Oh yes indeed, sorry for my misreading ("optional start is optional")...

getSerie() that returns a series of n Harshad numbers, optional start value not included

Isn't that already there?

Nice kata, thanks!
However the description should state clearly that start parameter for getSerie()/get_series function is exclusive (Harshad.get_series(10,1000) does not include 1000, which is a Harshad number).

Oh, my bad. I misread except for the start argument in getSerie() which is optional and should default to 0 as the getSerie()'s start parameter is already assigned to 0.

And how is that an issue? The author may leave the initial solution completely empty for you to write everything yourself.

The getSerie() method in JavaScript does not have start parameter to 0.

I had to initialize it myself to clear the tests.

Fun Kata. Curious, what are Harshad numbers used for?

Java Translation translated to Java

check the given input variables in getSerie
One is an optional agrument

I get undefined as a start value in function getSerie.

I get undefiend as a start value every test in JS

Fixed both in python and ruby

In the ::is_valid method I use an Integer instance method called digits on the number argument in order to break the number into an array of its constituent digits, and I get this error message: <NoMethodError: undefined method `digits' for 1:Fixnum>. But the Integer class is the parent of Fixnum, so the method 'digits' should be defined for the Fixnum 1. What gives? These tests pass in irb. Oh, by the way, I'm coding in Ruby.

A few hints:

• sieve of Erosthenes is solid and fast way to generate primes; however, a lot depends on the implementation. I suggest to write your version and time it to generate primes up to 10^6, and try to improve it. You can use an online IDE, e.g. https://repl.it ; further hints: half sieve, "rwh primes"
• test small code snippets on http://www.pythontutor.com -- the less steps it requires, the faster it will be
• do more easy katas, e.g. about digits sum: there are much quicker ways to get the digit sum of a number
• don't repeat yourself (DRY): reuse your existing code -- don't write a validity chacker for each method, just call the existing one
• etc