How to evaluate a NaN equality in javascript?

Question

How to evaluate this statement in javascript?

 isnan = NaN;
 notnan = "hello";
 typeof(notnan) === "number" && isNaN(notnan);

What is expected result? If the first part of the expression is `true` how could the second part of expression also be `true`? — guest271314, Aug 08 '17 at 05:14
in right side: two operands are evaluated by and operator then equality checked or equality checked with any operands in the right side then evaluate both results? — Hassan Mohagheghian, Aug 08 '17 at 05:23
How can the value be a number and not be a number at the same time? As far as am aware of quantum computing is not implemented at JavaScript at this time — guest271314, Aug 08 '17 at 05:25
Um whys NaN not working? Its typeof Number but not a number ... :/ — Jonas Wilms, Aug 08 '17 at 05:28
please check this statement: typeof(isnan) === NaN ; isnan is nan but results false. — Hassan Mohagheghian, Aug 08 '17 at 05:34
Because `isNaN` is a function. `NaN` is not a string. `NaN` is not a JavaScript type — guest271314, Aug 08 '17 at 05:36
He's trying to see if a string is a number in `typeof(notnan) === "number"`, I believe that is the issue. — Cernodile, Aug 08 '17 at 05:37
If you are expecting a `true` result for the expression you can use `!` operator `!(typeof(notnan) === "number") /* notnan is not a "number" : false, which we invert to true */ && isNaN(notnan) // again, notnan is not a number, true && true === true` — guest271314, Aug 08 '17 at 05:39

Cernodile · Answer 1 · 2017-08-08T05:36:43.400

0

This statment is illogical, because you're trying to see if it's a number and not a number in same statement, making two different booleans output.

typeof(notnan) === "number" && isNaN(notnan);

I believe you're looking for

if (!isNaN(notnan)) {
  // Code if it's a number
} else {
  // Code if it's not a number
}

IF you expect to flip it, you add ! to start it

!isNaN(notnan)
/*
* Returns opposite of isNaN, if it's a number then `true`
* else returns `false`.
*/

EDIT:

 typeof(notnan) === "number" && isNaN(notnan);

Reason this doesn't work is because notnan is a string, therefore it returns false and doesn't evaluate properly. isNaN alone works perfectly fine.

edited Aug 08 '17 at 05:36

answered Aug 08 '17 at 05:23

Cernodile

139
7

for check a variable (literal) is Nan we must check this variable is typeof number because NaN is number by itself then ckeck this variable is typeof NaN. – Hassan Mohagheghian Aug 08 '17 at 05:27
@hassanmohagheghiyan What's the point of checking such thing if isNaN function itself returns either true or false depending on input. You could use parseInt() to ensure it's a number being processed. – Cernodile Aug 08 '17 at 05:30
please check typeof(isnan) === NaN that results flase! – Hassan Mohagheghian Aug 08 '17 at 05:37
`isnan` isn't a function, and NaN equal to itself is false. To properly detect if it's not a number, please use `isNaN()` function. – Cernodile Aug 08 '17 at 05:39

score 0 · Answer 2 · answered Aug 08 '17 at 05:26

run this it will say something,

isnan = NaN;
notnan = "hello";
if (typeof(notnan) === "number" && isNaN(notnan)){
  console.log('yes');
}
else {
  console.log('typeof(notnan) is :',typeof(notnan), '\n isNaN(notnan) :', isNaN(notnan),'\n this is,',typeof(notnan)=== "number",'-',isNaN(notnan), 'so this condition became false...')
}

How to evaluate a NaN equality in javascript?

2 Answers2