> So it's Not-a-Number but isNumber returns true?

NaN is not a number (the concept) but it is a Number (the type).

> That would imply then that isNumber is named incorrectly.

Arguably, it is Number (the type) that is named incorrectly (a type called Number that includes a value called Not-a-Number seems problematic.) Maybe "Number" should be called "MaybeNumber".

And, perhaps more importantly, the fact that JS doesn't have a convenient type membership operator for primitive types is problematic, as if there was a convenient parallel to "instanceOf" for primitive types -- or if "instanceOf" just worked for them -- then libraries wouldn't need to provide something like what underscores isNumber does at all, and then any isNumber function would probably test "is the argument a number" rather than "is the argument a Number".

It's consistent with the names of other type-checking functions. isNumber checks if a value is an instance of the Number class just like isDate checks if a value is an instance of the Date class. If it were isNumericType, they'd get questions about why the function doesn't follow the existing naming convention.

The only reason this is confusing for people is because they aren't familiar with IEEE 754 floating-point numbers and their representations. But since floats are used in almost every programming language, it's something that any programmer should be familiar with. Once they learn exactly what NaN means in that context, then the confusion should disappear.