igorpavlov igorpavlov - 4 months ago 8
Javascript Question

JavaScript - should Infinity and NaN be numbers?

Few experiments



I am going to write some Maths expressions in JS and wondered:

typeof(Infinity) // "number", in Maths it is NOT a number
typeof(NaN) // "number", how NaN (Not A Number) can actually be a number?


And just a few more experiments:

Infinity === Infinity // TRUE
Infinity * 2 === Infinity * 5 // TRUE, so 2===5 if Infinity !== 0?


However, this makes sense to me:

NaN * 2 === NaN * 5 // FALSE


Questions




  • Why JS is designed so? Was there a particular reason for that?

  • Are there potentials threats when writing Maths expressions in JS?


Answer

Why JS is designed so? Was there a particular reason for that?

JavaScript takes both NaN (including the fact that it's never equal to itself) and infinity directly from IEEE-754, the go-to standard for floating point numbers in computing, which is also used by many other languages. (Specifically, it uses a "quiet NaN".) IEEE-754, in turn, gets a lot of this from mathematic theory (e.g., Infinity + 1 === Infinity, apparently this is considered true for some kinds of numbers, but I am not a mathematician; I would assume the designers of the standard had a reason for following that definition).

Are there potentials threats when writing Maths expressions in JS?

I wouldn't call them threats, but: NaN will propagate throughout any calculation where it comes up. Other than that, you have the usual issues with precision discussed in this question and its answers.