From 6597a472b30419493f123bff1e9194e4c09e488e Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Markus=20T=C3=B6r=C3=B6k?= <markus.toeroek@gmail.com>
Date: Tue, 10 Apr 2018 23:33:38 +0200
Subject: [PATCH] Add "use strict" directive (#7411)

* #7349
- Add "use strict" directive to transformed class bodies

* Update tests (#7349)

* Add test for program with use strict directive (#7349)

* Update tests (#7349)

* Update source mapping (#7349)

* Add test for parent blockStatement with use strict directive (#7349)

* Update tests (#7349)

* Update tests (#7349)

* Update tests (#7349)

* Add test for constructor-only class (#7349)

* Constructor only classes are strict, too

But constructor only that use non-simple parameters must use a strict function wrapper.

* Fix test

* Wrapper not needed if class is already strict

* Revert change to lerna.json
---
 .../misc/regression-1155/output.js            |  2 +
 .../source-maps/class/output.js               |  2 +
 .../source-maps/class/source-mappings.json    |  4 +-
 .../fixtures/public-loose/computed/output.js  |  2 +
 .../constructor-collision/output.js           |  2 +
 .../fixtures/public-loose/derived/output.js   |  2 +
 .../fixtures/public-loose/foobar/output.js    |  2 +
 .../public-loose/instance-computed/output.js  |  2 +
 .../public-loose/instance-undefined/output.js |  2 +
 .../fixtures/public-loose/instance/output.js  |  2 +
 .../public-loose/regression-T6719/output.js   |  2 +
 .../public-loose/static-infer-name/output.js  |  2 +
 .../public-loose/static-undefined/output.js   |  2 +
 .../fixtures/public-loose/static/output.js    |  2 +
 .../public-loose/super-call/output.js         |  4 ++
 .../public-loose/super-expression/output.js   |  2 +
 .../public-loose/super-statement/output.js    |  2 +
 .../super-with-collision/output.js            |  2 +
 .../test/fixtures/public/computed/output.js   |  2 +
 .../public/constructor-collision/output.js    |  2 +
 .../test/fixtures/public/derived/output.js    |  2 +
 .../test/fixtures/public/foobar/output.js     |  2 +
 .../public/instance-computed/output.js        |  2 +
 .../public/instance-undefined/output.js       |  2 +
 .../test/fixtures/public/instance/output.js   |  2 +
 .../public/regression-T6719/output.js         |  2 +
 .../public/static-infer-name/output.js        |  2 +
 .../public/static-undefined/output.js         |  2 +
 .../test/fixtures/public/static/output.js     |  2 +
 .../test/fixtures/public/super-call/output.js |  4 ++
 .../public/super-expression/output.js         |  2 +
 .../fixtures/public/super-statement/output.js |  2 +
 .../public/super-with-collision/output.js     |  2 +
 .../test/fixtures/regression/6154/output.js   |  2 +
 .../test/fixtures/regression/T6719/output.js  |  2 +
 .../test/fixtures/regression/7030/output.js   |  2 +
 .../test/fixtures/regression/T6984/output.js  |  2 +
 .../src/transformClass.js                     | 41 +++++++++++++++++--
 .../fixtures/extend-builtins/loose/output.js  |  2 +
 .../extend-builtins/shadowed/output.js        |  4 ++
 .../fixtures/extend-builtins/spec/output.js   |  2 +
 .../loose-classCallCheck/class/output.js      |  4 +-
 .../with-constructor/output.js                |  4 ++
 .../with-superClass/output.js                 |  6 ++-
 .../loose/accessing-super-class/output.js     |  2 +
 .../accessing-super-properties/output.js      |  2 +
 .../input.js                                  |  6 +++
 .../output.js                                 | 15 +++++++
 .../loose/calling-super-properties/output.js  |  2 +
 .../loose/constructor-order/output.js         |  2 +
 .../output.js                                 |  2 +
 .../output.js                                 |  2 +
 .../output.js                                 |  2 +
 .../test/fixtures/loose/literal-key/output.js |  2 +
 .../method-return-type-annotation/output.js   |  2 +
 .../loose/method-reuse-prototype/output.js    |  2 +
 .../input.js                                  |  7 ++++
 .../output.js                                 | 13 ++++++
 .../output.js                                 |  4 ++
 .../test/fixtures/loose/super-class/output.js |  2 +
 .../loose/super-function-fallback/output.js   |  2 +
 .../test/fixtures/regression/5817/output.js   |  2 +
 .../test/fixtures/regression/T2494/output.js  |  2 +
 .../test/fixtures/regression/T2997/output.js  |  4 ++
 .../test/fixtures/regression/T6712/output.js  |  2 +
 .../test/fixtures/regression/T6755/output.js  |  2 +
 .../test/fixtures/regression/T7010/output.js  |  2 +
 .../test/fixtures/regression/T7537/output.js  |  4 ++
 .../spec/accessing-super-class/output.js      |  2 +
 .../spec/accessing-super-properties/output.js |  2 +
 .../spec/calling-super-properties/output.js   |  2 +
 .../fixtures/spec/computed-methods/output.js  |  2 +
 .../constructor-binding-collision/output.js   |  2 +
 .../fixtures/spec/constructor-only/input.js   |  5 +++
 .../fixtures/spec/constructor-only/output.js  |  5 +++
 .../test/fixtures/spec/constructor/output.js  |  6 +++
 .../output.js                                 |  2 +
 .../output.js                                 |  2 +
 .../output.js                                 |  2 +
 .../output.js                                 |  2 +
 .../output.js                                 |  2 +
 .../output.js                                 |  2 +
 .../output.js                                 |  2 +
 .../spec/inferred-expression-name/output.js   |  2 +
 .../spec/instance-getter-and-setter/output.js |  2 +
 .../fixtures/spec/instance-getter/output.js   |  2 +
 .../fixtures/spec/instance-method/output.js   |  2 +
 .../fixtures/spec/instance-setter/output.js   |  2 +
 .../method-return-type-annotation/output.js   |  2 +
 .../test/fixtures/spec/plain-class/output.js  |  2 +
 .../spec/preserves-directives/output.js       |  3 ++
 .../spec/relaxed-method-coercion/output.js    |  2 +
 .../test/fixtures/spec/statement/output.js    |  6 +++
 .../test/fixtures/spec/static/output.js       |  2 +
 .../spec/super-class-anonymous/output.js      | 20 +++++++++
 .../output.js                                 |  4 ++
 .../test/fixtures/spec/super-class/output.js  |  2 +
 .../spec/super-function-fallback/output.js    |  2 +
 .../output.js                                 |  2 +
 .../output.js                                 |  2 +
 .../output.js                                 |  2 +
 .../output.js                                 |  2 +
 .../output.js                                 |  2 +
 .../output.js                                 |  2 +
 .../output.js                                 |  2 +
 .../output.js                                 |  2 +
 .../output.js                                 |  2 +
 .../output.js                                 |  2 +
 .../output.js                                 |  2 +
 .../output.js                                 |  2 +
 .../output.js                                 |  2 +
 .../regression/class-prop-types/output.js     |  2 +
 .../output.js                                 |  2 +
 .../output.js                                 |  2 +
 .../function-name/class-method/output.js      |  2 +
 .../fixtures/jscript/simple-class/output.js   |  2 +
 .../parameters/default-iife-4253/output.js    | 18 +++++---
 .../parameters/default-iife-self/output.js    | 36 +++++++++++-----
 .../parameters/rest-arrow-functions/output.js |  2 +
 .../parameters/rest-nested-iife/output.js     |  2 +
 .../output.js                                 |  2 +
 .../test/fixtures/runtime/class/output.js     |  2 +
 .../fixtures/runtime/no-helpers/output.js     |  2 +
 .../useBuiltIns-useESModules/output.js        |  2 +
 .../use-options/useBuiltIns/output.js         |  2 +
 .../use-options/useESModules/output.js        |  2 +
 packages/babel-standalone/test/babel.js       |  2 +-
 127 files changed, 410 insertions(+), 23 deletions(-)
 create mode 100644 packages/babel-plugin-transform-classes/test/fixtures/loose/blockstatement-with-use-strict-directive/input.js
 create mode 100644 packages/babel-plugin-transform-classes/test/fixtures/loose/blockstatement-with-use-strict-directive/output.js
 create mode 100644 packages/babel-plugin-transform-classes/test/fixtures/loose/program-with-use-strict-directive/input.js
 create mode 100644 packages/babel-plugin-transform-classes/test/fixtures/loose/program-with-use-strict-directive/output.js
 create mode 100644 packages/babel-plugin-transform-classes/test/fixtures/spec/constructor-only/input.js
 create mode 100644 packages/babel-plugin-transform-classes/test/fixtures/spec/constructor-only/output.js

diff --git a/packages/babel-core/test/fixtures/transformation/misc/regression-1155/output.js b/packages/babel-core/test/fixtures/transformation/misc/regression-1155/output.js
index f1349b84f7..78220af85d 100644
--- a/packages/babel-core/test/fixtures/transformation/misc/regression-1155/output.js
+++ b/packages/babel-core/test/fixtures/transformation/misc/regression-1155/output.js
@@ -1,6 +1,8 @@
 var Foo =
 /*#__PURE__*/
 function (_Bar) {
+  "use strict";
+
   babelHelpers.inherits(Foo, _Bar);
 
