Keep a trailing comma in type parameter in .tsx files (#6115)

CHANGELOG.unreleased.md

@@ -65,3 +65,22 @@ Examples:
     )
   );
   ```
+
+- TypeScript: Keep trailing comma in tsx type parameters ([#6115] by [@sosukesuzuki])
+
+  Previously, a trailing comma after single type parameter in arrow function was cleaned up. The formatted result is valid as ts, but is invalid as tsx. Prettier master fixes this issue.
+
+  <!-- prettier-ignore -->
+  ```tsx
+  // Input
+  type G<T> = any;
+  const myFunc = <T,>(arg1: G<T>) => false;
+
+  // Output (Prettier stable)
+  type G<T> = any;
+  const myFunc = <T>(arg1: G<T>) => false;
+
+  // Output (prettier master)
+  type G<T> = any;
+  const myFunc = <T,>(arg1: G<T>) => false;
+  ```

src/language-js/printer-estree.js

@@ -3000,6 +3000,19 @@ function printPathNoParens(path, options, print, args) {
         parts.push(" = ", path.call(print, "default"));
       }
 
+      // Keep comma if the file extension is .tsx and
+      // has one type parameter that isn't extend with any types.
+      // Because, otherwise formatted result will be invalid as tsx.
+      if (
+        parent.params &&
+        parent.params.length === 1 &&
+        options.filepath &&
+        options.filepath.match(/\.tsx/) &&
+        !n.constraint
+      ) {
+        parts.push(",");
+      }
+
       return concat(parts);
     }
     case "TypeofTypeAnnotation":

tests/typescript_tsx/__snapshots__/jsfmt.spec.js.snap

@@ -103,6 +103,24 @@ printWidth: 80
 ================================================================================
 `;
 
+exports[`type-parameters.tsx 1`] = `
+====================================options=====================================
+parsers: ["typescript"]
+printWidth: 80
+                                                                                | printWidth
+=====================================input======================================
+const functionName1 = <T,>(arg) => false;
+const functionName2 = <T extends any>(arg) => false;
+const functionName3 = <T, S>(arg) => false;
+
+=====================================output=====================================
+const functionName1 = <T,>(arg) => false;
+const functionName2 = <T extends any>(arg) => false;
+const functionName3 = <T, S>(arg) => false;
+
+================================================================================
+`;
+
 exports[`url.tsx 1`] = `
 ====================================options=====================================
 parsers: ["typescript"]

tests/typescript_tsx/type-parameters.tsx

@@ -0,0 +1,3 @@
+const functionName1 = <T,>(arg) => false;
+const functionName2 = <T extends any>(arg) => false;
+const functionName3 = <T, S>(arg) => false;