isNaN

The isNaN function in JavaScript is a global function that determines whether a value is NaN (Not-a-Number). It returns true if the value is NaN, and false otherwise. It's important to understand how isNaN works, as it can sometimes produce unexpected results due to its type coercion behavior.

When isNaN is called with a value, it first attempts to convert that value to a number. If the conversion results in NaN, then isNaN returns true. For example, isNaN("hello") returns true because the string "hello" cannot be converted to a valid number. Similarly, isNaN(undefined) returns true.

However, if the value is already a number, or if it can be successfully converted to a number, isNaN will return false. For instance, isNaN(123) returns false, and isNaN("123") also returns false because the string "123" is converted to the number 123. The special value NaN itself, when passed to isNaN, also results in true.

A more reliable way to check if a value is strictly NaN is to use Number.isNaN(). This method does not perform type coercion and will only return true if the provided value is actually the primitive NaN value. For example, Number.isNaN("hello") returns false, whereas isNaN("hello") returns true. Therefore, Number.isNaN() is generally preferred for accurately identifying NaN values.