Add parentheses for immediately-constructed fn/class (#5996)

CHANGELOG.unreleased.md

@@ -42,6 +42,22 @@ Examples:
 
 -->
 
+- JavaScript: Add parentheses for immediately-constructed fn/class ([#5996] by [@bakkot])
+
+  ```
+  // Input
+  new class {};
+  new function() {}
+
+  // Output (Prettier stable)
+  new class {}();
+  new function() {}();
+
+  // Output (Prettier master)
+  new (class {})();
+  new (function() {})();
+  ```
+
 - Config: Support shared configurations ([#5963] by [@azz])
 
   Sharing a Prettier configuration is simple: just publish a module that exports a configuration object, say `@company/prettier-config`, and reference it in your `package.json`:

src/language-js/needs-parens.js

@@ -610,6 +610,7 @@ function needsParens(path, options) {
 
     case "FunctionExpression":
       switch (parent.type) {
+        case "NewExpression":
         case "CallExpression":
           return name === "callee"; // Not strictly necessary, but it's clearer to the reader if IIFEs are wrapped in parentheses.
         case "TaggedTemplateExpression":
@@ -649,7 +650,14 @@ function needsParens(path, options) {
       }
 
     case "ClassExpression":
-      return parent.type === "ExportDefaultDeclaration";
+      switch (parent.type) {
+        case "ExportDefaultDeclaration":
+          return true;
+        case "NewExpression":
+          return name === "callee" && parent.callee === node;
+        default:
+          return false;
+      }
 
     case "OptionalMemberExpression":
       return parent.type === "MemberExpression";

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

@@ -203,6 +203,22 @@ class C {
 ================================================================================
 `;
 
+exports[`new.js 1`] = `
+====================================options=====================================
+parsers: ["flow", "babel", "typescript"]
+printWidth: 80
+                                                                                | printWidth
+=====================================input======================================
+new class {};
+new Ctor(class {});
+
+=====================================output=====================================
+new (class {})();
+new Ctor(class {});
+
+================================================================================
+`;
+
 exports[`property.js 1`] = `
 ====================================options=====================================
 parsers: ["flow", "babel", "typescript"]

tests/classes/new.js

@@ -0,0 +1,2 @@
+new class {};
+new Ctor(class {});

tests/flow/async/__snapshots__/jsfmt.spec.js.snap

@@ -209,7 +209,7 @@ class C {
 var e = async function() {};
 var et = async function<T>(a: T) {};
 
-var n = new async function() {}();
+var n = new (async function() {})();
 
 var o = { async m() {} };
 var ot = { async m<T>(a: T) {} };
@@ -552,9 +552,9 @@ var et = async function<T>(a: T) {
   await 1;
 };
 
-var n = new async function() {
+var n = new (async function() {
   await 1;
-}();
+})();
 
 var o = {
   async m() {

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

@@ -24,12 +24,12 @@ typeof function() {};
 export default (function() {})();
 (function() {})()\`\`;
 (function() {})\`\`;
-new function() {}();
+new (function() {})();
 (function() {});
 a = function f() {} || b;
 (function() {} && a);
 a + function() {};
-new function() {}();
+new (function() {})();
 
 ================================================================================
 `;