Help me, please!
Why is the answer for string "The quick brown fox jumped over the lazy developer." is "The quick brown fox jump-\ned over the lazy develop-\ner." instead of 'The quick brown fox jump-\ned over the lazy develope-\nr.'?

JS: chai.config.truncateThreshold should be set to falsy value (0 / false)

Using splicing might not be the intended method or at least it can be hard to visualize how the string is expanding. An easier approach would be to concatenate the string from scratch and initiate different sub-approaches when encountering a space or upon pre-encounterance.

• An example in the description is required with explanations in comments specifically for strings-based katas that can become very long!

• Description does not mention whether input or strings of length less than 25 will be given. If so, such fixed and sample tests should be present

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Although I agree that it would make more sense, the expected result follows the rules of the description and kata so this is more a suggestion than an issue.

It looks like there is a bug in the test cases.
For input
'abcdefghijklmnopqrstuvw xyz'
the expected result is
'abcdefghijklmnopqrstuvw -\nxyz'
But it seems to me that the following result is more correct:
'abcdefghijklmnopqrstuvw \nxyz' (with one or two spaces, but not with a hyphen)

Either mark this kata as "puzzle" or add specification about edge cases mentioned in other comments.

random tests with author solution added. also added explicit edge case of the line being 25 characters long.

fixed

Nope, there's still no sample tests in sample test block ....

Sample and fixed tests have no error messages.

No random tests.

Update kata to Node 12. Added sample tests. Updated the test suite with useful messaging.

There are no random tests -- and never were.