feat: Port ESLint core rule no-constant-binary-expression

[delino]

Summary

This PR ports the ESLint core rule no-constant-binary-expression to rslint, disallowing expressions where the operation doesn't affect the value.

Follow-up task of PR #73.

Implementation Details

• ✅ Created new rule in internal/rules/no_constant_binary_expression/
• ✅ Detects constant binary expressions with always-predictable results
• ✅ Detects logical short-circuit operators (&&, ||, ??) with constant operands
• ✅ Detects comparisons to newly constructed objects
• ✅ Detects comparisons with constant nullishness
• ✅ Detects comparisons with constant boolean values
• ✅ Registered in global rule registry

Rule Behavior

The rule prevents binary expressions where the operation doesn't affect the outcome. This includes comparisons that always evaluate to true/false and logical expressions that always or never short-circuit.

Invalid Patterns

// Constant short-circuit
[] && greeting;
true && hello;
({}) ?? foo;

// Constant comparisons
[] == true;
({}) === null;
true === true;

// Newly constructed object comparisons - can never be equal
x === {};
x === [];
[a] == [a];
({}) == [];

Valid Patterns

// Variable references (not constant)
bar && foo;
foo == true;
x === null;

// Function calls (not constant)
foo() && bar;
delete bar.baz && foo;

// Template literals with expressions
`${bar}` && foo;

// Assignment expressions
(x += 1) && foo;

Test Coverage

• ✅ Ported comprehensive test cases from ESLint's test suite
• ✅ 18 valid test cases covering various scenarios
• ✅ 68 invalid test cases with expected error detection
• ✅ Tests include:
  • Constant short-circuit with &&, ||, ?? operators
  • Constant binary operands with ==, !=, ===, !== operators
  • Comparisons to newly constructed objects
  • Boolean, string, and numeric literal comparisons
  • Built-in constructor calls (Boolean, String, Number)
  • Unary negation operators
  • Shadowed built-in functions
  • Template literals with variables

Test Plan

• Rule implementation follows rslint patterns
• All test cases ported from ESLint
• Tests pass (requires full build environment with submodules)
• Manual testing with example code
• Integration with existing linter setup

References

• ESLint Rule: https://eslint.org/docs/latest/rules/no-constant-binary-expression
• ESLint Source: https://github.com/eslint/eslint/blob/main/lib/rules/no-constant-binary-expression.js
• ESLint Tests: https://github.com/eslint/eslint/blob/main/tests/lib/rules/no-constant-binary-expression.js
• Related PR feat: Port ESLint core rule no-const-assign #73: feat: Port ESLint core rule no-const-assign #73

Files Changed

• internal/config/config.go - Added rule registration (2 lines)
• internal/rules/no_constant_binary_expression/no_constant_binary_expression.go - Complete rule implementation (353 lines)
• internal/rules/no_constant_binary_expression/no_constant_binary_expression_test.go - Comprehensive test suite (430 lines)

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[kdy1]

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[kdy1]

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Objective

Port the ESLint core rule no-constant-binary-expression to rslint (Go implementation).

Documentation & Resources

• Rule documentation: https://eslint.org/docs/latest/rules/no-constant-binary-expression
• ESLint source code: https://github.com/eslint/eslint/blob/main/lib/rules/no-constant-binary-expression.js
• ESLint test cases: https://github.com/eslint/eslint/blob/main/tests/lib/rules/no-constant-binary-expression.js

Rule Description

Disallows expressions where the operation doesn't affect the value.

Implementation Requirements

1. Create directory internal/rules/no_constant_binary_expression/
2. Implement rule from scratch in no_constant_binary_expression.go
3. Copy all ESLint test cases
4. Create no_constant_binary_expression_test.go
5. Register the rule

Test Requirements

• Port ALL test cases
• All tests must pass

Success Criteria

• Full implementation
• All tests passing

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[claude]

Pull Request Review: no-constant-binary-expression

Summary

This PR successfully ports the ESLint core rule no-constant-binary-expression to rslint. The implementation is well-structured and follows the project's conventions. Overall, this is high-quality work with comprehensive test coverage.

