The merge function has been deprecated. You should consider updating to mergeLeft (it will make the placeholder redundant).

Using Ramda has no added value, and its use is not even tested.

Please specify and test a new Array of new Objects has to be returned.

The behaviour for empty input lists is specified only in an Example test. That is not the place for it - it should be in the Description.

It's also very illogical - null is to be returned if at least one element is not divisible by three. This is not the case. The mean of an empty list should be undefined, I think.

Can existing books have errorNotice: false or they are guaranteed to not have errorNotice?

Books with errors should be corrected - Expected: '[{ title: \'Foo\', errors: true, errorNotice: true }, { title: \'Bar\' }]', instead got: '[{ errorNotice: true, title: \'Foo\', errors: true }, { title: \'Bar\' }]'

assertSimilar shouldn't be used for objects because property enumeration order is unspecified.

• Needs random tests
• The behaviour for empty arrays are not specified
• The use of Ramda is not enforced

You can use Test.assertDeepEquals instead of Test.assertSimilar, which gets rid of all the unnecessary string escaping.

• Please add random tests
• Solutions like this should not pass ;-)