value

The value to be tested.

true if the given value is NaN after being converted to a number; otherwise, false.

isNaN() is a function property of the global object.

For number values, isNaN() tests if the number is the value NaN. When the argument to the isNaN() function is not of type Number, the value is first coerced to a number, and the resulting value is then compared against NaN.

This behavior of isNaN() for non-numeric arguments can be confusing! For example, an empty string is coerced to 0, while a boolean is coerced to 0 or 1; both values are intuitively "not numbers", but they don't evaluate to NaN, so isNaN() returns false. Therefore, isNaN() answers neither the question "is the input the floating point NaN value" nor the question "is the input not a number".

Number.isNaN() is a more reliable way to test whether a value is the number value NaN or not. Alternatively, the expression x !== x can be used, and neither of the solutions is subject to the false positives that make the global isNaN() unreliable. To test if a value is a number, use typeof x === "number".

The isNaN() function answers the question "is the input functionally equivalent to NaN when used in a number context". If isNaN(x) returns false, you can use x in an arithmetic expression as if it's a valid number that's not NaN. If isNaN(x) returns true, x will get coerced to NaN and make most arithmetic expressions return NaN (because NaN propagates). You can use this, for example, to test whether an argument to a function is arithmetically processable (usable "like" a number), and handle values that are not number-like by throwing an error, providing a default value, etc. This way, you can have a function that makes use of the full versatility JavaScript provides by implicitly converting values depending on context.

Note how isNaN() returns true for values that are not the value NaN but are not numbers either:

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: 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; Date objects can be converted to a number (timestamp)
isNaN(new Date().toString()); // true; the string representation of a Date object cannot be parsed as a number

// Arrays
isNaN([]); // false; the primitive representation is "", which coverts to the number 0
isNaN([1]); // false; the primitive representation is "1"
isNaN([1, 2]); // true; the primitive representation is "1,2", which cannot be parsed as number

• NaN
• Number.isNaN()