use-isnan.md

require calls to isNaN() when checking for NaN (use-isnan)

In JavaScript, NaN is a special value of the Number type. It's used to represent any of the "not-a-number" values represented by the double-precision 64-bit format as specified by the IEEE Standard for Binary Floating-Point Arithmetic.

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, use Number.isNaN() or global isNaN() functions to test whether a value is NaN.

Rule Details

This rule disallows comparisons to 'NaN'.

Examples of incorrect code for this rule:

/*eslint use-isnan: "error"*/

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

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

Examples of correct code for this rule:

/*eslint use-isnan: "error"*/

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

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

Further Reading

• Use the isNaN function to compare with NaN