   function Foo(options) {
diff --git a/packages/babel-core/test/fixtures/transformation/source-maps/class/output.js b/packages/babel-core/test/fixtures/transformation/source-maps/class/output.js
index c68f687b3a..76b7ca7f89 100644
--- a/packages/babel-core/test/fixtures/transformation/source-maps/class/output.js
+++ b/packages/babel-core/test/fixtures/transformation/source-maps/class/output.js
@@ -1,6 +1,8 @@
 var Test =
 /*#__PURE__*/
 function () {
+  "use strict";
+
   function Test() {
     babelHelpers.classCallCheck(this, Test);
   }
diff --git a/packages/babel-core/test/fixtures/transformation/source-maps/class/source-mappings.json b/packages/babel-core/test/fixtures/transformation/source-maps/class/source-mappings.json
index 5d8c4d8816..b53208fbe4 100644
--- a/packages/babel-core/test/fixtures/transformation/source-maps/class/source-mappings.json
+++ b/packages/babel-core/test/fixtures/transformation/source-maps/class/source-mappings.json
@@ -4,7 +4,7 @@
     "column": 10
   },
   "generated": {
-    "line": 11,
+    "line": 13,
     "column": 15
   }
-}]
+}]
\ No newline at end of file
diff --git a/packages/babel-plugin-proposal-class-properties/test/fixtures/public-loose/computed/output.js b/packages/babel-plugin-proposal-class-properties/test/fixtures/public-loose/computed/output.js
index 0c45f99e1d..9389374c65 100644
--- a/packages/babel-plugin-proposal-class-properties/test/fixtures/public-loose/computed/output.js
+++ b/packages/babel-plugin-proposal-class-properties/test/fixtures/public-loose/computed/output.js
@@ -24,6 +24,8 @@ var _ref4 = `template${expression}`;
 var MyClass =
 /*#__PURE__*/
 function () {
+  "use strict";
+
   function MyClass() {
     babelHelpers.classCallCheck(this, MyClass);
     this[null] = "null";
diff --git a/packages/babel-plugin-proposal-class-properties/test/fixtures/public-loose/constructor-collision/output.js b/packages/babel-plugin-proposal-class-properties/test/fixtures/public-loose/constructor-collision/output.js
index b15e5d8f86..e1a32c84b8 100644
--- a/packages/babel-plugin-proposal-class-properties/test/fixtures/public-loose/constructor-collision/output.js
+++ b/packages/babel-plugin-proposal-class-properties/test/fixtures/public-loose/constructor-collision/output.js
@@ -1,6 +1,8 @@
 var foo = "bar";
 
 var Foo = function Foo() {
+  "use strict";
+
   babelHelpers.classCallCheck(this, Foo);
   this.bar = foo;
   var _foo = "foo";
diff --git a/packages/babel-plugin-proposal-class-properties/test/fixtures/public-loose/derived/output.js b/packages/babel-plugin-proposal-class-properties/test/fixtures/public-loose/derived/output.js
index 3554a0e804..c4ec254b61 100644
--- a/packages/babel-plugin-proposal-class-properties/test/fixtures/public-loose/derived/output.js
+++ b/packages/babel-plugin-proposal-class-properties/test/fixtures/public-loose/derived/output.js
@@ -1,6 +1,8 @@
 var Foo =
 /*#__PURE__*/
 function (_Bar) {
+  "use strict";
+
   babelHelpers.inherits(Foo, _Bar);
 
   function Foo(...args) {
diff --git a/packages/babel-plugin-proposal-class-properties/test/fixtures/public-loose/foobar/output.js b/packages/babel-plugin-proposal-class-properties/test/fixtures/public-loose/foobar/output.js
index 0fab0c8e95..3f43d7eb0f 100644
--- a/packages/babel-plugin-proposal-class-properties/test/fixtures/public-loose/foobar/output.js
+++ b/packages/babel-plugin-proposal-class-properties/test/fixtures/public-loose/foobar/output.js
@@ -1,6 +1,8 @@
 var Child =
 /*#__PURE__*/
 function (_Parent) {
+  "use strict";
+
   babelHelpers.inherits(Child, _Parent);
 
   function Child() {
diff --git a/packages/babel-plugin-proposal-class-properties/test/fixtures/public-loose/instance-computed/output.js b/packages/babel-plugin-proposal-class-properties/test/fixtures/public-loose/instance-computed/output.js
index d565431f92..0e98b482c7 100644
--- a/packages/babel-plugin-proposal-class-properties/test/fixtures/public-loose/instance-computed/output.js
+++ b/packages/babel-plugin-proposal-class-properties/test/fixtures/public-loose/instance-computed/output.js
@@ -2,6 +2,8 @@ function test(x) {
   var _x = x;
 
   var F = function F() {
+    "use strict";
+
     babelHelpers.classCallCheck(this, F);
     this[_x] = 1;
   };
diff --git a/packages/babel-plugin-proposal-class-properties/test/fixtures/public-loose/instance-undefined/output.js b/packages/babel-plugin-proposal-class-properties/test/fixtures/public-loose/instance-undefined/output.js
index 7266d5fa3e..fce9e3540e 100644
--- a/packages/babel-plugin-proposal-class-properties/test/fixtures/public-loose/instance-undefined/output.js
+++ b/packages/babel-plugin-proposal-class-properties/test/fixtures/public-loose/instance-undefined/output.js
@@ -1,4 +1,6 @@
 var Foo = function Foo() {
+  "use strict";
+
   babelHelpers.classCallCheck(this, Foo);
   this.bar = void 0;
 };
diff --git a/packages/babel-plugin-proposal-class-properties/test/fixtures/public-loose/instance/output.js b/packages/babel-plugin-proposal-class-properties/test/fixtures/public-loose/instance/output.js
index f832ea3565..0a72dc1bf7 100644
--- a/packages/babel-plugin-proposal-class-properties/test/fixtures/public-loose/instance/output.js
+++ b/packages/babel-plugin-proposal-class-properties/test/fixtures/public-loose/instance/output.js
@@ -1,4 +1,6 @@
 var Foo = function Foo() {
+  "use strict";
+
   babelHelpers.classCallCheck(this, Foo);
   this.bar = "foo";
 };
diff --git a/packages/babel-plugin-proposal-class-properties/test/fixtures/public-loose/regression-T6719/output.js b/packages/babel-plugin-proposal-class-properties/test/fixtures/public-loose/regression-T6719/output.js
index c815369e9e..f8aebbe026 100644
--- a/packages/babel-plugin-proposal-class-properties/test/fixtures/public-loose/regression-T6719/output.js
+++ b/packages/babel-plugin-proposal-class-properties/test/fixtures/public-loose/regression-T6719/output.js
@@ -4,6 +4,8 @@ function withContext(ComposedComponent) {
   return _temp = _class =
   /*#__PURE__*/
   function (_Component) {
+    "use strict";
+
     babelHelpers.inherits(WithContext, _Component);
 
     function WithContext() {
diff --git a/packages/babel-plugin-proposal-class-properties/test/fixtures/public-loose/static-infer-name/output.js b/packages/babel-plugin-proposal-class-properties/test/fixtures/public-loose/static-infer-name/output.js
index b9dfd403a2..293485cf34 100644
--- a/packages/babel-plugin-proposal-class-properties/test/fixtures/public-loose/static-infer-name/output.js
+++ b/packages/babel-plugin-proposal-class-properties/test/fixtures/public-loose/static-infer-name/output.js
@@ -1,5 +1,7 @@
 var _class, _temp;
 
 var Foo = (_temp = _class = function Foo() {
+  "use strict";
+
   babelHelpers.classCallCheck(this, Foo);
 }, _class.num = 0, _temp);
diff --git a/packages/babel-plugin-proposal-class-properties/test/fixtures/public-loose/static-undefined/output.js b/packages/babel-plugin-proposal-class-properties/test/fixtures/public-loose/static-undefined/output.js
index 94707d5451..463ea3130e 100644
--- a/packages/babel-plugin-proposal-class-properties/test/fixtures/public-loose/static-undefined/output.js
+++ b/packages/babel-plugin-proposal-class-properties/test/fixtures/public-loose/static-undefined/output.js
@@ -1,4 +1,6 @@
 var Foo = function Foo() {
+  "use strict";
+
   babelHelpers.classCallCheck(this, Foo);
 };
 
diff --git a/packages/babel-plugin-proposal-class-properties/test/fixtures/public-loose/static/output.js b/packages/babel-plugin-proposal-class-properties/test/fixtures/public-loose/static/output.js
index 4d0c09379d..50eea8c486 100644
--- a/packages/babel-plugin-proposal-class-properties/test/fixtures/public-loose/static/output.js
+++ b/packages/babel-plugin-proposal-class-properties/test/fixtures/public-loose/static/output.js
@@ -1,4 +1,6 @@
 var Foo = function Foo() {
+  "use strict";
+
   babelHelpers.classCallCheck(this, Foo);
 };
 
diff --git a/packages/babel-plugin-proposal-class-properties/test/fixtures/public-loose/super-call/output.js b/packages/babel-plugin-proposal-class-properties/test/fixtures/public-loose/super-call/output.js
index 201ad4d5dc..36274bc9f9 100644
--- a/packages/babel-plugin-proposal-class-properties/test/fixtures/public-loose/super-call/output.js
+++ b/packages/babel-plugin-proposal-class-properties/test/fixtures/public-loose/super-call/output.js
@@ -1,6 +1,8 @@
 var A =
 /*#__PURE__*/
 function () {
+  "use strict";
+
   function A() {
     babelHelpers.classCallCheck(this, A);
   }
@@ -17,6 +19,8 @@ function () {
 var B =
 /*#__PURE__*/
 function (_A) {
+  "use strict";
+
   babelHelpers.inherits(B, _A);
 
   function B(...args) {
diff --git a/packages/babel-plugin-proposal-class-properties/test/fixtures/public-loose/super-expression/output.js b/packages/babel-plugin-proposal-class-properties/test/fixtures/public-loose/super-expression/output.js
index 0aff7fc69f..d682e4a6ed 100644
--- a/packages/babel-plugin-proposal-class-properties/test/fixtures/public-loose/super-expression/output.js
+++ b/packages/babel-plugin-proposal-class-properties/test/fixtures/public-loose/super-expression/output.js
@@ -1,6 +1,8 @@
 var Foo =
 /*#__PURE__*/
 function (_Bar) {
+  "use strict";
+
   babelHelpers.inherits(Foo, _Bar);
 
