Add --checkJsFiles

src/compiler/commandLineParser.ts

@@ -481,6 +481,12 @@ namespace ts {
                 name: "plugin",
                 type: "object"
             }
+        },
+        {
+            name: "checkJsFiles",
+            type: "boolean",
+            experimental: true,
+            description: Diagnostics.Report_errors_in_js_files
         }
     ];
 

src/compiler/diagnosticMessages.json

@@ -3362,5 +3362,9 @@
     "Octal literals are not allowed in enums members initializer. Use the syntax '{0}'.": {
         "category": "Error",
         "code": 8018
+    },
+    "Report errors in .js files.": {
+        "category": "Message",
+        "code": 8019
     }
 }

src/compiler/program.ts

@@ -907,7 +907,7 @@ namespace ts {
                 // For JavaScript files, we don't want to report semantic errors.
                 // Instead, we'll report errors for using TypeScript-only constructs from within a
                 // JavaScript file when we get syntactic diagnostics for the file.
-                const checkDiagnostics = isSourceFileJavaScript(sourceFile) ? [] : typeChecker.getDiagnostics(sourceFile, cancellationToken);
+                const checkDiagnostics = !options.checkJsFiles && isSourceFileJavaScript(sourceFile) ? [] : typeChecker.getDiagnostics(sourceFile, cancellationToken);
                 const fileProcessingDiagnosticsInFile = fileProcessingDiagnostics.getDiagnostics(sourceFile.fileName);
                 const programDiagnosticsInFile = programDiagnostics.getDiagnostics(sourceFile.fileName);
 

src/compiler/types.ts

@@ -3312,6 +3312,7 @@
         alwaysStrict?: boolean;  // Always combine with strict property
         baseUrl?: string;
         charset?: string;
+        checkJsFiles?: boolean;
         /* @internal */ configFilePath?: string;
         declaration?: boolean;
         declarationDir?: string;