#use-isnan

#Configuration

import { defineConfig, js } from '@rslint/core';

export default defineConfig([
  js.configs.recommended,
  {
    rules: {
      'use-isnan': 'error',
    },
  },
]);

#Rule Details

Requires calls to isNaN() when checking for NaN. Because NaN is unique in JavaScript in that it is not equal to anything, including itself, the results of comparisons to NaN are confusing: NaN === NaN is false. Therefore, use Number.isNaN() or the global isNaN() function to test whether a value is NaN.

#Options

• enforceForSwitchCase (default: true): Disallows switch(NaN) and case NaN: in switch statements.
• enforceForIndexOf (default: false): Disallows calling indexOf and lastIndexOf with NaN as an argument.

Examples of incorrect code for this rule:

if (foo == NaN) {
}

if (foo === NaN) {
}

if (foo !== NaN) {
}

switch (NaN) {
  case foo:
    break;
}

switch (foo) {
  case NaN:
    break;
}

Examples of correct code for this rule:

if (isNaN(foo)) {
}

if (Number.isNaN(foo)) {
}

if (!isNaN(foo)) {
}

#Original Documentation

• ESLint use-isnan