You are mutating the input and that's what's causing your error. Do not change the input.

With the following test values:
[ { day: 'mon', from: '11:00', to: '23:00' },
{ day: 'tue', from: '11:00', to: '23:00' },
{ day: 'wed', from: '11:00', to: '23:00' },
{ day: 'thu', from: '11:00', to: '23:00' },
{ day: 'fri', from: '11:00', to: '23:00' },
{ day: 'sat', from: '11:00', to: '23:00' },
{ day: 'sun', from: '11:00', to: '23:00' } ]

I obtained this message:
"Expected: 'MON: 11:00 - 23:00\nSUN: 11:00 - 23:00', instead got: 'MON - SUN: 11:00 - 23:00'"

The expected results seem to be quite wrong since in the sentence of the Kata is remarked the next fact:
"If two or more days of the week in a row have the same working hours they should be concatenated and have the following format: MON - WED: 11:00 - 23:00."

It's nearly impossible to change a kata's rank once it has been set. The process of doing so is disruptive and labor-intensive and for that reason, it is almost never done. Something would have to be egregiously wrong with a kata's rank to get re-ranked (like a 1-kyu rank with an 8-kyu difficulty level).

Agree with other warriors on difficulty of the kata. 5kyu is a good spot

I still dont get how this can be kyu 6... but human readable time is kyu 5? I feel like this needs more time and brains to solve

This issue caused some very strange expected results on the tests.

That took a bit of time to solve. Good kata, but low reward for the effort. IMO, 5 kyu would be appropriate?

This is too hard to be a 6.

Nice kata to practice working on objects. Time-consuming but worthwhile. Could be 5 kyu.

How does this work?

Hello!

Reasult shows:
Expected: 'MON: 11:00 - 23:00\nTUE - SUN: null - null', instead got: 'MON: 11:00 - 23:00'
Expected: 'MON: 11:00 - 23:00\nTUE - SAT: null - null\nSUN: 11:00 - 23:00', instead got: 'MON: 11:00 - 23:00\nSUN: 11:00 - 23:00'
Expected: 'MON - TUE: 11:00 - 23:00\nWED - FRI: null - null\nSAT - SUN: 11:00 - 23:00', instead got: 'MON - TUE: 11:00 - 23:00\nSAT - SUN: 11:00 - 23:00'

Why expected output has unknown word: null??? 'null' really confuses me.

Returning empty array for empty timetable makes no sense whatsoever. It should return empty string.

Test cases starting from the 5th are vulnerable to input modification.