Fix last TA module of a lesson type cannot be removed

[zehata]

• Fixed TA modules cannot be deselected if they are the only one or the last one of their type

Context

Resolves #4280 . As reported by quite a number of users starting at https://t.me/NUSMods/13309 , my implementation in #4225 causes the behavior to be different from the original specification: the last lesson of a lesson type cannot be removed for a TA module.

Implementation

• Modified the logic to allow any lessons to be removed as long as there is at least 1 left in the module
• Refactored the logic into a util for more convenient testing
• Added tests for the intended behavior of the timetable hydration logic

Lessons that are the only one of their lesson type can now be deselected

Lessons that are the last one of their module cannot be deselected

Lessons in non-TA modules are not affected by this change

Other Information

[vercel]

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[codecov]

Codecov Report

❌ Patch coverage is 80.85106% with 9 lines in your changes missing coverage. Please review.
✅ Project coverage is 57.00%. Comparing base (988c6fd) to head (28cea77).
⚠️ Report is 163 commits behind head on master.

Files with missing lines	Patch %	Lines
website/src/views/timetable/TimetableContent.tsx	10.00%	9 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4279      +/-   ##
==========================================
+ Coverage   54.52%   57.00%   +2.47%     
==========================================
  Files         274      297      +23     
  Lines        6076     6933     +857     
  Branches     1455     1674     +219     
==========================================
+ Hits         3313     3952     +639     
- Misses       2763     2981     +218     

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Project	Deployment	Review	Updated (UTC)
nusmods-export	Ready	Preview, Comment	Jan 6, 2026 2:29am
nusmods-website	Ready	Preview, Comment	Jan 6, 2026 2:29am

[zehata]

@leslieyip02 Btw, while writing this patch, I realized that there is some refactoring that should be done, but it might be better to do it together with changing TimetableContent to a functional component.

Specifically, currently I am passing a lot of parameters to the getInteractableLessons and hydrateInteractability functions, and I think it would be better to refactor these props as an interface instead

Something I would like to ask for help for is to check if the test cases I wrote for getInteractableLessons is missing any test cases.

[leslieyip02]

The deployment preview seems to work fine. Should be good once we add a few more test cases.

Regarding the refactor, I'll leave it up to you but I think it's not too urgent right now.

[zehata]

@leslieyip02 Thanks! I will work on fixing the regression and test cases so we can push the fix out to users first. The refactor will come later.

[zehata]

While reading your comment
#4279 (comment) I realized that I really should be writing these test in the conditional style instead, so I basically rewrote the entire test. There will be quite a lot of new things to review, sorry about that.

[zehata]

@leslieyip02 I forgot to mention, while writing the tests, I realized we don't have a sample module similar to CG1111A where there are modules with multiple classNos, and each classNo has multiple lessons.

I don't think it's urgent, so I think will add that on once this patch is out.

[leslieyip02]

LGTM.