This PR implements the missinggetJSSyntacticDiagnosticsForFile function and updatesgetSyntacticDiagnosticsForFile to properly handle JavaScript files by detecting TypeScript-only constructs and reporting them as syntactic errors.

Changes Made

Core Implementation

• AddedadditionalSyntacticDiagnostics field toSourceFile struct with getter/setter methods for caching JS syntactic diagnostics
• ImplementedgetJSSyntacticDiagnosticsForFile function that recursively walks the AST to detect TypeScript-only constructs in JavaScript files
• UpdatedgetSyntacticDiagnosticsForFile to callgetJSSyntacticDiagnosticsForFile for JS files and concatenate results with parse diagnostics

TypeScript-only Constructs Detected

The implementation detects and reports errors for:

• Type annotations on parameters, variables, functions, properties, and methods
• Type-only imports/exports (import type,export type,import =,export =)
• Declaration types (interfaces, type aliases, enums, modules, namespaces)
• Type expressions (non-null assertions!, type assertionsas, satisfies expressions)
• TypeScript modifiers (public,private,protected,readonly,declare,abstract,override, etc.)
• Optional markers (? on parameters)
• Type parameters andtype arguments (<T>,Array<string>())
• Signature declarations (functions without bodies)
• Implements clauses
• Parameter modifiers (constructor parameter properties)

Test Coverage

• Added comprehensive test case (jsSyntacticDiagnostics.ts) covering 40+ different TypeScript constructs
• 285+ baseline files updated across the test suite showing improved diagnostic coverage
• All existing tests pass with no regressions

Example Output

// JavaScript file with TypeScript constructsfunctionfunc(x:number):string{// ❌ Type annotations flaggedreturnx.toString();}interfacePerson{// ❌ Interface declaration flagged    name:string;}classMyClass{publicname:string;// ❌ Public modifier flaggedprivateage:number;// ❌ Private modifier flagged}letvalue=getValue()!;// ❌ Non-null assertion flagged

The implementation follows the TypeScript reference compiler closely and provides lazy evaluation with caching for performance, matching the original TypeScript behavior exactly.

Fixes#1386.

💬 Share your feedback on Copilot coding agent for the chance to win a $200 gift card! Clickhere to start the survey.