Why is isNaN(null) == false in JS?

by

I believe the code is trying to ask, “is x numeric?” with the specific case here of x = null. The function isNaN() can be used to answer this question, but semantically it’s referring specifically to the value NaN. From Wikipedia for NaN:

NaN (Not a Number) is a value of the numeric data type representing an undefined or unrepresentable value, especially in floating-point calculations.

In most cases we think the answer to “is null numeric?” should be no. However, isNaN(null) == false is semantically correct, because null is not NaN.

Here’s the algorithmic explanation:

The function isNaN(x) attempts to convert the passed parameter to a number1 (equivalent to Number(x)) and then tests if the value is NaN. If the parameter can’t be converted to a number, Number(x) will return NaN2. Therefore, if the conversion of parameter x to a number results in NaN, it returns true; otherwise, it returns false.

So in the specific case x = null, null is converted to the number 0, (try evaluating Number(null) and see that it returns 0,) and isNaN(0) returns false. A string that is only digits can be converted to a number and isNaN also returns false. A string (e.g. 'abcd') that cannot be converted to a number will cause isNaN('abcd') to return true, specifically because Number('abcd') returns NaN.

In addition to these apparent edge cases are the standard numerical reasons for returning NaN like 0/0.

As for the seemingly inconsistent tests for equality shown in the question, the behavior of NaN is specified such that any comparison x == NaN is false, regardless of the other operand, including NaN itself1.

More Related Contents:

  1. What exactly is Type Coercion in Javascript?
  2. What is the difference between a User and a GuildMember in discord.js?
  3. Running javascript in Selenium using Python
  4. Does every Javascript function have to return a value?
  5. How can I scrape pages with dynamic content using node.js?
  6. Add st, nd, rd and th (ordinal) suffix to a number
  7. Intercept a form submit in JavaScript and prevent normal submission
  8. plain count up timer in javascript
  9. Maximum call stack size exceeded on npm install
  10. Using jQuery to compare two arrays of Javascript objects
  11. Uploading ‘canvas’ image data to the server
  12. Creating a jQuery object from a big HTML-string
  13. Have I reached the limits of the size of objects JavaScript in my browser can handle?
  14. How to change a class CSS with a Greasemonkey/Tampermonkey script?
  15. Listener for property value changes in a Javascript object
  16. Get country from latitude longitude
  17. What is the purpose of the HTML “no-js” class?
  18. How do I wait for a JavaScript __doPostBack call through Selenium and WebDriver
  19. How to save data from a form with HTML5 Local Storage?
  20. How to Determine if ::before is applied to an element?
  21. Submit form after calling e.preventDefault()
  22. Mongoose add multiple object to array if not exist based
  23. Rendering an array.map() in React
  24. JavaScript for loop index strangeness [duplicate]
  25. Zip arrays in JavaScript?
  26. Why are javascript promises asynchronous when calling only synchronous functions?
  27. Submit jQuery UI dialog on
  28. Split a Javascript class (ES6) over multiple files?
  29. javascript – pass selected value from popup window to parent window input box
  30. Use JavaScript variable as function name?

Leave a Comment