feat(eslint-plugin): [no-unnecessary-condition] flag unnecessary type predicates based on assignability #11735

azat-io · 2025-11-03T00:34:37Z

PR Checklist

Addresses an existing open issue: fixes Enhancement: [no-unnecessary-condition] detect unnecessary calls to Array.isArray #11716
That issue was marked as accepting prs
Steps in Contributing were taken

Overview

Adds detection of redundant Array.isArray() calls when checkTypePredicates is enabled.

Reports error when the argument is already typed as an array (including readonly arrays).

typescript-eslint · 2025-11-03T00:34:41Z

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kirkwaiblinger · 2025-11-03T09:20:18Z

packages/eslint-plugin/tests/rules/no-unnecessary-condition.test.ts

+
+    {
+      code: `
+const items: number[] | null = [1, 2, 3];


You'll want to do it like this, I think, here and elsewhere:

Suggested change

const items: number[] | null = [1, 2, 3];

declare const items: number[] | null;

TS ignores the | null because it can see that it's not null (playgroun)

kirkwaiblinger · 2025-11-03T09:38:42Z

packages/eslint-plugin/src/rules/no-unnecessary-condition.ts

If I replace this entire diff with

- if (typeOfArgument === typeGuardAssertedArgument.type) { + if ( + checker.isTypeAssignableTo( + typeOfArgument, + typeGuardAssertedArgument.type, + ) + ) {

(which is what was intended in #11716 (comment)), the only test case that doesn't pass (apart from spurious reasons) is

function processArray(arr: readonly string[]) { if (Array.isArray(arr)) { return arr.length; } }

So, I'm still thinking that changing to the assignability API makes sense, at least as a first step. In other words, let's take on the assignability API change before worrying about the edge case of readonly arrays with Array.isArray() (which, FWIW, I think is basically a corollary of microsoft/TypeScript#17002). What do you think?

Also cc @ronami

Done! Changed to use checker.isTypeAssignableTo() for Array.isArray

@azat-io almost! I really mean the entire diff, though, not just the Array.isArray case. The thought is to change all unnecessary type predicate detection to be based on assignability.

Note that this change would require accompanying report message changes and additional testing

Done! Changed to use isTypeAssignableTo() for all type predicates. Removed isArrayIsArrayCall() function and updated the tests accordingly.

codecov · 2025-11-03T10:41:15Z

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.67%. Comparing base (189a7f7) to head (e1abd45).

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #11735   +/-   ##
=======================================
  Coverage   90.66%   90.67%           
=======================================
  Files         518      518           
  Lines       52444    52478   +34     
  Branches     8690     8701   +11     
=======================================
+ Hits        47548    47584   +36     
+ Misses       4882     4880    -2     
  Partials       14       14

Flag	Coverage Δ
unittest	`90.67% <100.00%> (+<0.01%)`	⬆️

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

Files with missing lines	Coverage Δ
...slint-plugin/src/rules/no-unnecessary-condition.ts	`99.59% <100.00%> (+0.01%)`	⬆️
...ckages/rule-tester/src/utils/flat-config-schema.ts	`40.65% <ø> (ø)`

... and 1 file with indirect coverage changes

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kirkwaiblinger

I don't have time to do a fully detailed review right now, but jotting down the notes that I'll want to come back to. Feel free to look into these in the meantime:

We'll probably need to update the text of the typeGuardAlreadyIsType message.
The getConstrainedTypeAtLocation may actually be wrong/unnecessary now. Needs tests with generics to prove one way or another.
Like with no-unsafe-type-assertion, we should double-check and add testing around object literals and excess property checking.
Needs further tests around subtype cases.

…rray.isArray` calls

…ableTo` API for `Array.isArray` checks

…ype predicates

azat-io · 2025-11-09T21:33:15Z

Dialed the change back to focus solely on Array.isArray: the rule still reports when the argument type exactly matches the predicate type, and now additionally reports redundant Array.isArray checks when the argument is assignable to the guard’s array type (excluding any/unknown). Added valid/invalid coverage for unions, generics, and tuples so the new path stays constrained.

kirkwaiblinger reviewed Nov 3, 2025

View reviewed changes

kirkwaiblinger changed the title ~~feat(eslint-plugin): [no-unnecessary-condition] detect unnecessary Array.isArray calls~~ feat(eslint-plugin): [no-unnecessary-condition] flag unnecessary type predicates based on assignability Nov 3, 2025

kirkwaiblinger requested changes Nov 3, 2025

View reviewed changes

azat-io force-pushed the feat/no-unnecessary-condition-array branch 2 times, most recently from a9234f9 to 5bb6d0a Compare November 9, 2025 19:08

azat-io added 3 commits November 9, 2025 22:17

feat(eslint-plugin): [no-unnecessary-condition] detect unnecessary `A…

439da04

…rray.isArray` calls

refactor(eslint-plugin): [no-unnecessary-condition] use `isTypeAssign…

6157a5a

…ableTo` API for `Array.isArray` checks

refactor(no-unnecessary-condition): use assignability check for all t…

c13f7b7

…ype predicates

azat-io force-pushed the feat/no-unnecessary-condition-array branch 4 times, most recently from 7224f53 to c0cfba1 Compare November 9, 2025 20:42

fix(eslint-plugin): refine type guard assignability handling

e1abd45

azat-io force-pushed the feat/no-unnecessary-condition-array branch from c0cfba1 to e1abd45 Compare November 9, 2025 21:06

	const items: number[] \| null = [1, 2, 3];
	declare const items: number[] \| null;

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feat(eslint-plugin): [no-unnecessary-condition] flag unnecessary type predicates based on assignability #11735

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feat(eslint-plugin): [no-unnecessary-condition] flag unnecessary type predicates based on assignability #11735

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azat-io commented Nov 3, 2025

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