Why does typeof NaN return 'number'?

Question

Just out of curiosity.

It doesn\'t seem very logical that typeof NaN is number. Just like NaN === NaN or NaN == NaN returning

Answer 1

typeof NaN returns 'number' because:

• ECMAScript spec says the Number type includes NaN:

  4.3.20 Number type

  set of all possible Number values including the special “Not-a-Number” (NaN) values, positive infinity, and negative infinity

• So typeof returns accordingly:

  11.4.3 The typeof Operator

  The production UnaryExpression : typeof UnaryExpression is evaluated as follows:

  1. Let val be the result of evaluating UnaryExpression.
  2. If Type(val) is Reference, then
     1. If IsUnresolvableReference(val) is true, return "undefined".
     2. Let val be GetValue(val).
  3. Return a String determined by Type(val) according to Table 20.
     ​

                  Table 20 — typeof Operator Results
  ==================================================================
  |        Type of val         |              Result               |
  ==================================================================
  | Undefined                  | "undefined"                       |
  |----------------------------------------------------------------|
  | Null                       | "object"                          |
  |----------------------------------------------------------------|
  | Boolean                    | "boolean"                         |
  |----------------------------------------------------------------|
  | Number                     | "number"                          |
  |----------------------------------------------------------------|
  | String                     | "string"                          |
  |----------------------------------------------------------------|
  | Object (native and does    | "object"                          |
  | not implement [[Call]])    |                                   |
  |----------------------------------------------------------------|
  | Object (native or host and | "function"                        |
  | does implement [[Call]])   |                                   |
  |----------------------------------------------------------------|
  | Object (host and does not  | Implementation-defined except may |
  | implement [[Call]])        | not be "undefined", "boolean",    |
  |                            | "number", or "string".            |
  ------------------------------------------------------------------
  

This behavior is in accordance with IEEE Standard for Floating-Point Arithmetic (IEEE 754):

4.3.19 Number value

primitive value corresponding to a double-precision 64-bit binary format IEEE 754 value

4.3.23 NaN

number value that is a IEEE 754 “Not-a-Number” value

8.5 The Number Type

The Number type has exactly 18437736874454810627 (that is, 2⁵³−2⁶⁴+3) values, representing the double-precision 64-bit format IEEE 754 values as specified in the IEEE Standard for Binary Floating-Point Arithmetic, except that the 9007199254740990 (that is, 2⁵³−2) distinct “Not-a-Number” values of the IEEE Standard are represented in ECMAScript as a single special NaN value. (Note that the NaN value is produced by the program expression NaN.)