What is the purpose of the `isNaN()` function?

Q: What is the purpose of the `isNaN()` function?

isNaN(123); // false (It is a number) isNaN("123"); // false (String "123" can be converted to number 123) isNaN(NaN); // true (It is NaN) isNaN("Hello"); // true (String "Hello" cannot be a number) isNaN(undefined); // true (undefined converts to NaN) isNaN() doesn't just check if a value is currently NaN; it checks if the value cannot be converted into a number. This leads to some very famous JavaScript quirks.

Difficulty: 3/10

type coercion, validation, numeric parsing

The `isNaN()` function is used to determine whether a value is NaN (Not-a-Number) or not. It returns `true` if the value is not a valid number; otherwise, it returns `false`.

javascript

isNaN() doesn't just check if a value is currently NaN; it checks if the value cannot be converted into a number. This leads to some very famous JavaScript quirks.

Scenario Questions

0-2 years experience

1How would you check that a value entered in a form field is a number before you add it to a total?
2What does isNaN('123') return and why?
3If you call isNaN(undefined), what result do you get and what does it mean?

Follow-up Questions

Can you show a case where isNaN returns true for a value you expected to be numeric?
How does JavaScript's type coercion affect the result of isNaN?
What modern alternative would you use instead of the global isNaN?

What is the purpose of the `isNaN()` function?

The isNaN() function is used to determine whether a value is NaN (Not-a-Number) or not. It returns true if the value is not a valid number; otherwise, it returns false.

Scenario Questions

Follow-up Questions

The `isNaN()` function is used to determine whether a value is NaN (Not-a-Number) or not. It returns `true` if the value is not a valid number; otherwise, it returns `false`.