Code Quality ✅

Strengths

1. Clear Structure: The code is well-organized with helper functions that have clear, single responsibilities
2. Good Comments: Function documentation explains the purpose of each helper
3. Consistent Naming: Follows Go conventions and matches the existing codebase patterns
4. Proper Error Handling: Checks for nil values appropriately throughout

Conventions Adherence ✅

The implementation follows the established patterns seen in other rules:

• Message builders use rule.RuleMessage structure
• Uses rule.CreateRule with rule.RuleListeners
• Properly registered in internal/config/config.go
• Test structure matches the project's rule_tester framework

Potential Issues & Improvements

1. Shadowed Built-in Constructors ⚠️

Location: no_constant_binary_expression.go:131-141, 171-179

The rule identifies Boolean(), String(), and Number() calls as constant when their arguments are constant. However, it doesn't check if these identifiers have been shadowed by local variables.

Your tests include this case (line 37-38 in test file), marking them as valid, which suggests the implementation should handle shadowing. However, the current implementation doesn't appear to check for this.

Impact: Low - This is an edge case that rarely occurs in practice

Recommendation: Consider adding scope/symbol checking to verify these are the global built-ins, not local variables. This would require accessing ctx.TypeChecker to resolve symbols.

2. Missing Operator in Error Message

Location: no_constant_binary_expression.go:10-13

The constantBinaryOperand message doesn't include which operator caused the issue, which could make debugging harder for users.

Recommendation: Consider including the operator in the error message for better developer experience.

3. Code Organization - Minor

Location: no_constant_binary_expression.go:256-285

The findBinaryExpressionConstantOperand function has some code duplication in the null/undefined checking block (lines 276-284).

Recommendation: Minor refactor for maintainability (not blocking)

Test Coverage ✅

Excellent Coverage

• 18 valid test cases covering edge cases
• 68 invalid test cases with comprehensive error checking
• Tests cover:
  • ✅ All three short-circuit operators (&&, ||, ??)
  • ✅ All equality operators (==, !=, ===, !==)
  • ✅ Newly constructed objects
  • ✅ Boolean, string, numeric literals
  • ✅ Built-in constructor calls
  • ✅ Unary negation
  • ✅ Shadowed built-ins
  • ✅ Template literals

Suggested Additional Test Cases

1. Type Coercion Edge Cases: NaN, Infinity
2. Nested Expressions: nested constant expressions
3. BigInt Literals (if supported)
4. Void Operator with function calls: void foo() && bar

Performance Considerations ✅

The implementation is efficient:

• Uses early returns to avoid unnecessary checks
• Switch statements for kind checking are performant
• No unnecessary AST traversals
• Helper functions are appropriately scoped

No performance concerns identified.

Security Concerns ✅

This is a linting rule with no security implications. The code:

• Doesn't execute user code
• Doesn't perform file I/O
• Doesn't make network requests
• Only analyzes AST structure

No security concerns identified.

Missing Features (Low Priority)

Based on ESLint's implementation:

1. Class Expressions: Should be treated as "always new"

   • Currently not explicitly handled in isAlwaysNew.

2. Function Expressions: Already handled via KindFunctionExpression ✅

Recommendations Summary

High Priority

• ✅ None - the implementation is solid

Medium Priority

1. Add symbol resolution for shadowed built-ins to match the test expectations (lines 37-38 in test file)
2. Verify class expressions are handled correctly in isAlwaysNew

Low Priority (Nice to Have)

1. Include operator in constantBinaryOperand error message
2. Add suggested edge case tests (NaN, Infinity, BigInt)
3. Minor refactor of findBinaryExpressionConstantOperand to reduce duplication

Conclusion

This is excellent work! The implementation is clean, well-tested, and follows project conventions. The rule correctly handles the vast majority of cases, and the comprehensive test suite gives confidence in its correctness.

