NaN
1 function w(w){console.log(w)} 2 w(null == null); 3 w(null === null); 4 w(NaN == NaN); //false 5 w(NaN === NaN); //false 6 w(NaN); //NaN 7 w('2' == 2) //true 8 w('2' === 2); //false 9 w(null == undefined) //true 10 w(null === undefined) //false
ECMA-262_ECMAScript®2015 Language Specification
6.1.6
The Number type has exactly 18437736874454810627(that is, 264-253+3) values,
representing the double-precision 64-bit format IEEE 754-2008values 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.)
In some implementations, external code might be able to detect a difference between various Not-a-Number values, but such behaviour is implementation-dependent; to ECMAScript code, all NaN values are indistinguishable from each other.
