Because NaN is unique in JavaScript by not being equal to anything, including itself, the results of comparisons to NaN are confusing:

• NaN === NaN or NaN == NaN evaluate to false
• NaN !== NaN or NaN != NaN evaluate to true

Therefore, this rule makes you use the isNaN() or Number.isNaN() to judge the value is NaN or not.

Invalid:

if (foo == NaN) {
  // ...
}

if (foo != NaN) {
  // ...
}

switch (NaN) {
  case foo:
    // ...
}

switch (foo) {
  case NaN:
    // ...
}

Valid:

if (isNaN(foo)) {
  // ...
}

if (!isNaN(foo)) {
  // ...
}