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

The ECMAScript (JavaScript) standard specifies that Numbers are IEEE 754 floats, which include NaN as a possible value.

ECMA 262 5e Section 4.3.19: Number value

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

ECMA 262 5e Section 4.3.23: NaN

Number value that is a IEEE 754 "Not-a-Number" value.

IEEE 754 on Wikipedia

The IEEE Standard for Floating-Point Arithmetic is a technical standard established by the Institute of Electrical and Electronics Engineers and the most widely used standard for floating-point computation [...]

The standard defines

• arithmetic formats: sets of binary and decimal floating-point data, which consist of finite numbers (including signed zeros and subnormal numbers), infinities, and special "not a number" values (NaNs)

[...]