   function Foo() {
diff --git a/packages/babel-plugin-proposal-class-properties/test/fixtures/public-loose/super-statement/output.js b/packages/babel-plugin-proposal-class-properties/test/fixtures/public-loose/super-statement/output.js
index e286be95c0..c63a614d02 100644
--- a/packages/babel-plugin-proposal-class-properties/test/fixtures/public-loose/super-statement/output.js
+++ b/packages/babel-plugin-proposal-class-properties/test/fixtures/public-loose/super-statement/output.js
@@ -1,6 +1,8 @@
 var Foo =
 /*#__PURE__*/
 function (_Bar) {
+  "use strict";
+
   babelHelpers.inherits(Foo, _Bar);
 
   function Foo() {
diff --git a/packages/babel-plugin-proposal-class-properties/test/fixtures/public-loose/super-with-collision/output.js b/packages/babel-plugin-proposal-class-properties/test/fixtures/public-loose/super-with-collision/output.js
index bd91aee83d..778fd34564 100644
--- a/packages/babel-plugin-proposal-class-properties/test/fixtures/public-loose/super-with-collision/output.js
+++ b/packages/babel-plugin-proposal-class-properties/test/fixtures/public-loose/super-with-collision/output.js
@@ -1,4 +1,6 @@
 var A = function A(_force) {
+  "use strict";
+
   babelHelpers.classCallCheck(this, A);
   this.force = force;
   this.foo = babelHelpers.get(babelHelpers.getPrototypeOf(A.prototype), "method", babelHelpers.assertThisInitialized(this)).call(this);
diff --git a/packages/babel-plugin-proposal-class-properties/test/fixtures/public/computed/output.js b/packages/babel-plugin-proposal-class-properties/test/fixtures/public/computed/output.js
index a52e83bc2f..155ee19677 100644
--- a/packages/babel-plugin-proposal-class-properties/test/fixtures/public/computed/output.js
+++ b/packages/babel-plugin-proposal-class-properties/test/fixtures/public/computed/output.js
@@ -24,6 +24,8 @@ var _ref4 = `template${expression}`;
 var MyClass =
 /*#__PURE__*/
 function () {
+  "use strict";
+
   function MyClass() {
     babelHelpers.classCallCheck(this, MyClass);
     Object.defineProperty(this, null, {
diff --git a/packages/babel-plugin-proposal-class-properties/test/fixtures/public/constructor-collision/output.js b/packages/babel-plugin-proposal-class-properties/test/fixtures/public/constructor-collision/output.js
index 9d80c3926c..cd3bee9780 100644
--- a/packages/babel-plugin-proposal-class-properties/test/fixtures/public/constructor-collision/output.js
+++ b/packages/babel-plugin-proposal-class-properties/test/fixtures/public/constructor-collision/output.js
@@ -1,6 +1,8 @@
 var foo = "bar";
 
 var Foo = function Foo() {
+  "use strict";
+
   babelHelpers.classCallCheck(this, Foo);
   Object.defineProperty(this, "bar", {
     configurable: true,
diff --git a/packages/babel-plugin-proposal-class-properties/test/fixtures/public/derived/output.js b/packages/babel-plugin-proposal-class-properties/test/fixtures/public/derived/output.js
index 332f2cb090..29668c34fd 100644
--- a/packages/babel-plugin-proposal-class-properties/test/fixtures/public/derived/output.js
+++ b/packages/babel-plugin-proposal-class-properties/test/fixtures/public/derived/output.js
@@ -1,6 +1,8 @@
 var Foo =
 /*#__PURE__*/
 function (_Bar) {
+  "use strict";
+
   babelHelpers.inherits(Foo, _Bar);
 
   function Foo(...args) {
diff --git a/packages/babel-plugin-proposal-class-properties/test/fixtures/public/foobar/output.js b/packages/babel-plugin-proposal-class-properties/test/fixtures/public/foobar/output.js
index 6404800a00..db8ae705a9 100644
--- a/packages/babel-plugin-proposal-class-properties/test/fixtures/public/foobar/output.js
+++ b/packages/babel-plugin-proposal-class-properties/test/fixtures/public/foobar/output.js
@@ -1,6 +1,8 @@
 var Child =
 /*#__PURE__*/
 function (_Parent) {
+  "use strict";
+
   babelHelpers.inherits(Child, _Parent);
 
   function Child() {
diff --git a/packages/babel-plugin-proposal-class-properties/test/fixtures/public/instance-computed/output.js b/packages/babel-plugin-proposal-class-properties/test/fixtures/public/instance-computed/output.js
index 8e21a1801f..7edfd976f9 100644
--- a/packages/babel-plugin-proposal-class-properties/test/fixtures/public/instance-computed/output.js
+++ b/packages/babel-plugin-proposal-class-properties/test/fixtures/public/instance-computed/output.js
@@ -2,6 +2,8 @@ function test(x) {
   var _x = x;
 
   var F = function F() {
+    "use strict";
+
     babelHelpers.classCallCheck(this, F);
     Object.defineProperty(this, _x, {
       configurable: true,
diff --git a/packages/babel-plugin-proposal-class-properties/test/fixtures/public/instance-undefined/output.js b/packages/babel-plugin-proposal-class-properties/test/fixtures/public/instance-undefined/output.js
index a039021e41..e9996a624b 100644
--- a/packages/babel-plugin-proposal-class-properties/test/fixtures/public/instance-undefined/output.js
+++ b/packages/babel-plugin-proposal-class-properties/test/fixtures/public/instance-undefined/output.js
@@ -1,4 +1,6 @@
 var Foo = function Foo() {
+  "use strict";
+
   babelHelpers.classCallCheck(this, Foo);
   Object.defineProperty(this, "bar", {
     configurable: true,
diff --git a/packages/babel-plugin-proposal-class-properties/test/fixtures/public/instance/output.js b/packages/babel-plugin-proposal-class-properties/test/fixtures/public/instance/output.js
index caa2e35bbc..ef0a7c734e 100644
--- a/packages/babel-plugin-proposal-class-properties/test/fixtures/public/instance/output.js
+++ b/packages/babel-plugin-proposal-class-properties/test/fixtures/public/instance/output.js
@@ -1,4 +1,6 @@
 var Foo = function Foo() {
+  "use strict";
+
   babelHelpers.classCallCheck(this, Foo);
   Object.defineProperty(this, "bar", {
     configurable: true,
diff --git a/packages/babel-plugin-proposal-class-properties/test/fixtures/public/regression-T6719/output.js b/packages/babel-plugin-proposal-class-properties/test/fixtures/public/regression-T6719/output.js
index 874bc077bb..4e45d2b07f 100644
--- a/packages/babel-plugin-proposal-class-properties/test/fixtures/public/regression-T6719/output.js
+++ b/packages/babel-plugin-proposal-class-properties/test/fixtures/public/regression-T6719/output.js
@@ -4,6 +4,8 @@ function withContext(ComposedComponent) {
   return _temp = _class =
   /*#__PURE__*/
   function (_Component) {
+    "use strict";
+
     babelHelpers.inherits(WithContext, _Component);
 
     function WithContext() {
diff --git a/packages/babel-plugin-proposal-class-properties/test/fixtures/public/static-infer-name/output.js b/packages/babel-plugin-proposal-class-properties/test/fixtures/public/static-infer-name/output.js
index 479b45b8ef..2b91078682 100644
--- a/packages/babel-plugin-proposal-class-properties/test/fixtures/public/static-infer-name/output.js
+++ b/packages/babel-plugin-proposal-class-properties/test/fixtures/public/static-infer-name/output.js
@@ -1,6 +1,8 @@
 var _class, _temp;
 
 var Foo = (_temp = _class = function Foo() {
+  "use strict";
+
   babelHelpers.classCallCheck(this, Foo);
 }, Object.defineProperty(_class, "num", {
   configurable: true,
diff --git a/packages/babel-plugin-proposal-class-properties/test/fixtures/public/static-undefined/output.js b/packages/babel-plugin-proposal-class-properties/test/fixtures/public/static-undefined/output.js
index dd57556d7c..efcc95d4ed 100644
--- a/packages/babel-plugin-proposal-class-properties/test/fixtures/public/static-undefined/output.js
+++ b/packages/babel-plugin-proposal-class-properties/test/fixtures/public/static-undefined/output.js
@@ -1,4 +1,6 @@
 var Foo = function Foo() {
+  "use strict";
+
   babelHelpers.classCallCheck(this, Foo);
 };
 
diff --git a/packages/babel-plugin-proposal-class-properties/test/fixtures/public/static/output.js b/packages/babel-plugin-proposal-class-properties/test/fixtures/public/static/output.js
index 4f84872b8a..34b428919a 100644
--- a/packages/babel-plugin-proposal-class-properties/test/fixtures/public/static/output.js
+++ b/packages/babel-plugin-proposal-class-properties/test/fixtures/public/static/output.js
@@ -1,4 +1,6 @@
 var Foo = function Foo() {
+  "use strict";
+
   babelHelpers.classCallCheck(this, Foo);
 };
 
diff --git a/packages/babel-plugin-proposal-class-properties/test/fixtures/public/super-call/output.js b/packages/babel-plugin-proposal-class-properties/test/fixtures/public/super-call/output.js
index 365452158f..526591d0ef 100644
--- a/packages/babel-plugin-proposal-class-properties/test/fixtures/public/super-call/output.js
+++ b/packages/babel-plugin-proposal-class-properties/test/fixtures/public/super-call/output.js
@@ -1,6 +1,8 @@
 var A =
 /*#__PURE__*/
 function () {
+  "use strict";
+
   function A() {
     babelHelpers.classCallCheck(this, A);
   }
@@ -17,6 +19,8 @@ function () {
 var B =
 /*#__PURE__*/
 function (_A) {
+  "use strict";
+
   babelHelpers.inherits(B, _A);
 
   function B(...args) {
diff --git a/packages/babel-plugin-proposal-class-properties/test/fixtures/public/super-expression/output.js b/packages/babel-plugin-proposal-class-properties/test/fixtures/public/super-expression/output.js
index 942670c9bf..c47e317152 100644
--- a/packages/babel-plugin-proposal-class-properties/test/fixtures/public/super-expression/output.js
+++ b/packages/babel-plugin-proposal-class-properties/test/fixtures/public/super-expression/output.js
@@ -1,6 +1,8 @@
 var Foo =
 /*#__PURE__*/
 function (_Bar) {
+  "use strict";
+
   babelHelpers.inherits(Foo, _Bar);
 
