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useIsNan (since v1.0.0)

Diagnostic Category: lint/correctness/useIsNan

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.

Examples

Section titled Examples

Invalid

Section titled Invalid
123 == NaN
correctness/useIsNan.js:1:1 lint/correctness/useIsNan  FIXABLE  ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ✖ Use the Number.isNaN function to compare with NaN.
  
  > 1 │ 123 == NaN
      │ ^^^^^^^^^^
    2 │ 
  
  ℹ Unsafe fix: Use Number.isNaN() instead.
  
    1   │ - 123·==·NaN
      1 │ + Number.isNaN(123)
    2 2 │   
  

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

  ✖ Use the Number.isNaN function to compare with NaN.
  
  > 1 │ 123 != NaN
      │ ^^^^^^^^^^
    2 │ 
  
  ℹ Unsafe fix: Use Number.isNaN() instead.
  
    1   │ - 123·!=·NaN
      1 │ + !Number.isNaN(123)
    2 2 │   
  

switch(foo) { case (NaN): break; }
correctness/useIsNan.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"
correctness/useIsNan.js:1:1 lint/correctness/useIsNan  FIXABLE  ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ✖ Use the Number.isNaN function to compare with NaN.
  
  > 1 │ Number.NaN == "abc"
      │ ^^^^^^^^^^^^^^^^^^^
    2 │ 
  
  ℹ Unsafe fix: Use Number.isNaN() instead.
  
    1   │ - Number.NaN·==·"abc"
      1 │ + Number.isNaN("abc")
    2 2 │

Valid

Section titled Valid
if (Number.isNaN(123) !== true) {}


foo(Number.NaN / 2)


switch(foo) {}
Section titled Related links