In JavaScript my code fails the tests due to timeout. Fewer (or simpler) tests would fix that.

I suggest making this kata at least a bit more difficult. Here are some ideas:

• Adding a second argument and checking if the first is divisible by the second;
• Checking for negative numbers;

If no initialValue was provided, then previousValue will be equal to the first value in the array and currentValue will be equal to the second. It is a TypeError if the array contains no elements and initialValue is not provided.

Source: http://www.ecma-international.org/ecma-262/6.0/

I didn't want to deal with throwing errors - handling empty arrays when no initial value is present is not the that essential for this kata.

I'd suggest adding some more details on how non letter characters should be handled.

Fully agreed - I've changed the kata to ask for a more traditional behaviour - if the initial value is not set it should use the first element of the array as the first element.

Thanks for the tip. I'm using assertSimilar in the tests for arrays.

Thanks for noticing - fixed.