   function Foo() {
diff --git a/packages/babel-plugin-proposal-class-properties/test/fixtures/public/super-statement/output.js b/packages/babel-plugin-proposal-class-properties/test/fixtures/public/super-statement/output.js
index c28e24fbb3..87c98cac68 100644
--- a/packages/babel-plugin-proposal-class-properties/test/fixtures/public/super-statement/output.js
+++ b/packages/babel-plugin-proposal-class-properties/test/fixtures/public/super-statement/output.js
@@ -1,6 +1,8 @@
 var Foo =
 /*#__PURE__*/
 function (_Bar) {
+  "use strict";
+
   babelHelpers.inherits(Foo, _Bar);
 
   function Foo() {
diff --git a/packages/babel-plugin-proposal-class-properties/test/fixtures/public/super-with-collision/output.js b/packages/babel-plugin-proposal-class-properties/test/fixtures/public/super-with-collision/output.js
index f172e7c8b8..08c52c6d10 100644
--- a/packages/babel-plugin-proposal-class-properties/test/fixtures/public/super-with-collision/output.js
+++ b/packages/babel-plugin-proposal-class-properties/test/fixtures/public/super-with-collision/output.js
@@ -1,4 +1,6 @@
 var A = function A(_force) {
+  "use strict";
+
   babelHelpers.classCallCheck(this, A);
   Object.defineProperty(this, "force", {
     configurable: true,
diff --git a/packages/babel-plugin-proposal-class-properties/test/fixtures/regression/6154/output.js b/packages/babel-plugin-proposal-class-properties/test/fixtures/regression/6154/output.js
index d6b68f2eee..7cb8f5de65 100644
--- a/packages/babel-plugin-proposal-class-properties/test/fixtures/regression/6154/output.js
+++ b/packages/babel-plugin-proposal-class-properties/test/fixtures/regression/6154/output.js
@@ -17,6 +17,8 @@ function _getPrototypeOf(o) { _getPrototypeOf = Object.getPrototypeOf || functio
 function _assertThisInitialized(self) { if (self === void 0) { throw new ReferenceError("this hasn't been initialised - super() hasn't been called"); } return self; }
 
 var Test = function Test() {
+  "use strict";
+
   var _this2 = this;
 
   _classCallCheck(this, Test);
diff --git a/packages/babel-plugin-proposal-class-properties/test/fixtures/regression/T6719/output.js b/packages/babel-plugin-proposal-class-properties/test/fixtures/regression/T6719/output.js
index c815369e9e..f8aebbe026 100644
--- a/packages/babel-plugin-proposal-class-properties/test/fixtures/regression/T6719/output.js
+++ b/packages/babel-plugin-proposal-class-properties/test/fixtures/regression/T6719/output.js
@@ -4,6 +4,8 @@ function withContext(ComposedComponent) {
   return _temp = _class =
   /*#__PURE__*/
   function (_Component) {
+    "use strict";
+
     babelHelpers.inherits(WithContext, _Component);
 
     function WithContext() {
diff --git a/packages/babel-plugin-proposal-decorators/test/fixtures/regression/7030/output.js b/packages/babel-plugin-proposal-decorators/test/fixtures/regression/7030/output.js
index af11401cc0..aa7a196e26 100644
--- a/packages/babel-plugin-proposal-decorators/test/fixtures/regression/7030/output.js
+++ b/packages/babel-plugin-proposal-decorators/test/fixtures/regression/7030/output.js
@@ -30,6 +30,8 @@ function generateAsyncAction(type) {
 var A =
 /*#__PURE__*/
 function (_B) {
+  "use strict";
+
   _inherits(A, _B);
 
   function A(timestamp) {
diff --git a/packages/babel-plugin-proposal-function-bind/test/fixtures/regression/T6984/output.js b/packages/babel-plugin-proposal-function-bind/test/fixtures/regression/T6984/output.js
index 2d5326366b..6e30a3b425 100644
--- a/packages/babel-plugin-proposal-function-bind/test/fixtures/regression/T6984/output.js
+++ b/packages/babel-plugin-proposal-function-bind/test/fixtures/regression/T6984/output.js
@@ -11,6 +11,8 @@ function _two() {}
 let Test1 =
 /*#__PURE__*/
 function () {
+  "use strict";
+
   function Test1() {
     _classCallCheck(this, Test1);
   }
diff --git a/packages/babel-plugin-transform-classes/src/transformClass.js b/packages/babel-plugin-transform-classes/src/transformClass.js
index 8a2c9d6439..b4283eedf8 100644
--- a/packages/babel-plugin-transform-classes/src/transformClass.js
+++ b/packages/babel-plugin-transform-classes/src/transformClass.js
@@ -688,16 +688,51 @@ export default function transformClass(
       classState.staticPropBody.map(fn => fn(t.cloneNode(classState.classRef))),
     );
 
-    if (classState.classId && body.length === 1) {
+    const strictParent = path.findParent(path => {
+      if (path.isProgram() && path.node.sourceType === "module") {
+        return true;
+      }
+
+      if (path.isClassBody()) {
+        return true;
+      }
+
+      if (!path.isProgram() && !path.isBlockStatement()) {
+        return false;
+      }
+
+      return path.node.directives.some(
+        directive => directive.value.value === "use strict",
+      );
+    });
+
+    let constructorOnly = classState.classId && body.length === 1;
+    if (constructorOnly && !strictParent) {
+      for (const param of classState.construct.params) {
+        // It's illegal to put a use strict directive into the body of a function
+        // with non-simple parameters for some reason. So, we have to use a strict
+        // wrapper function.
+        if (!t.isIdentifier(param)) {
+          constructorOnly = false;
+          break;
+        }
+      }
+    }
+
+    const directives = constructorOnly ? body[0].body.directives : [];
+    if (!strictParent) {
+      directives.push(t.directive(t.directiveLiteral("use strict")));
+    }
+
+    if (constructorOnly) {
       // named class with only a constructor
       return t.toExpression(body[0]);
     }
 
     body.push(t.returnStatement(t.cloneNode(classState.classRef)));
-
     const container = t.arrowFunctionExpression(
       closureParams,
-      t.blockStatement(body),
+      t.blockStatement(body, directives),
     );
     return t.callExpression(container, closureArgs);
   }
diff --git a/packages/babel-plugin-transform-classes/test/fixtures/extend-builtins/loose/output.js b/packages/babel-plugin-transform-classes/test/fixtures/extend-builtins/loose/output.js
index 409e1211ac..b4cb5c179a 100644
--- a/packages/babel-plugin-transform-classes/test/fixtures/extend-builtins/loose/output.js
+++ b/packages/babel-plugin-transform-classes/test/fixtures/extend-builtins/loose/output.js
@@ -11,6 +11,8 @@ function _getPrototypeOf(o) { _getPrototypeOf = Object.getPrototypeOf || functio
 var List =
 /*#__PURE__*/
 function (_Array) {
+  "use strict";
+
   _inheritsLoose(List, _Array);
 
   function List() {
diff --git a/packages/babel-plugin-transform-classes/test/fixtures/extend-builtins/shadowed/output.js b/packages/babel-plugin-transform-classes/test/fixtures/extend-builtins/shadowed/output.js
index 19472774a1..dae6970860 100644
--- a/packages/babel-plugin-transform-classes/test/fixtures/extend-builtins/shadowed/output.js
+++ b/packages/babel-plugin-transform-classes/test/fixtures/extend-builtins/shadowed/output.js
@@ -1,10 +1,14 @@
 let Array = function Array() {
+  "use strict";
+
   babelHelpers.classCallCheck(this, Array);
 };
 
 let List =
 /*#__PURE__*/
 function (_Array) {
+  "use strict";
+
   babelHelpers.inherits(List, _Array);
 
   function List() {
diff --git a/packages/babel-plugin-transform-classes/test/fixtures/extend-builtins/spec/output.js b/packages/babel-plugin-transform-classes/test/fixtures/extend-builtins/spec/output.js
index df62b55860..708a9c4b80 100644
--- a/packages/babel-plugin-transform-classes/test/fixtures/extend-builtins/spec/output.js
+++ b/packages/babel-plugin-transform-classes/test/fixtures/extend-builtins/spec/output.js
@@ -17,6 +17,8 @@ function _getPrototypeOf(o) { _getPrototypeOf = Object.getPrototypeOf || functio
 var List =
 /*#__PURE__*/
 function (_Array) {
+  "use strict";
+
   _inherits(List, _Array);
 
   function List() {
diff --git a/packages/babel-plugin-transform-classes/test/fixtures/loose-classCallCheck/class/output.js b/packages/babel-plugin-transform-classes/test/fixtures/loose-classCallCheck/class/output.js
index 8311a6c3db..ad9405abde 100644
--- a/packages/babel-plugin-transform-classes/test/fixtures/loose-classCallCheck/class/output.js
+++ b/packages/babel-plugin-transform-classes/test/fixtures/loose-classCallCheck/class/output.js
@@ -1 +1,3 @@
-let A = function A() {};
+let A = function A() {
+  "use strict";
+};
diff --git a/packages/babel-plugin-transform-classes/test/fixtures/loose-classCallCheck/with-constructor/output.js b/packages/babel-plugin-transform-classes/test/fixtures/loose-classCallCheck/with-constructor/output.js
index b06aa9274b..1844688c05 100644
--- a/packages/babel-plugin-transform-classes/test/fixtures/loose-classCallCheck/with-constructor/output.js
+++ b/packages/babel-plugin-transform-classes/test/fixtures/loose-classCallCheck/with-constructor/output.js
@@ -1,10 +1,14 @@
 let A = function A() {
+  "use strict";
+
   console.log('a');
 };
 
 let B =
 /*#__PURE__*/
 function () {
+  "use strict";
+
   function B() {}
 
   var _proto = B.prototype;
diff --git a/packages/babel-plugin-transform-classes/test/fixtures/loose-classCallCheck/with-superClass/output.js b/packages/babel-plugin-transform-classes/test/fixtures/loose-classCallCheck/with-superClass/output.js
index a8e18067dd..7dfbc2cda9 100644
--- a/packages/babel-plugin-transform-classes/test/fixtures/loose-classCallCheck/with-superClass/output.js
+++ b/packages/babel-plugin-transform-classes/test/fixtures/loose-classCallCheck/with-superClass/output.js
@@ -2,11 +2,15 @@ function _assertThisInitialized(self) { if (self === void 0) { throw new Referen
 
 function _inheritsLoose(subClass, superClass) { subClass.prototype = Object.create(superClass.prototype); subClass.prototype.constructor = subClass; subClass.__proto__ = superClass; }
 
