What's the polyfill for logb in Javascript?

Question

I need to use something like logb from C++ but Google searches turn up absolutely nothing useful and I'm having trouble understanding how to replicate the function based on that C++ document.

Does anyone know how to make a polyfill or equivalent function for JS?

Answer 1

From the document you linked

Returns the logarithm of |x|, using FLT_RADIX as base for the logarithm.

On most platforms, FLT_RADIX is 2, and thus this function is equivalent to log2 for positive values.

So you just need to compute whats the log2(|x|).

Math.log(Math.abs(x))/Math.log(2)

---

Logarithmic functions have a base, the most common one is 10 since we work in a base 10 numeric system. Common bases for computers is a base 2 since it works in a binary system. Converting bases can be simply done by working with this formula.

Log_b(x) = Log_v(x) / Log_v(b)

Simply take the Log(x) in any base you want (10 for instance) and divide it by the Log(b).

Heres a relevant link about it:

http://www.mathwords.com/c/change_of_base_formula.htm https://www.khanacademy.org/math/algebra2/exponential-and-logarithmic-functions/change-of-base-formula-for-logarithms/a/logarithm-change-of-base-rule-intro

You might right a function like:

// get the log value of x using base b
function logb(x, b) {
  return Math.log(x)/Math.log(b);
}