NaN javascript type

Question

I have written below code in JS.

function foo() {

var s = "Jack";
s = parseInt(s);
console.log(s)
if (s!=NaN) {
    if (typeof (s) == "number")
        console.log("number");
    else if (typeof (s) == "string")
        console.log("string");
} else {
    console.log("Your entry is not a number");
}

}

As i am trying to parseInt a string, i got NaN as value.

We know that typeof(NaN) is "number" and numbers can be compared directly like(1==1) in IF condition. But this didn't work.

I tried with isNaN(s) and it worked where as s!=NaN failed.

why ?

Probably better duplicate: http://stackoverflow.com/q/559792/218196 . — Felix Kling, Jun 23 '16 at 07:04

score 1 · Accepted Answer · edited Jun 20 '20 at 09:12

1

The necessity of an isNaN function

Unlike all other possible values in JavaScript, it is not possible to rely on the equality operators (== and ===) to determine whether a value is NaN or not, because both NaN == NaN and NaN === NaN evaluate to false. Hence, the necessity of an isNaN function.[Ref]

Conclusion: NaNis never equals to anything including NaN itself hence rely on Number.isNaN(value) method.

console.log(NaN == NaN);
console.log(NaN === NaN);
console.log(NaN === undefined);
console.log(NaN === null);
console.log(NaN === 'TEST');
console.log(NaN === 0);

edited Jun 20 '20 at 09:12

Community

1
1

answered Jun 23 '16 at 06:59

Rayon

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That's what the OP did. The question is not how to test for NaN by why the equality operator doesn't work: *"I tried with isNaN(s) and it worked where as s!=NaN failed. Why?"* – Felix Kling Jun 23 '16 at 07:00
http://stackoverflow.com/questions/1565164/what-is-the-rationale-for-all-comparisons-returning-false-for-ieee754-nan-values – Pradeep Kr Kaushal Jun 23 '16 at 07:04

NaN javascript type

1 Answers1

The necessity of an isNaN function