-let B = function B() {};
+let B = function B() {
+  "use strict";
+};
 
 let A =
 /*#__PURE__*/
 function (_B) {
+  "use strict";
+
   _inheritsLoose(A, _B);
 
   function A(track) {
diff --git a/packages/babel-plugin-transform-classes/test/fixtures/loose/accessing-super-class/output.js b/packages/babel-plugin-transform-classes/test/fixtures/loose/accessing-super-class/output.js
index 89e2167cad..ceb0946730 100644
--- a/packages/babel-plugin-transform-classes/test/fixtures/loose/accessing-super-class/output.js
+++ b/packages/babel-plugin-transform-classes/test/fixtures/loose/accessing-super-class/output.js
@@ -1,6 +1,8 @@
 var Test =
 /*#__PURE__*/
 function (_Foo) {
+  "use strict";
+
   babelHelpers.inheritsLoose(Test, _Foo);
 
   function Test() {
diff --git a/packages/babel-plugin-transform-classes/test/fixtures/loose/accessing-super-properties/output.js b/packages/babel-plugin-transform-classes/test/fixtures/loose/accessing-super-properties/output.js
index c4f39bbb58..8ecd49b4ca 100644
--- a/packages/babel-plugin-transform-classes/test/fixtures/loose/accessing-super-properties/output.js
+++ b/packages/babel-plugin-transform-classes/test/fixtures/loose/accessing-super-properties/output.js
@@ -1,6 +1,8 @@
 var Test =
 /*#__PURE__*/
 function (_Foo) {
+  "use strict";
+
   babelHelpers.inheritsLoose(Test, _Foo);
 
   function Test() {
diff --git a/packages/babel-plugin-transform-classes/test/fixtures/loose/blockstatement-with-use-strict-directive/input.js b/packages/babel-plugin-transform-classes/test/fixtures/loose/blockstatement-with-use-strict-directive/input.js
new file mode 100644
index 0000000000..6ae68b11c2
--- /dev/null
+++ b/packages/babel-plugin-transform-classes/test/fixtures/loose/blockstatement-with-use-strict-directive/input.js
@@ -0,0 +1,6 @@
+function t() {
+    "use strict";
+    class Foo {
+      method() {}
+    }
+  }
\ No newline at end of file
diff --git a/packages/babel-plugin-transform-classes/test/fixtures/loose/blockstatement-with-use-strict-directive/output.js b/packages/babel-plugin-transform-classes/test/fixtures/loose/blockstatement-with-use-strict-directive/output.js
new file mode 100644
index 0000000000..9295d5720e
--- /dev/null
+++ b/packages/babel-plugin-transform-classes/test/fixtures/loose/blockstatement-with-use-strict-directive/output.js
@@ -0,0 +1,15 @@
+function t() {
+  "use strict";
+
+  var Foo =
+  /*#__PURE__*/
+  function () {
+    function Foo() {}
+
+    var _proto = Foo.prototype;
+
+    _proto.method = function method() {};
+
+    return Foo;
+  }();
+}
diff --git a/packages/babel-plugin-transform-classes/test/fixtures/loose/calling-super-properties/output.js b/packages/babel-plugin-transform-classes/test/fixtures/loose/calling-super-properties/output.js
index 81b0cc1098..2e6eba2520 100644
--- a/packages/babel-plugin-transform-classes/test/fixtures/loose/calling-super-properties/output.js
+++ b/packages/babel-plugin-transform-classes/test/fixtures/loose/calling-super-properties/output.js
@@ -1,6 +1,8 @@
 var Test =
 /*#__PURE__*/
 function (_Foo) {
+  "use strict";
+
   babelHelpers.inheritsLoose(Test, _Foo);
 
   function Test() {
diff --git a/packages/babel-plugin-transform-classes/test/fixtures/loose/constructor-order/output.js b/packages/babel-plugin-transform-classes/test/fixtures/loose/constructor-order/output.js
index 41baf4126a..388e99e459 100644
--- a/packages/babel-plugin-transform-classes/test/fixtures/loose/constructor-order/output.js
+++ b/packages/babel-plugin-transform-classes/test/fixtures/loose/constructor-order/output.js
@@ -1,6 +1,8 @@
 var x =
 /*#__PURE__*/
 function () {
+  "use strict";
+
   var _proto = x.prototype;
 
   _proto.f = function f() {
diff --git a/packages/babel-plugin-transform-classes/test/fixtures/loose/derived-constructor-must-call-super/output.js b/packages/babel-plugin-transform-classes/test/fixtures/loose/derived-constructor-must-call-super/output.js
index ecb4d54469..a221b7cfb7 100644
--- a/packages/babel-plugin-transform-classes/test/fixtures/loose/derived-constructor-must-call-super/output.js
+++ b/packages/babel-plugin-transform-classes/test/fixtures/loose/derived-constructor-must-call-super/output.js
@@ -1,6 +1,8 @@
 var Foo =
 /*#__PURE__*/
 function (_Bar) {
+  "use strict";
+
   babelHelpers.inheritsLoose(Foo, _Bar);
 
   function Foo() {
diff --git a/packages/babel-plugin-transform-classes/test/fixtures/loose/derived-constructor-no-super-return-falsey/output.js b/packages/babel-plugin-transform-classes/test/fixtures/loose/derived-constructor-no-super-return-falsey/output.js
index 8f91a1a798..c8ecc7ad2a 100644
--- a/packages/babel-plugin-transform-classes/test/fixtures/loose/derived-constructor-no-super-return-falsey/output.js
+++ b/packages/babel-plugin-transform-classes/test/fixtures/loose/derived-constructor-no-super-return-falsey/output.js
@@ -1,6 +1,8 @@
 var Child =
 /*#__PURE__*/
 function (_Base) {
+  "use strict";
+
   babelHelpers.inheritsLoose(Child, _Base);
 
   function Child() {
diff --git a/packages/babel-plugin-transform-classes/test/fixtures/loose/derived-constructor-no-super-return-object/output.js b/packages/babel-plugin-transform-classes/test/fixtures/loose/derived-constructor-no-super-return-object/output.js
index 9d6464d2f2..fe7950eb71 100644
--- a/packages/babel-plugin-transform-classes/test/fixtures/loose/derived-constructor-no-super-return-object/output.js
+++ b/packages/babel-plugin-transform-classes/test/fixtures/loose/derived-constructor-no-super-return-object/output.js
@@ -1,6 +1,8 @@
 var Child =
 /*#__PURE__*/
 function (_Base) {
+  "use strict";
+
   babelHelpers.inheritsLoose(Child, _Base);
 
   function Child() {
diff --git a/packages/babel-plugin-transform-classes/test/fixtures/loose/literal-key/output.js b/packages/babel-plugin-transform-classes/test/fixtures/loose/literal-key/output.js
index f03cd91ce8..15ab4f17a6 100644
--- a/packages/babel-plugin-transform-classes/test/fixtures/loose/literal-key/output.js
+++ b/packages/babel-plugin-transform-classes/test/fixtures/loose/literal-key/output.js
@@ -1,6 +1,8 @@
 var Foo =
 /*#__PURE__*/
 function () {
+  "use strict";
+
   function Foo() {}
 
   var _proto = Foo.prototype;
diff --git a/packages/babel-plugin-transform-classes/test/fixtures/loose/method-return-type-annotation/output.js b/packages/babel-plugin-transform-classes/test/fixtures/loose/method-return-type-annotation/output.js
index 70948a6738..a545674a57 100644
--- a/packages/babel-plugin-transform-classes/test/fixtures/loose/method-return-type-annotation/output.js
+++ b/packages/babel-plugin-transform-classes/test/fixtures/loose/method-return-type-annotation/output.js
@@ -2,6 +2,8 @@
 var C =
 /*#__PURE__*/
 function () {
+  "use strict";
+
   function C() {}
 
   var _proto = C.prototype;
diff --git a/packages/babel-plugin-transform-classes/test/fixtures/loose/method-reuse-prototype/output.js b/packages/babel-plugin-transform-classes/test/fixtures/loose/method-reuse-prototype/output.js
index fbfea1f9dd..49978484eb 100644
--- a/packages/babel-plugin-transform-classes/test/fixtures/loose/method-reuse-prototype/output.js
+++ b/packages/babel-plugin-transform-classes/test/fixtures/loose/method-reuse-prototype/output.js
@@ -1,6 +1,8 @@
 var Test =
 /*#__PURE__*/
 function () {
+  "use strict";
+
   function Test() {}
 
