MDN URL: https://developer.mozilla.org/en-US/docs/Web/JavaScript/A_re-introduction_to_JavaScript

What information was incorrect, unhelpful, or incomplete?

In the article (Under Number) it says that "You can test for NaN using the built-in isNaN() function:"
However, this fuction also returns true on lots of other inputs, other than NaN

isNaN('hello') // true
isNaN(function(){}) // true
isNaN({hello: 'world'}) // true

But other than that, it is inconsistent as well:

isNaN([1]) // false
isNaN([1,2]) // true
isNaN('1') // false

Specific section or headline?

under Numbers / You can test for NaN using the built-in isNaN() function:

What did you expect to see?

Instead of isNaN, Number.isNaN produces the expected result:

Number.isNaN('hello') // false
Number.isNaN(function(){}) // false
Number.isNaN({hello: 'world'}) // false
Number.isNaN([1,2]) // false

Browser support is a little bit worse for Number.isNaN when compared to isNaN but it's supported in all the current browsers, plus that if you actually need it, you can always use a polyfill, as seen on
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Number/isNaN#polyfill

Did you test this? If so, how?

Using Chrome DevTools