# use-isnan
Disallow incorrect comparisons against NaN.
NaN is a special Number value used to represent "not a number" results in calculations.
This value is specified in the IEEE Standard for Binary Floating-Point-Arithmetic.
In JavaScript, NaN is unique, it is not equal to anything, including itself! therefore
any comparisons to it will either always yield true or false. Therefore you should
use isNaN(/* num */) instead to test if a value is NaN. This rule is aimed at removing this footgun.
# Invalid Code Examples
if (foo == NaN) {
// unreachable
}
if (NaN != NaN) {
// always runs
}
# Correct Code Examples
if (isNaN(foo)) {
/* */
}
if (!isNaN(foo)) {
/* */
}
# Config
| Name | Type | Description |
|---|---|---|
enforceForSwitchCase | bool | Switch statements use === internally to match an expression, therefore switch (NaN) and case NaN will never match.This rule disables uses like that which are always incorrect (true by default) |
enforceForIndexOf | bool | Index functions like indexOf and lastIndexOf use === internally, therefore matching them against NaN will alwaysyield -1. This option disallows using indexOf(NaN) and lastIndexOf(NaN) (false by default) |
More incorrect examples
123 == NaN;
123 === NaN;
NaN === "abc";
NaN == "abc";
123 != NaN;
123 !== NaN;
NaN !== "abc";
NaN != "abc";
NaN < "abc";
"abc" < NaN;
NaN > "abc";
"abc" > NaN;
NaN <= "abc";
"abc" <= NaN;
NaN >= "abc";
"abc" >= NaN;
More correct examples
var x = NaN;
isNaN(NaN) === true;
isNaN(123) !== true;
Number.isNaN(NaN) === true;
Number.isNaN(123) !== true;
foo(NaN + 1);
foo(1 + NaN);
foo(NaN - 1)
foo(1 - NaN)
foo(NaN * 2)
foo(2 * NaN)
foo(NaN / 2)
foo(2 / NaN)
var x; if (x = NaN) { }
foo.indexOf(NaN)
foo.lastIndexOf(NaN)