   var _proto = Test.prototype;
diff --git a/packages/babel-plugin-transform-classes/test/fixtures/loose/program-with-use-strict-directive/input.js b/packages/babel-plugin-transform-classes/test/fixtures/loose/program-with-use-strict-directive/input.js
new file mode 100644
index 0000000000..4bbf0638b3
--- /dev/null
+++ b/packages/babel-plugin-transform-classes/test/fixtures/loose/program-with-use-strict-directive/input.js
@@ -0,0 +1,7 @@
+"use strict";
+
+class Foo {
+    method() {
+        
+    }
+}
\ No newline at end of file
diff --git a/packages/babel-plugin-transform-classes/test/fixtures/loose/program-with-use-strict-directive/output.js b/packages/babel-plugin-transform-classes/test/fixtures/loose/program-with-use-strict-directive/output.js
new file mode 100644
index 0000000000..54034fca62
--- /dev/null
+++ b/packages/babel-plugin-transform-classes/test/fixtures/loose/program-with-use-strict-directive/output.js
@@ -0,0 +1,13 @@
+"use strict";
+
+var Foo =
+/*#__PURE__*/
+function () {
+  function Foo() {}
+
+  var _proto = Foo.prototype;
+
+  _proto.method = function method() {};
+
+  return Foo;
+}();
diff --git a/packages/babel-plugin-transform-classes/test/fixtures/loose/super-class-id-member-expression/output.js b/packages/babel-plugin-transform-classes/test/fixtures/loose/super-class-id-member-expression/output.js
index 7d4a5eceee..1a90473147 100644
--- a/packages/babel-plugin-transform-classes/test/fixtures/loose/super-class-id-member-expression/output.js
+++ b/packages/babel-plugin-transform-classes/test/fixtures/loose/super-class-id-member-expression/output.js
@@ -1,6 +1,8 @@
 var BaseController =
 /*#__PURE__*/
 function (_Chaplin$Controller) {
+  "use strict";
+
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   function BaseController() {
@@ -13,6 +15,8 @@ function (_Chaplin$Controller) {
 var BaseController2 =
 /*#__PURE__*/
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+  "use strict";
+
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diff --git a/packages/babel-plugin-transform-classes/test/fixtures/loose/super-class/output.js b/packages/babel-plugin-transform-classes/test/fixtures/loose/super-class/output.js
index 232a9f97e3..779cd3877c 100644
--- a/packages/babel-plugin-transform-classes/test/fixtures/loose/super-class/output.js
+++ b/packages/babel-plugin-transform-classes/test/fixtures/loose/super-class/output.js
@@ -1,6 +1,8 @@
 var Test =
 /*#__PURE__*/
 function (_Foo) {
+  "use strict";
+
   babelHelpers.inheritsLoose(Test, _Foo);
 
   function Test() {
diff --git a/packages/babel-plugin-transform-classes/test/fixtures/loose/super-function-fallback/output.js b/packages/babel-plugin-transform-classes/test/fixtures/loose/super-function-fallback/output.js
index 090bb67b1b..078d48fe93 100644
--- a/packages/babel-plugin-transform-classes/test/fixtures/loose/super-function-fallback/output.js
+++ b/packages/babel-plugin-transform-classes/test/fixtures/loose/super-function-fallback/output.js
@@ -1,3 +1,5 @@
 var Test = function Test() {
+  "use strict";
+
   Function.prototype.hasOwnProperty.call(this, "test");
 };
diff --git a/packages/babel-plugin-transform-classes/test/fixtures/regression/5817/output.js b/packages/babel-plugin-transform-classes/test/fixtures/regression/5817/output.js
index 51fb5e2723..af6350f201 100644
--- a/packages/babel-plugin-transform-classes/test/fixtures/regression/5817/output.js
+++ b/packages/babel-plugin-transform-classes/test/fixtures/regression/5817/output.js
@@ -1,6 +1,8 @@
 var A =
 /*#__PURE__*/
 function (_B) {
+  "use strict";
+
   babelHelpers.inherits(A, _B);
 
   function A() {
diff --git a/packages/babel-plugin-transform-classes/test/fixtures/regression/T2494/output.js b/packages/babel-plugin-transform-classes/test/fixtures/regression/T2494/output.js
index e343968a10..2ec0ec90d8 100644
--- a/packages/babel-plugin-transform-classes/test/fixtures/regression/T2494/output.js
+++ b/packages/babel-plugin-transform-classes/test/fixtures/regression/T2494/output.js
@@ -2,6 +2,8 @@ var x = {
   Foo:
   /*#__PURE__*/
   function (_Foo) {
+    "use strict";
+
     babelHelpers.inherits(_class, _Foo);
 
     function _class() {
diff --git a/packages/babel-plugin-transform-classes/test/fixtures/regression/T2997/output.js b/packages/babel-plugin-transform-classes/test/fixtures/regression/T2997/output.js
index 0437a00e7b..5779429055 100644
--- a/packages/babel-plugin-transform-classes/test/fixtures/regression/T2997/output.js
+++ b/packages/babel-plugin-transform-classes/test/fixtures/regression/T2997/output.js
@@ -1,10 +1,14 @@
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+  "use strict";
+
   babelHelpers.classCallCheck(this, A);
 };
 
 var B =
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+  "use strict";
+
   babelHelpers.inherits(B, _A);
 
   function B() {
diff --git a/packages/babel-plugin-transform-classes/test/fixtures/regression/T6712/output.js b/packages/babel-plugin-transform-classes/test/fixtures/regression/T6712/output.js
index 08cac4b578..0ab1f6cb2b 100644
--- a/packages/babel-plugin-transform-classes/test/fixtures/regression/T6712/output.js
+++ b/packages/babel-plugin-transform-classes/test/fixtures/regression/T6712/output.js
@@ -1,6 +1,8 @@
 var A =
 /*#__PURE__*/
 function () {
+  "use strict";
+
   function A() {
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diff --git a/packages/babel-plugin-transform-classes/test/fixtures/regression/T6755/output.js b/packages/babel-plugin-transform-classes/test/fixtures/regression/T6755/output.js
index 6b2fd1e691..bf1bf08282 100644
--- a/packages/babel-plugin-transform-classes/test/fixtures/regression/T6755/output.js
+++ b/packages/babel-plugin-transform-classes/test/fixtures/regression/T6755/output.js
@@ -1,6 +1,8 @@
 var Example =
 /*#__PURE__*/
 function () {
+  "use strict";
+
   function Example() {}
 
   var _proto = Example.prototype;
diff --git a/packages/babel-plugin-transform-classes/test/fixtures/regression/T7010/output.js b/packages/babel-plugin-transform-classes/test/fixtures/regression/T7010/output.js
index 4f8dfbdae2..724512cb32 100644
--- a/packages/babel-plugin-transform-classes/test/fixtures/regression/T7010/output.js
+++ b/packages/babel-plugin-transform-classes/test/fixtures/regression/T7010/output.js
@@ -1,6 +1,8 @@
 var Foo =
 /*#__PURE__*/
 function () {
+  "use strict";
+
   function Foo(val) {
     babelHelpers.classCallCheck(this, Foo);
     this._val = val;
diff --git a/packages/babel-plugin-transform-classes/test/fixtures/regression/T7537/output.js b/packages/babel-plugin-transform-classes/test/fixtures/regression/T7537/output.js
index 4f7e6d5549..d9ca778729 100644
--- a/packages/babel-plugin-transform-classes/test/fixtures/regression/T7537/output.js
+++ b/packages/babel-plugin-transform-classes/test/fixtures/regression/T7537/output.js
@@ -15,12 +15,16 @@ function _instanceof(left, right) { if (right != null && typeof Symbol !== "unde
 function _classCallCheck(instance, Constructor) { if (!_instanceof(instance, Constructor)) { throw new TypeError("Cannot call a class as a function"); } }
 
 var B = function B() {
+  "use strict";
+
   _classCallCheck(this, B);
 };
 
 var A =
 /*#__PURE__*/
 function (_B) {
+  "use strict";
+
   _inherits(A, _B);
 
   function A(track) {
diff --git a/packages/babel-plugin-transform-classes/test/fixtures/spec/accessing-super-class/output.js b/packages/babel-plugin-transform-classes/test/fixtures/spec/accessing-super-class/output.js
index d6d2dcd716..fd268f2e93 100644
--- a/packages/babel-plugin-transform-classes/test/fixtures/spec/accessing-super-class/output.js
+++ b/packages/babel-plugin-transform-classes/test/fixtures/spec/accessing-super-class/output.js
@@ -1,6 +1,8 @@
 var Test =
 /*#__PURE__*/
 function (_Foo) {
+  "use strict";
+
   babelHelpers.inherits(Test, _Foo);
 
   function Test() {
diff --git a/packages/babel-plugin-transform-classes/test/fixtures/spec/accessing-super-properties/output.js b/packages/babel-plugin-transform-classes/test/fixtures/spec/accessing-super-properties/output.js
index df07a5fe01..aef35bbb31 100644
--- a/packages/babel-plugin-transform-classes/test/fixtures/spec/accessing-super-properties/output.js
+++ b/packages/babel-plugin-transform-classes/test/fixtures/spec/accessing-super-properties/output.js
@@ -1,6 +1,8 @@
 var Test =
 /*#__PURE__*/
 function (_Foo) {
+  "use strict";
+
   babelHelpers.inherits(Test, _Foo);
 
   function Test() {
diff --git a/packages/babel-plugin-transform-classes/test/fixtures/spec/calling-super-properties/output.js b/packages/babel-plugin-transform-classes/test/fixtures/spec/calling-super-properties/output.js
index 5ff3862b0e..2451b695d1 100644
--- a/packages/babel-plugin-transform-classes/test/fixtures/spec/calling-super-properties/output.js
+++ b/packages/babel-plugin-transform-classes/test/fixtures/spec/calling-super-properties/output.js
@@ -1,6 +1,8 @@
 var Test =
 /*#__PURE__*/
 function (_Foo) {
+  "use strict";
+
   babelHelpers.inherits(Test, _Foo);
 
   function Test() {
diff --git a/packages/babel-plugin-transform-classes/test/fixtures/spec/computed-methods/output.js b/packages/babel-plugin-transform-classes/test/fixtures/spec/computed-methods/output.js
index cd00a3ce2d..63761f596e 100644
--- a/packages/babel-plugin-transform-classes/test/fixtures/spec/computed-methods/output.js
+++ b/packages/babel-plugin-transform-classes/test/fixtures/spec/computed-methods/output.js
@@ -1,6 +1,8 @@
 var Foo =
 /*#__PURE__*/
 function () {
+  "use strict";
+
   function Foo() {
     babelHelpers.classCallCheck(this, Foo);
   }
diff --git a/packages/babel-plugin-transform-classes/test/fixtures/spec/constructor-binding-collision/output.js b/packages/babel-plugin-transform-classes/test/fixtures/spec/constructor-binding-collision/output.js
index 5247dab668..176b0b84d7 100644
--- a/packages/babel-plugin-transform-classes/test/fixtures/spec/constructor-binding-collision/output.js
+++ b/packages/babel-plugin-transform-classes/test/fixtures/spec/constructor-binding-collision/output.js
@@ -1,4 +1,6 @@
 var Example = function Example() {
+  "use strict";
+
   babelHelpers.classCallCheck(this, Example);
 
