Why
isNaN(undefined) !== Number.isNaN(undefined)
is true.
Why true
console.log(isNaN(undefined)) //is trueBut if I add Number.
console.log(Number.isNaN(undefined)) // is false
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TSR
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3Why do you think it would be otherwise? – Robert Columbia Jul 31 '19 at 13:43
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Because they are both isNaN function, why they return different value if I add Number. – TSR Jul 31 '19 at 13:45
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2isNaN is not the same as Number.isNaN – Chris Li Jul 31 '19 at 13:46
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1Did you mean `!==`? Because `==!` in code will NOT do the same thing at all - it's an equality check `==` followed by a logical NOT `!` – VLAZ Jul 31 '19 at 13:46
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2[Confusion between isNaN and Number.isNaN in javascript](https://stackoverflow.com/questions/33164725) – adiga Jul 31 '19 at 13:47
2 Answers
2
The Number.isNaN function "is a more robust version" of the global isNaN function. Specifically, the rules for the global isNaN function are odd and that's why Number.isNaN is typically preferred. See https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/isNaN#Description.
According to that, isNaN essentially checks whether your value is not a number, whereas Number.isNaN checks whether your value is NaN. Of course, not all nonnumeric values are NaN - "test" for example, is not NaN but is also not a number.
Ian
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2"*isNaN essentially checks whether your value is not a number*" more specifically it checks if the value you supply *when converted to a number* produces `NaN`. So, `isNaN("123")` will report `false` because `Number("123")` does not result in `NaN`, although the value itself is definitely *not* a number but a string. However, `isNaN("hello")` returns `true` because `Number("hello")` does produce `NaN`. In short, yes - confusing. – VLAZ Jul 31 '19 at 13:54
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isNaN and !anything will always give you Booleans.
parseFloat('27') gives you the Number 27.
27 is not false, even with type conversion.
Quentin
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