feat(eslint-plugin): [prefer-nullish-coalescing]: add support for assignment expressions

[JoshuaKGoldberg]

PR Checklist

• Addresses an existing open issue: fixes Enhancement: [prefer-nullish-coalescing] recognize "||=" #5229
• That issue was marked as accepting prs
• Steps in CONTRIBUTING.md were taken

Overview

Adds ||= to ??= support to the rule by reusing much of the existing rule's logic.

Note that this is a breaking change, as it targets a significant new area of syntax. We'll probably have to wait until 6.X to merge. 😕

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[typescript-eslint]

Thanks for the PR, @JoshuaKGoldberg!

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 #5234 (9678f6c) into v6 (c4e0d86) will increase coverage by 0.11%.
The diff coverage is 100.00%.

Additional details and impacted files

@@            Coverage Diff             @@
##               v6    #5234      +/-   ##
==========================================
+ Coverage   91.33%   91.44%   +0.11%     
==========================================
  Files         361      355       -6     
  Lines       12044    12060      +16     
  Branches     3506     3531      +25     
==========================================
+ Hits        11000    11028      +28     
+ Misses        748      733      -15     
- Partials      296      299       +3     

Flag	Coverage Δ
unittest	91.44% <100.00%> (+0.11%)	⬆️

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

Impacted Files	Coverage Δ
...lint-plugin/src/rules/prefer-nullish-coalescing.ts	100.00% <100.00%> (ø)
packages/scope-manager/src/analyze.ts	57.89% <0.00%> (-7.49%)	⬇️
packages/scope-manager/src/ScopeManager.ts	77.21% <0.00%> (-2.54%)	⬇️
packages/typescript-estree/src/convert.ts	97.59% <0.00%> (-0.30%)	⬇️
...ackages/scope-manager/src/referencer/Referencer.ts	95.90% <0.00%> (-0.07%)	⬇️
packages/utils/src/ts-eslint/CLIEngine.ts	33.33% <0.00%> (ø)
packages/utils/src/eslint-utils/RuleCreator.ts	75.00% <0.00%> (ø)
...ges/typescript-estree/src/create-program/shared.ts	83.33% <0.00%> (ø)
...pt-estree/src/create-program/createWatchProgram.ts	78.06% <0.00%> (ø)
...-estree/src/create-program/createProjectProgram.ts	93.47% <0.00%> (ø)
... and 14 more

[jakebailey]

Thanks for working on this! I'm currently running this on the TS codebase (as in some benchmarks, even our downleveled code for ?? with explicit undefined/null checks is faster than the boolean conversion; benchmark). I'll just pull this into my local fork for now, if it's not going in any time soon.

(I have to fork in order to skip the rule when the left side is potentially a string/number/boolean; applying the fixes from this rule without review breaks a few things as the behavior is not equivalent.)

[bradzacher]

note that technically a ||= b is equivalent to a || (a = b), not a = a || b.
(similarly a ??= b is a ?? (a = b), not a = a ?? b)

This is a very slight difference that makes them marginally faster because you only assign if it's required.