nan.js

isNaN(NaN)        // true
isNaN(undefined)  // true
isNaN({})         // true

isNaN(true)       // false
isNaN(null)       // false
isNaN(37)         // false

// strings
isNaN('37')      // false: "37" is converted to the number 37 which is not NaN
isNaN('37.37')   // false: "37.37" is converted to the number 37.37 which is not NaN
isNaN("37,5")    // true
isNaN('123ABC')  // true:  parseInt("123ABC") is 123 but Number("123ABC") is NaN
isNaN('')        // false: the empty string is converted to 0 which is not NaN
isNaN(' ')       // false: a string with spaces is converted to 0 which is not NaN

// dates
isNaN(new Date())                // false
isNaN(new Date().toString())     // true