   var _Example;
diff --git a/packages/babel-plugin-transform-classes/test/fixtures/spec/constructor-only/input.js b/packages/babel-plugin-transform-classes/test/fixtures/spec/constructor-only/input.js
new file mode 100644
index 0000000000..f5e21bc1d7
--- /dev/null
+++ b/packages/babel-plugin-transform-classes/test/fixtures/spec/constructor-only/input.js
@@ -0,0 +1,5 @@
+class Foo {
+    constructor() {
+
+    }
+}
\ No newline at end of file
diff --git a/packages/babel-plugin-transform-classes/test/fixtures/spec/constructor-only/output.js b/packages/babel-plugin-transform-classes/test/fixtures/spec/constructor-only/output.js
new file mode 100644
index 0000000000..43ce45f806
--- /dev/null
+++ b/packages/babel-plugin-transform-classes/test/fixtures/spec/constructor-only/output.js
@@ -0,0 +1,5 @@
+var Foo = function Foo() {
+  "use strict";
+
+  babelHelpers.classCallCheck(this, Foo);
+};
diff --git a/packages/babel-plugin-transform-classes/test/fixtures/spec/constructor/output.js b/packages/babel-plugin-transform-classes/test/fixtures/spec/constructor/output.js
index f43f3601bd..2ebe633512 100644
--- a/packages/babel-plugin-transform-classes/test/fixtures/spec/constructor/output.js
+++ b/packages/babel-plugin-transform-classes/test/fixtures/spec/constructor/output.js
@@ -1,4 +1,6 @@
 var Test = function Test() {
+  "use strict";
+
   babelHelpers.classCallCheck(this, Test);
   this.state = "test";
 };
@@ -6,6 +8,8 @@ var Test = function Test() {
 var Foo =
 /*#__PURE__*/
 function (_Bar) {
+  "use strict";
+
   babelHelpers.inherits(Foo, _Bar);
 
   function Foo() {
@@ -21,6 +25,8 @@ function (_Bar) {
 }(Bar);
 
 var ConstructorScoping = function ConstructorScoping() {
+  "use strict";
+
   babelHelpers.classCallCheck(this, ConstructorScoping);
   var bar;
   {
diff --git a/packages/babel-plugin-transform-classes/test/fixtures/spec/delay-arrow-function-for-bare-super-derived/output.js b/packages/babel-plugin-transform-classes/test/fixtures/spec/delay-arrow-function-for-bare-super-derived/output.js
index 18e3b10e80..83a4d9b341 100644
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+++ b/packages/babel-plugin-transform-classes/test/fixtures/spec/delay-arrow-function-for-bare-super-derived/output.js
@@ -1,6 +1,8 @@
 var Foo =
 /*#__PURE__*/
 function (_Bar) {
+  "use strict";
+
   babelHelpers.inherits(Foo, _Bar);
 
   function Foo() {
diff --git a/packages/babel-plugin-transform-classes/test/fixtures/spec/derived-constructor-must-call-super-2/output.js b/packages/babel-plugin-transform-classes/test/fixtures/spec/derived-constructor-must-call-super-2/output.js
index 3fdb9ef413..876987aef3 100644
--- a/packages/babel-plugin-transform-classes/test/fixtures/spec/derived-constructor-must-call-super-2/output.js
+++ b/packages/babel-plugin-transform-classes/test/fixtures/spec/derived-constructor-must-call-super-2/output.js
@@ -1,6 +1,8 @@
 var Foo =
 /*#__PURE__*/
 function (_Bar) {
+  "use strict";
+
   babelHelpers.inherits(Foo, _Bar);
 
   function Foo() {
diff --git a/packages/babel-plugin-transform-classes/test/fixtures/spec/derived-constructor-must-call-super-3/output.js b/packages/babel-plugin-transform-classes/test/fixtures/spec/derived-constructor-must-call-super-3/output.js
index 3211f729d8..1859492a7b 100644
--- a/packages/babel-plugin-transform-classes/test/fixtures/spec/derived-constructor-must-call-super-3/output.js
+++ b/packages/babel-plugin-transform-classes/test/fixtures/spec/derived-constructor-must-call-super-3/output.js
@@ -1,6 +1,8 @@
 var Foo =
 /*#__PURE__*/
 function (_Bar) {
+  "use strict";
+
   babelHelpers.inherits(Foo, _Bar);
 
   function Foo() {
diff --git a/packages/babel-plugin-transform-classes/test/fixtures/spec/derived-constructor-must-call-super-4/output.js b/packages/babel-plugin-transform-classes/test/fixtures/spec/derived-constructor-must-call-super-4/output.js
index 0bd914d207..3ffe7db5e6 100644
--- a/packages/babel-plugin-transform-classes/test/fixtures/spec/derived-constructor-must-call-super-4/output.js
+++ b/packages/babel-plugin-transform-classes/test/fixtures/spec/derived-constructor-must-call-super-4/output.js
@@ -1,6 +1,8 @@
 var Foo =
 /*#__PURE__*/
 function (_Bar) {
+  "use strict";
+
   babelHelpers.inherits(Foo, _Bar);
 
   function Foo() {
diff --git a/packages/babel-plugin-transform-classes/test/fixtures/spec/derived-constructor-must-call-super/output.js b/packages/babel-plugin-transform-classes/test/fixtures/spec/derived-constructor-must-call-super/output.js
index d1029e22e3..1a1fed3e09 100644
--- a/packages/babel-plugin-transform-classes/test/fixtures/spec/derived-constructor-must-call-super/output.js
+++ b/packages/babel-plugin-transform-classes/test/fixtures/spec/derived-constructor-must-call-super/output.js
@@ -1,6 +1,8 @@
 var Foo =
 /*#__PURE__*/
 function (_Bar) {
+  "use strict";
+
   babelHelpers.inherits(Foo, _Bar);
 
   function Foo() {
diff --git a/packages/babel-plugin-transform-classes/test/fixtures/spec/derived-constructor-no-super-return-falsey/output.js b/packages/babel-plugin-transform-classes/test/fixtures/spec/derived-constructor-no-super-return-falsey/output.js
index 59f3410c93..231c297d7d 100644
--- a/packages/babel-plugin-transform-classes/test/fixtures/spec/derived-constructor-no-super-return-falsey/output.js
+++ b/packages/babel-plugin-transform-classes/test/fixtures/spec/derived-constructor-no-super-return-falsey/output.js
@@ -1,6 +1,8 @@
 var Child =
 /*#__PURE__*/
 function (_Base) {
+  "use strict";
+
   babelHelpers.inherits(Child, _Base);
 
   function Child() {
diff --git a/packages/babel-plugin-transform-classes/test/fixtures/spec/derived-constructor-no-super-return-object/output.js b/packages/babel-plugin-transform-classes/test/fixtures/spec/derived-constructor-no-super-return-object/output.js
index fb91ff5148..f0f8887eda 100644
--- a/packages/babel-plugin-transform-classes/test/fixtures/spec/derived-constructor-no-super-return-object/output.js
+++ b/packages/babel-plugin-transform-classes/test/fixtures/spec/derived-constructor-no-super-return-object/output.js
@@ -1,6 +1,8 @@
 var Child =
 /*#__PURE__*/
 function (_Base) {
+  "use strict";
+
   babelHelpers.inherits(Child, _Base);
 
