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fix(eslint-plugin): [prefer-optional-chain] skip optional chaining when it could change the result #11702

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Merged
merged 6 commits into from
Oct 19, 2025
+243 −719
Merged

fix(eslint-plugin): [prefer-optional-chain] skip optional chaining when it could change the result #11702

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66b1c1a
fix : remove isAlwaysTruthyOperand
mdm317 Oct 16, 2025
a382696
test : fix base case
mdm317 Oct 16, 2025
ca323ed
test : add some case for array/index access
mdm317 Oct 16, 2025
87605b0
fix : lint err
mdm317 Oct 16, 2025
837db2a
refactor : undeclaredVar
mdm317 Oct 17, 2025
fd98e65
fix : weird test csae
mdm317 Oct 17, 2025
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63 changes: 5 additions & 58 deletions packages/eslint-plugin/src/rules/prefer-optional-chain-utils/analyzeChain.ts
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@kirkwaiblinger kirkwaiblinger Oct 17, 2025 •
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Looks like there's a minor bug somewhere with:

declare const foo: { bar: number | null } | undefined;
if (foo === undefined || foo.bar === null) { // suggests to foo?.bar === null, which is not equivalent.
} else {
  foo.bar.toExponential(); // after accepting fix, this crashes if `foo` is `undefined`.
}

I guess this should either not report or it could maybe try to use foo?.bar == null? But this seems kind of bad since it is impossible to work with for an === nullishValue-prefering user

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@mdm317 mdm317 Oct 17, 2025

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typescript-eslint/packages/eslint-plugin/src/rules/prefer-optional-chain-utils/analyzeChain.ts

Lines 277 to 284 in 3f5fbf6

if (
includesType(parserServices, operand.comparedName, ts.TypeFlags.Null)
) {
// we know the next operand is not a `null` check and that this
// operand includes `null` - which means that making this an
// optional chain would change the runtime behavior of the expression
return null;
}

I believe this bug didn’t originate from this PR — it seems to have existed beforehand.
Since I wasn’t the original author, it might take some time to fix it properly.

If possible, we could apply a quick hotfix for the current issue first and then open a new issue to track it, but it’s just a suggestion — feel free to decide as you think best.

In any case, I’ll start digging into it now.

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@kirkwaiblinger kirkwaiblinger Oct 17, 2025

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That's fine if we defer it!

Just out of curiosity, do you mean to say it didn't originate in this PR, ie #11702, or in #11533? (either way we can defer it just good to know)

Original file line number Diff line number Diff line change
Expand Up @@ -10,7 +10,7 @@ import type {
} from '@typescript-eslint/utils/ts-eslint';

import { AST_NODE_TYPES } from '@typescript-eslint/utils';
import { isFalsyType, unionConstituents } from 'ts-api-utils';
import { unionConstituents } from 'ts-api-utils';
import * as ts from 'typescript';

import type { LastChainOperand, ValidOperand } from './gatherLogicalOperands';
Expand Down Expand Up @@ -48,40 +48,6 @@ function includesType(
return false;
}

function isAlwaysTruthyOperand(
comparedName: TSESTree.Node,
nullishComparisonType: NullishComparisonType,
parserServices: ParserServicesWithTypeInformation,
): boolean {
const ANY_UNKNOWN_FLAGS = ts.TypeFlags.Any | ts.TypeFlags.Unknown;
const comparedNameType = parserServices.getTypeAtLocation(comparedName);

if (isTypeFlagSet(comparedNameType, ANY_UNKNOWN_FLAGS)) {
return false;
}
switch (nullishComparisonType) {
case NullishComparisonType.Boolean:
case NullishComparisonType.NotBoolean: {
const types = unionConstituents(comparedNameType);
return types.every(type => !isFalsyType(type));
}
case NullishComparisonType.NotStrictEqualUndefined:
case NullishComparisonType.NotStrictEqualNull:
case NullishComparisonType.StrictEqualNull:
case NullishComparisonType.StrictEqualUndefined:
return !isTypeFlagSet(
comparedNameType,
ts.TypeFlags.Null | ts.TypeFlags.Undefined,
);
case NullishComparisonType.NotEqualNullOrUndefined:
case NullishComparisonType.EqualNullOrUndefined:
return !isTypeFlagSet(
comparedNameType,
ts.TypeFlags.Null | ts.TypeFlags.Undefined,
);
}
}

function isValidAndLastChainOperand(
ComparisonValueType: TSESTree.Node,
comparisonType: ComparisonType,
Expand Down Expand Up @@ -710,20 +676,6 @@ export function analyzeChain(
}
break;
}
case NullishComparisonType.StrictEqualNull:
case NullishComparisonType.NotStrictEqualNull: {
if (
comparisonResult === NodeComparisonResult.Subset &&
isAlwaysTruthyOperand(
lastOperand.comparedName,
lastOperand.comparisonType,
parserServices,
)
) {
lastChain = operand;
}
break;
}
}
}
maybeReportThenReset();
Expand Down Expand Up @@ -768,16 +720,11 @@ export function analyzeChain(
: isValidOrLastChainOperand;
if (
comparisonResult === NodeComparisonResult.Subset &&
(isAlwaysTruthyOperand(
lastOperand.comparedName,
lastOperand.comparisonType,
isValidLastChainOperand(
lastChainOperand.comparisonValue,
lastChainOperand.comparisonType,
parserServices,
) ||
isValidLastChainOperand(
lastChainOperand.comparisonValue,
lastChainOperand.comparisonType,
parserServices,
))
)
) {
lastChain = lastChainOperand;
}
Expand Down
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