of this and compare result against itself)

this is JavaScript(a good and bizarre language)

回答3:

Equality operator (== and ===) cannot be used to test a value against NaN. Use Number.isNaN() or isNaN() instead.

NaN === NaN;        // false
Number.NaN === NaN; // false
isNaN(NaN);         // true
isNaN(Number.NaN);  // true