fix(eslint-plugin): [prefer-readonly] report if a member's property is reassigned

[islandryu]

PR Checklist

• Addresses an existing open issue: fixes Bug: [prefer-readonly] does not report if a member's property is reassigned #6005
• That issue was marked as accepting prs
• Steps in CONTRIBUTING.md were taken

Overview

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[typescript-eslint]

Thanks for the PR, @islandryu!

typescript-eslint is a 100% community driven project, and we are incredibly grateful that you are contributing to that community.

The core maintainers work on this in their personal time, so please understand that it may not be possible for them to review your work immediately.

Thanks again!

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[codecov]

Codecov Report

Merging #6043 (be983e9) into main (d69fdf4) will increase coverage by 0.00%.
The diff coverage is 100.00%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6043   +/-   ##
=======================================
  Coverage   91.27%   91.28%           
=======================================
  Files         366      366           
  Lines       12362    12363    +1     
  Branches     3616     3617    +1     
=======================================
+ Hits        11284    11285    +1     
  Misses        768      768           
  Partials      310      310           

Flag	Coverage Δ
unittest	91.28% <100.00%> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
...ackages/eslint-plugin/src/rules/prefer-readonly.ts	99.09% <100.00%> (+<0.01%)	⬆️

[JoshuaKGoldberg]

Code change looks great to start, thanks for sending this in @islandryu! 🙌 Just requesting changes for a bit more test coverage.

[JoshuaKGoldberg]

A few more test cases to consider...

this.testObj.prop;
this.testObj?.prop;
this.testObj!.prop;
this.testObj!.prop = '';
this.testObj.prop.prop = '';
this.testObj.prop.doesSomething();
this.testObj?.prop.prop;
this.testObj?.prop?.prop;
this.testObj.prop?.prop;
this.testObj!.prop?.prop;

For any change to rule logic, it's generally good to try to add both valid and invalid test cases.

[JoshuaKGoldberg]

@islandryu by "test cases" I do mean cases plural - we generally try to keep these separate, so that each test case tests one case. It's easy to miss incorrect or mismatched errors when they're all lumped together. Could you split them up please?

[islandryu]

@JoshuaKGoldberg
Thank you!
I've added a test case!

[JoshuaKGoldberg]

Sorry, I might not have been very clear - I'm hoping for quite a few cases to test this. It's a behavior that has a lot of edge cases we should test for individually. Thanks!

[JoshuaKGoldberg]

@islandryu by "test cases" I do mean cases plural - we generally try to keep these separate, so that each test case tests one case. It's easy to miss incorrect or mismatched errors when they're all lumped together. Could you split them up please?

[islandryu]

@JoshuaKGoldberg
Thank you.
Is it correct like this?

[JoshuaKGoldberg]

Yup, this is great - thanks for sending this in! 🙌