isNaN() vs Number.isNaN() in JavaScript

What is the global NaN

NaN is a property of the global object that represents a Not-A-Number.

What is isNaN(value)

a function that checks if value is NaN after trying converting it to a number.

What is Number.isNaN(value)

a static method of the Number class determines whether the passed value is the number value NaN

The difference

the main difference is that isNaN try to converts the passed value to a number then checks if it is NaN, while the Number.isNaN() check the passed value as it is, so non-numbers always return false.

examples:

isNaN('test'); // true // it trys to convert "test" to a number returns NaN then check if the converted value is NaN
Number.isNaN('test'); // false  // check directly if the converted value is NaN

// value is NaN
isNaN(NaN); // true
Number.isNaN(NaN); // true

// value is undefined
isNaN(undefined); // true
Number.isNaN(undefined); // false

// value is null
isNaN(null); // false // is coerced to 0
Number.isNaN(null); // false

// value is a boolean
isNaN(true); // false // is coerced to 1
Number.isNaN(true); // false

// value is a boolean
isNaN(false); // false // is coerced to 0
Number.isNaN(false); // false

// value is an object
isNaN({}); // true
Number.isNaN({}); // false

// value is a number
isNaN(37); // false
Number.isNaN(37); // false

// value is a date
isNaN(new Date()); // false // coerced to a number (timestamp)
Number.isNaN(new Date()); // false

// value is "NaN"
isNaN("NaN"); // true // coerced to NaN
Number.isNaN("NaN"); // false

// value is empty string
isNaN(" "); // false // coerced to 0
Number.isNaN(" "); // false

// value is empty array
isNaN([]); // false // coerced to 0
Number.isNaN([]); // false

Number.isNaN is a more safe and robust version of isNaN.

You can find more details:

NaN

isNaN()

Number.isNaN()