Approval Status: ✅ Approved with Minor Suggestions

The code is production-ready as-is. The suggestions above are optimizations and edge case improvements that can be addressed in follow-up work if desired.

Great job on this port! 🎉

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🤖 Review generated with Claude Code

[claude]

Code Review: no-constant-binary-expression Rule

Summary

This PR successfully ports the ESLint no-constant-binary-expression rule to rslint. The implementation is well-structured and comprehensive, with excellent test coverage (18 valid + 68 invalid test cases). Overall, this is high-quality work with just a few minor suggestions for improvement.

Code Quality ✅

Strengths

• Well-organized code structure: Helper functions are clearly named and properly documented
• Good separation of concerns: Each helper function has a single, well-defined responsibility
• Consistent with codebase patterns: Follows the same structure as other rules (e.g., no_compare_neg_zero, no_cond_assign)
• Clear comments: All helper functions have descriptive comments explaining their purpose

Minor Suggestions

1. Message string formatting (no_constant_binary_expression.go:19)

   • The message construction uses string concatenation which could be simplified

   // Current:
   Description: "Unexpected constant " + property + " on the left-hand side of a `" + property + "` expression.",
   
   // Consider using fmt.Sprintf for better readability:
   Description: fmt.Sprintf("Unexpected constant %s on the left-hand side of a `%s` expression.", property, property),

2. DRY principle in equality operator checks (no_constant_binary_expression.go:284-285)

   • The same condition is repeated twice. Consider extracting to a helper:

   func isEqualityOperator(operator ast.Kind) bool {
       return operator == ast.KindEqualsEqualsToken || 
              operator == ast.KindExclamationEqualsToken ||
              operator == ast.KindEqualsEqualsEqualsToken || 
              operator == ast.KindExclamationEqualsEqualsToken
   }

Potential Bugs or Issues ⚠️

Logic Concerns

1. Potential false negatives with template literals (no_constant_binary_expression.go:112)

   • Template literals with expressions (${bar}) are correctly excluded from constants, but template literals without substitutions (NoSubstitutionTemplateLiteral) are marked as constants
   • This seems correct based on the test case at line 26 of the test file showing `${bar}` && foo as valid
   • However, consider edge cases like tagged template literals which might need special handling

2. Built-in function shadowing detection (no_constant_binary_expression.go:135-141)

   • The code checks if Boolean, String, or Number are called with constant arguments
   • This doesn't account for shadowed built-ins (user defines their own Boolean function)
   • The test file shows awareness of this issue (lines 38-39), but the implementation doesn't handle it
   • Recommendation: Add scope analysis to verify these are actually the built-in functions, or document this limitation

3. Recursive call in hasConstantNullishness (no_constant_binary_expression.go:183-191)

   • The hasConstantNullishness function recursively calls itself for nested ?? operators
   • Consider potential stack overflow for deeply nested expressions: a ?? b ?? c ?? d ?? ...
   • While unlikely in practice, adding a depth limit would make this more robust

Performance Considerations 💡

Generally Efficient

The implementation is performant:

• Uses single-pass AST traversal (visitor pattern)
• Helper functions use early returns to avoid unnecessary computation
• Switch statements are O(1) for constant checks

Potential Optimizations

1. Redundant nil checks (throughout)

   • Many functions check if node == nil at the start, then check if prefix != nil after calling As*() methods
   • The double-checking is defensive but could be streamlined if the AST API guarantees are documented

2. Repeated AST conversions (no_constant_binary_expression.go:305-356)

   • node.AsBinaryExpression() is called once per binary expression node
   • This is already optimal, but ensure the As*() methods are cheap (ideally just type assertions)

Security Concerns 🔒

No security issues identified.

