As above, a unit test exists checking logBase 10 100, however if this had checked logBase 10 1000, it would have caught the fact that logBase 10 x implemented as log x / log 10 (using JavaScript's natural logarithm) can yield results slightly less than an integer even for small power of 10 inputs. This unfortunately complicates a common use case for logBase 10: finding it's floor in order to determine a exponent when formatting a number in scientific notation.

JavaScript's log10 is provided separately specifically to address this imprecision, and indeed it returns an integer for all powers of 10 ranging from -311 to 308 (i.e., all those representable as normal IEEE 754 binary64 floats and even a few of the subnormals). Accordingly, I respectfully recommend either adding a corresponding log10 function in Elm or a special case for logBase 10 that instead calls JavaScript's log10.