Only replace implements with a comma if the heritage clauses are sensible

Author: amcasey

src/services/codefixes/fixExtendsInterfaceBecomesImplements.ts

@@ -28,21 +28,25 @@ namespace ts.codefix {
 
     function doChanges(changes: textChanges.ChangeTracker, sourceFile: SourceFile, extendsToken: Node, heritageClauses: ReadonlyArray<HeritageClause>): void {
         changes.replaceRange(sourceFile, { pos: extendsToken.getStart(), end: extendsToken.end }, createToken(SyntaxKind.ImplementsKeyword));
-        // We replace existing keywords with commas.
-        for (let i = 1; i < heritageClauses.length; i++) {
-            const keywordToken = heritageClauses[i].getFirstToken()!;
-            const keywordFullStart = keywordToken.getFullStart();
-            changes.replaceRange(sourceFile, { pos: keywordFullStart, end: keywordFullStart }, createToken(SyntaxKind.CommaToken));
+
+        // If there is already an implements clause, replace the implements keyword with a comma.
+        if (heritageClauses.length === 2 &&
+            heritageClauses[0].token === SyntaxKind.ExtendsKeyword &&
+            heritageClauses[1].token === SyntaxKind.ImplementsKeyword) {
+
+            const implementsToken = heritageClauses[1].getFirstToken()!;
+            const implementsFullStart = implementsToken.getFullStart();
+            changes.replaceRange(sourceFile, { pos: implementsFullStart, end: implementsFullStart }, createToken(SyntaxKind.CommaToken));
 
             // Rough heuristic: delete trailing whitespace after keyword so that it's not excessive.
             // (Trailing because leading might be indentation, which is more sensitive.)
             const text = sourceFile.text;
-            let end = keywordToken.end;
+            let end = implementsToken.end;
             while (end < text.length && ts.isWhiteSpaceSingleLine(text.charCodeAt(end))) {
                 end++;
             }
 
-            changes.deleteRange(sourceFile, { pos: keywordToken.getStart(), end });
+            changes.deleteRange(sourceFile, { pos: implementsToken.getStart(), end });
         }
     }
 }