The rule is purely static analysis and doesn't:

• Execute user code
• Access the file system
• Make network requests
• Process untrusted input in an unsafe way

Test Coverage ✅

Excellent Coverage

The test suite is comprehensive:

• 18 valid test cases covering legitimate use patterns
• 68 invalid test cases covering all error conditions
• Tests organized by error type (constantShortCircuit, constantBinaryOperand, alwaysNew, bothAlwaysNew)

Test Quality

• Tests cover all three short-circuit operators (&&, ||, ??)
• Tests cover all four equality operators (==, !=, ===, !==)
• Edge cases are well-covered:
  • Unary negation (!foo && bar)
  • Built-in constructors (Boolean(x), String(x), Number(x))
  • Newly constructed objects (x === {}, [] === [])
  • Template literals
  • Shadowed built-ins

Missing Test Cases (Optional Enhancements)

1. Nested nullish coalescing: a ?? (b ?? c) - to verify recursion works
2. Mixed operators: true && false || x - to ensure proper handling
3. Edge case: void 0 === undefined - both are undefined but constant
4. Function expression: (function(){}) === x - tests function expressions

Integration & Registration ✅

The rule is properly registered in internal/config/config.go:

• Import added correctly (line 85)
• Registration call added in proper alphabetical order (line 437)

The dictionary update (scripts/dictionary.txt) properly adds "nullishness" to the spell-check dictionary.

Recommendations

High Priority

1. Add scope analysis or document the limitation regarding shadowed built-in functions
2. Consider adding a depth limit to hasConstantNullishness for deeply nested expressions

Medium Priority

3. Use fmt.Sprintf for message formatting (better readability)
4. Extract isEqualityOperator helper to reduce duplication

Low Priority

5. Add a few more edge case tests (nested ??, function expressions)
6. Add documentation comments for the main rule variable explaining the rule's purpose

Conclusion

This is a well-implemented, production-ready rule. The code quality is high, the test coverage is excellent, and it follows the repository's conventions. The suggestions above are mostly minor improvements and defensive enhancements rather than critical issues.

Recommendation: Approve with minor suggestions ✅

Great work on this port! The implementation demonstrates a solid understanding of both the ESLint rule and the rslint architecture.

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Reviewed with care by Claude Code 🤖

[claude]

Code Review: PR #74 - Port ESLint core rule no-constant-binary-expression

Thank you for this well-structured PR! The implementation ports the ESLint no-constant-binary-expression rule to rslint. Here's my detailed review:

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✅ Strengths

Code Quality

1. Excellent Code Organization: The implementation is well-structured with clear helper functions (isNullOrUndefined, isStaticBoolean, isAlwaysNew, etc.) that each have a single, well-defined responsibility.

2. Comprehensive Pattern Detection: The rule correctly identifies multiple categories of constant binary expressions:

   • Constant short-circuit operators (&&, ||, ??)
   • Constant boolean comparisons (loose and strict)
   • Comparisons to newly constructed objects
   • Constant nullish values

3. Proper Parenthesized Expression Handling: Good attention to detail in recursively handling KindParenthesizedExpression across all helper functions (lines 52-54, 68-70, 106-108, etc.).

4. Clear Error Messages: The message builders provide descriptive, user-friendly error messages that clearly explain the issue.

Test Coverage

• 86 test cases total (18 valid, 68 invalid) - excellent coverage
• Tests cover edge cases like:
  • Template literals with expressions
  • Assignment expressions
  • Delete operations
  • Shadowed built-in functions
  • Various operator combinations

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🔍 Issues & Suggestions

1. Potential False Positive with Unary Negation (Medium Priority)

Location: no_constant_binary_expression.go:143-149

case ast.KindPrefixUnaryExpression:
    prefix := node.AsPrefixUnaryExpression()
    if prefix != nil {
        if prefix.Operator == ast.KindExclamationToken {
            return true  // Any !value is considered constant
        }
        return isConstant(prefix.Operand)
    }

Issue: The logic at line 148 returns true for ANY expression with ! operator, regardless of whether the operand is constant. This means !someVariable is treated as constant.

