What is the usage of js isNaN?

In JavaScript, the `isNaN()` function is primarily used to determine if a given value is Not-a-Number. It's crucial for handling scenarios where mathematical operations or type conversions might result in an invalid numeric representation.

The `isNaN()` function first attempts to convert the input value to a number. If the conversion results in `NaN`, the function returns `true`; otherwise, it returns `false`. It's important to note that `isNaN()` has a quirk: it can return `true` for non-numeric values that can't be converted to numbers, such as strings or objects. For example, `isNaN("hello")` returns `true` because "hello" cannot be parsed into a number. On the other hand, `isNaN("123")` returns `false` because "123" can be successfully converted to the number 123. Likewise, `isNaN(123)` returns `false` since it is already a number.

A better alternative to the global `isNaN()` is `Number.isNaN()`, introduced in ECMAScript 2015 (ES6). This method does not perform any type coercion, and it only returns true if the value is exactly the `NaN` value, and false in any other case. Therefore, `Number.isNaN("hello")` will return `false`, because the value is not `NaN`. When dealing with user inputs or data where you expect numbers, it's generally safer to use `Number.isNaN()` to avoid unexpected behavior caused by type coercion. However, the basic `isNaN()` function is still valid and can be appropriate in certain situations if type coercion is the desired behaviour.

For example, imagine a form where a user enters their age. The input value will initially be a string. To ensure the user provided a valid number you might use either `isNaN()` or `Number.isNaN()`, but knowing the difference is critical for proper validation. Using `isNaN(userAge)` ensures that the provided age can be converted to number and it's not `NaN`, which can be suitable in some case. On the other hand, `Number.isNaN(Number(userAge))` ensures that the user input is exactly a `NaN` value after being converted to a number, this can be more strict and accurate.