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useIsNan

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Diagnostic Category: lint/correctness/useIsNan

Since: v1.0.0

Sources:

Require calls to isNaN() when checking for NaN.

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.

Note that Number.isNaN() and isNaN() do not have the same behavior. When the argument to isNaN() is not a number, the value is first coerced to a number. Number.isNaN() does not perform this coercion. Therefore, it is a more reliable way to test whether a value is NaN.

123 == NaN
code-block.js:1:1 lint/correctness/useIsNan  FIXABLE  ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

Use the Number.isNaN function to compare with NaN.

> 1 │ 123 == NaN
^^^^^^^^^^
2 │

Unsafe fix: Use Number.isNaN() instead.

1 - 123·==·NaN
1+ Number.isNaN(123)
2 2

123 != NaN
code-block.js:1:1 lint/correctness/useIsNan  FIXABLE  ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

Use the Number.isNaN function to compare with NaN.

> 1 │ 123 != NaN
^^^^^^^^^^
2 │

Unsafe fix: Use Number.isNaN() instead.

1 - 123·!=·NaN
1+ !Number.isNaN(123)
2 2

switch(foo) { case (NaN): break; }
code-block.js:1:20 lint/correctness/useIsNan ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

‘case NaN’ can never match. Use Number.isNaN before the switch.

> 1 │ switch(foo) { case (NaN): break; }
^^^^^
2 │

Number.NaN == "abc"
code-block.js:1:1 lint/correctness/useIsNan  FIXABLE  ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

Use the Number.isNaN function to compare with NaN.

> 1 │ Number.NaN == “abc”
^^^^^^^^^^^^^^^^^^^
2 │

Unsafe fix: Use Number.isNaN() instead.

1 - Number.NaN·==·abc
1+ Number.isNaN(abc)
2 2

if (Number.isNaN(123) !== true) {}
foo(Number.NaN / 2)
switch(foo) {}