While the test case !foo && bar (line 394 of test file) expects this to be flagged, this seems inconsistent with the ESLint behavior. According to the rule's purpose, !foo should only be constant if foo is constant.

Impact: This could lead to false positives for code like:

!userInput && doSomething()  // Would incorrectly flag this

Recommendation: Consider changing line 148 to:

// ! (logical not) produces a boolean if operand is constant
if prefix.Operator == ast.KindExclamationToken {
    return isConstant(prefix.Operand)
}

2. Missing Scope Analysis for Built-in Functions (Known Limitation)

Location: Test file lines 399-434

The PR correctly identifies and documents this limitation with TODO comments. The code cannot distinguish between shadowed and non-shadowed Boolean(), String(), and Number() functions.

Current behavior:

• Boolean(x) && foo - Not flagged (could be user function)
• Boolean(true) && foo - Flagged (constant argument)

Recommendation:

• The current conservative approach is good for avoiding false positives
• Consider adding a comment in the main implementation explaining this design choice
• Future enhancement: Add scope analysis via TypeChecker to detect shadowing

3. Duplicate Logic in hasConstantNullishness (Minor - Code Quality)

Location: no_constant_binary_expression.go:198-208

The logic for checking Boolean(), String(), Number() is duplicated across multiple functions:

• isConstant() (lines 154-165)
• hasConstantNullishness() (lines 198-208)

Recommendation: Extract this into a helper function to reduce duplication:

func isBuiltInTypeConversion(node *ast.Node) bool {
    if node == nil || node.Kind != ast.KindCallExpression {
        return false
    }
    call := node.AsCallExpression()
    if call != nil && call.Expression != nil && call.Expression.Kind == ast.KindIdentifier {
        name := call.Expression.Text()
        return name == "Boolean" || name == "String" || name == "Number"
    }
    return false
}

4. Template Literals Coverage (Low Priority)

Location: Valid test cases

The valid test cases include template literals with expressions: `${bar}` && foo

However, I don't see handling for KindTemplateExpression in the isConstant() function. This might be implicitly handled correctly (template literals with expressions are not constant), but it would be clearer to explicitly handle this case to prevent future bugs.

Recommendation: Add explicit handling in isConstant():

case ast.KindTemplateExpression:
    // Template literals with expressions are not constant
    return false

---

⚡ Performance Considerations

The implementation is efficient:

• O(1) lookups for operator checks using switch statements
• No unnecessary traversals - only checks immediate node properties
• Minimal recursion - only for parenthesized expressions

Potential optimization (very minor): The multiple switch statements in the main handler (lines 352-365, 368-395) could potentially be combined, but current structure prioritizes readability.

---

🔒 Security Considerations

No security concerns identified. The rule performs static analysis only and doesn't execute or evaluate code.

---

📋 Additional Observations

Good Practices Followed:

1. ✅ Follows existing codebase patterns (compared to no_compare_neg_zero.go and no_const_assign.go)
2. ✅ Proper nil checks throughout
3. ✅ Good separation of concerns
4. ✅ Comprehensive comments explaining complex logic
5. ✅ Dictionary updated with "nullishness" term

Registration:

✅ Correctly registered in internal/config/config.go

---

🎯 Recommendations Summary

Must Fix:

• Review the unary negation logic (issue chore(ci): Add Claude Code action #1) - verify this matches intended ESLint behavior

Should Consider:

• Extract duplicate built-in type conversion logic
• Add explicit template literal handling for clarity

Nice to Have:

• Add implementation comments about the shadowing limitation
• Add scope analysis support in future PR

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Final Assessment

This is a high-quality implementation that demonstrates:

• Strong understanding of the ESLint rule semantics
• Good Go coding practices
• Thorough testing approach
• Proper error handling

The code is production-ready with one potential issue to verify (unary negation behavior). Great work overall! 🎉

Recommendation: Approve with minor suggestions for improvement.

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Review generated by Claude Code