JavaScript: NaN

Meaning of NaN

NaN means Not a Number. It is typically returned when converting a value to number that doesn't make sense.

// convert string to number
// if arg does not make sense, result is NaN
console.log( Number.isNaN( Number( "xyz") ) );

NaN is a Property

NaN is the value of the property key "NaN" of the Global Object.

console.log( Number.isNaN( window["NaN"] ) );

NaN is a Literal Expression

NaN is a literal value. For example, you can write let x = NaN;

[see Number Constructor]

Type of NaN

Type of NaN is "number".

console.log( typeof NaN === "number" );

Test Equality of NaN

NaN === NaN return false.

console.log( (NaN === NaN) === false);

To test equality of NaN, use Object.is .

To test if a value is NaN, use Number.isNaN.