Why Does JavaScript Return NaN? Understanding NaN and isNaN()

NaN (Not a Number) is a special numeric value in ECMAScript used to represent an operation that should produce a number but does not, preventing errors such as division by zero from halting execution.

NaN has two unusual characteristics: any operation involving NaN returns NaN, and NaN is not equal to any value, including itself; for example, NaN == NaN evaluates to false.

The ECMAScript isNaN() function accepts any type of argument and determines whether the value is “not a number.” It first attempts to convert the argument to a number; values that cannot be converted cause isNaN() to return true.

The example tests five different inputs: NaN (returns true), the number 10 and the string “10” (both return false because the string is convertible), the string “blue” (returns true), and the boolean true (converted to 1, returns false).

When isNaN() is called on an object, JavaScript first invokes the object's valueOf() method; if the result cannot be converted to a number, it then calls toString() and tests that result, following the standard execution flow for built‑in functions and operators.