   function Child() {
diff --git a/packages/babel-plugin-transform-classes/test/fixtures/spec/inferred-expression-name/output.js b/packages/babel-plugin-transform-classes/test/fixtures/spec/inferred-expression-name/output.js
index f2bb238d0a..dc3d9a57e1 100644
--- a/packages/babel-plugin-transform-classes/test/fixtures/spec/inferred-expression-name/output.js
+++ b/packages/babel-plugin-transform-classes/test/fixtures/spec/inferred-expression-name/output.js
@@ -1,5 +1,7 @@
 var o = {
   foo: function foo() {
+    "use strict";
+
     babelHelpers.classCallCheck(this, foo);
   }
 };
diff --git a/packages/babel-plugin-transform-classes/test/fixtures/spec/instance-getter-and-setter/output.js b/packages/babel-plugin-transform-classes/test/fixtures/spec/instance-getter-and-setter/output.js
index 4e6b1addd2..fb7c1e0a44 100644
--- a/packages/babel-plugin-transform-classes/test/fixtures/spec/instance-getter-and-setter/output.js
+++ b/packages/babel-plugin-transform-classes/test/fixtures/spec/instance-getter-and-setter/output.js
@@ -1,6 +1,8 @@
 var Test =
 /*#__PURE__*/
 function () {
+  "use strict";
+
   function Test() {
     babelHelpers.classCallCheck(this, Test);
   }
diff --git a/packages/babel-plugin-transform-classes/test/fixtures/spec/instance-getter/output.js b/packages/babel-plugin-transform-classes/test/fixtures/spec/instance-getter/output.js
index 3623db226b..7881728022 100644
--- a/packages/babel-plugin-transform-classes/test/fixtures/spec/instance-getter/output.js
+++ b/packages/babel-plugin-transform-classes/test/fixtures/spec/instance-getter/output.js
@@ -1,6 +1,8 @@
 var Test =
 /*#__PURE__*/
 function () {
+  "use strict";
+
   function Test() {
     babelHelpers.classCallCheck(this, Test);
   }
diff --git a/packages/babel-plugin-transform-classes/test/fixtures/spec/instance-method/output.js b/packages/babel-plugin-transform-classes/test/fixtures/spec/instance-method/output.js
index 240b4c1f05..88d63804cf 100644
--- a/packages/babel-plugin-transform-classes/test/fixtures/spec/instance-method/output.js
+++ b/packages/babel-plugin-transform-classes/test/fixtures/spec/instance-method/output.js
@@ -1,6 +1,8 @@
 var Test =
 /*#__PURE__*/
 function () {
+  "use strict";
+
   function Test() {
     babelHelpers.classCallCheck(this, Test);
   }
diff --git a/packages/babel-plugin-transform-classes/test/fixtures/spec/instance-setter/output.js b/packages/babel-plugin-transform-classes/test/fixtures/spec/instance-setter/output.js
index 62c12ab32a..86ccf7f552 100644
--- a/packages/babel-plugin-transform-classes/test/fixtures/spec/instance-setter/output.js
+++ b/packages/babel-plugin-transform-classes/test/fixtures/spec/instance-setter/output.js
@@ -1,6 +1,8 @@
 var Test =
 /*#__PURE__*/
 function () {
+  "use strict";
+
   function Test() {
     babelHelpers.classCallCheck(this, Test);
   }
diff --git a/packages/babel-plugin-transform-classes/test/fixtures/spec/method-return-type-annotation/output.js b/packages/babel-plugin-transform-classes/test/fixtures/spec/method-return-type-annotation/output.js
index e25af88244..3452df1054 100644
--- a/packages/babel-plugin-transform-classes/test/fixtures/spec/method-return-type-annotation/output.js
+++ b/packages/babel-plugin-transform-classes/test/fixtures/spec/method-return-type-annotation/output.js
@@ -2,6 +2,8 @@
 var C =
 /*#__PURE__*/
 function () {
+  "use strict";
+
   function C() {
     babelHelpers.classCallCheck(this, C);
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diff --git a/packages/babel-plugin-transform-classes/test/fixtures/spec/plain-class/output.js b/packages/babel-plugin-transform-classes/test/fixtures/spec/plain-class/output.js
index c18eb5aa04..19a9beffaf 100644
--- a/packages/babel-plugin-transform-classes/test/fixtures/spec/plain-class/output.js
+++ b/packages/babel-plugin-transform-classes/test/fixtures/spec/plain-class/output.js
@@ -1,3 +1,5 @@
 var Test = function Test() {
+  "use strict";
+
   babelHelpers.classCallCheck(this, Test);
 };
diff --git a/packages/babel-plugin-transform-classes/test/fixtures/spec/preserves-directives/output.js b/packages/babel-plugin-transform-classes/test/fixtures/spec/preserves-directives/output.js
index df1be86af7..2e1347219e 100644
--- a/packages/babel-plugin-transform-classes/test/fixtures/spec/preserves-directives/output.js
+++ b/packages/babel-plugin-transform-classes/test/fixtures/spec/preserves-directives/output.js
@@ -1,5 +1,6 @@
 var MyCtrl = function MyCtrl(a) {
   "any directive prologue";
+  "use strict";
 
   babelHelpers.classCallCheck(this, MyCtrl);
   foo;
@@ -8,6 +9,7 @@ var MyCtrl = function MyCtrl(a) {
 var MyCtrl2 = function MyCtrl2(a) {
   "a";
   "b";
+  "use strict";
 
   babelHelpers.classCallCheck(this, MyCtrl2);
   foo;
@@ -15,6 +17,7 @@ var MyCtrl2 = function MyCtrl2(a) {
 
 var MyCtrl3 = function MyCtrl3(a) {
   "a";
+  "use strict";
 
   babelHelpers.classCallCheck(this, MyCtrl3);
   foo;
diff --git a/packages/babel-plugin-transform-classes/test/fixtures/spec/relaxed-method-coercion/output.js b/packages/babel-plugin-transform-classes/test/fixtures/spec/relaxed-method-coercion/output.js
index 13ebcf8dc3..c233a15a0e 100644
--- a/packages/babel-plugin-transform-classes/test/fixtures/spec/relaxed-method-coercion/output.js
+++ b/packages/babel-plugin-transform-classes/test/fixtures/spec/relaxed-method-coercion/output.js
@@ -2,6 +2,8 @@
 var Foo =
 /*#__PURE__*/
 function () {
+  "use strict";
+
   function Foo() {
     babelHelpers.classCallCheck(this, Foo);
   }
diff --git a/packages/babel-plugin-transform-classes/test/fixtures/spec/statement/output.js b/packages/babel-plugin-transform-classes/test/fixtures/spec/statement/output.js
index b7cfb0487c..29c3bfc821 100644
--- a/packages/babel-plugin-transform-classes/test/fixtures/spec/statement/output.js
+++ b/packages/babel-plugin-transform-classes/test/fixtures/spec/statement/output.js
@@ -1,9 +1,13 @@
 var BaseView = function BaseView() {
+  "use strict";
+
   babelHelpers.classCallCheck(this, BaseView);
   this.autoRender = true;
 };
 
 var BaseView = function BaseView() {
+  "use strict";
+
   babelHelpers.classCallCheck(this, BaseView);
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 };
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@@ -1,6 +1,8 @@
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index da7a6bd2c0..8a994335e8 100644
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+++ b/packages/babel-plugin-transform-classes/test/fixtures/spec/super-class/output.js
@@ -1,6 +1,8 @@
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index a78ae7926c..301cbfcf3c 100644
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+++ b/packages/babel-plugin-transform-classes/test/fixtures/spec/super-function-fallback/output.js
@@ -1,4 +1,6 @@
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+
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@@ -1,6 +1,8 @@
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diff --git a/packages/babel-plugin-transform-classes/test/fixtures/spec/super-reference-in-prop-exression/output.js b/packages/babel-plugin-transform-classes/test/fixtures/spec/super-reference-in-prop-exression/output.js
index 5d1dd6f69c..eeb5168030 100644
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@@ -1,6 +1,8 @@
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+  "use strict";
+
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diff --git a/packages/babel-plugin-transform-classes/test/fixtures/spec/this-not-allowed-before-super-in-derived-classes-2/output.js b/packages/babel-plugin-transform-classes/test/fixtures/spec/this-not-allowed-before-super-in-derived-classes-2/output.js
index a38f951718..ec1d652c8d 100644
--- a/packages/babel-plugin-transform-classes/test/fixtures/spec/this-not-allowed-before-super-in-derived-classes-2/output.js
+++ b/packages/babel-plugin-transform-classes/test/fixtures/spec/this-not-allowed-before-super-in-derived-classes-2/output.js
@@ -1,6 +1,8 @@
 var Foo =
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 function (_Bar) {
+  "use strict";
+
   babelHelpers.inherits(Foo, _Bar);
 
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diff --git a/packages/babel-plugin-transform-classes/test/fixtures/spec/this-not-allowed-before-super-in-derived-classes-3/output.js b/packages/babel-plugin-transform-classes/test/fixtures/spec/this-not-allowed-before-super-in-derived-classes-3/output.js
index 753e40bf0a..06e41b47d3 100644
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@@ -1,6 +1,8 @@
 var Foo =
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+
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diff --git a/packages/babel-plugin-transform-classes/test/fixtures/spec/this-not-allowed-before-super-in-derived-classes-4/output.js b/packages/babel-plugin-transform-classes/test/fixtures/spec/this-not-allowed-before-super-in-derived-classes-4/output.js
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@@ -1,6 +1,8 @@
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+
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diff --git a/packages/babel-plugin-transform-classes/test/fixtures/spec/this-not-allowed-before-super-in-derived-classes-5/output.js b/packages/babel-plugin-transform-classes/test/fixtures/spec/this-not-allowed-before-super-in-derived-classes-5/output.js
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@@ -1,6 +1,8 @@
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+
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@@ -1,6 +1,8 @@
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diff --git a/packages/babel-plugin-transform-flow-strip-types/test/fixtures/regression/class-prop-types/output.js b/packages/babel-plugin-transform-flow-strip-types/test/fixtures/regression/class-prop-types/output.js
index a63c4ab872..864e3000b9 100644
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+++ b/packages/babel-plugin-transform-flow-strip-types/test/fixtures/regression/class-prop-types/output.js
@@ -1,3 +1,5 @@
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+
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diff --git a/packages/babel-plugin-transform-flow-strip-types/test/fixtures/regression/transformed-class-method-loose-return-type-annotation/output.js b/packages/babel-plugin-transform-flow-strip-types/test/fixtures/regression/transformed-class-method-loose-return-type-annotation/output.js
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@@ -1,6 +1,8 @@
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diff --git a/packages/babel-plugin-transform-function-name/test/fixtures/function-name/class-method/output.js b/packages/babel-plugin-transform-function-name/test/fixtures/function-name/class-method/output.js
index 6c554f273d..80ef49fb81 100644
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@@ -1,6 +1,8 @@
 let Foo =
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+
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diff --git a/packages/babel-plugin-transform-jscript/test/fixtures/jscript/simple-class/output.js b/packages/babel-plugin-transform-jscript/test/fixtures/jscript/simple-class/output.js
index 7d28d4011e..df1e015e5e 100644
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+++ b/packages/babel-plugin-transform-jscript/test/fixtures/jscript/simple-class/output.js
@@ -1,5 +1,7 @@
 var Test = function () {
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+    "use strict";
+
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diff --git a/packages/babel-plugin-transform-parameters/test/fixtures/parameters/default-iife-4253/output.js b/packages/babel-plugin-transform-parameters/test/fixtures/parameters/default-iife-4253/output.js
index 47997457af..4432ba6ae3 100644
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 Ref.nextID = 0;
diff --git a/packages/babel-plugin-transform-parameters/test/fixtures/parameters/default-iife-self/output.js b/packages/babel-plugin-transform-parameters/test/fixtures/parameters/default-iife-self/output.js
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+++ b/packages/babel-plugin-transform-parameters/test/fixtures/parameters/default-iife-self/output.js
@@ -1,11 +1,27 @@
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-  babelHelpers.classCallCheck(this, X);
-  this.x = x;
-};
+  function Ref() {
+    var ref = arguments.length > 0 && arguments[0] !== undefined ? arguments[0] : Ref;
+    babelHelpers.classCallCheck(this, Ref);
+    this.ref = ref;
+  }
+
+  return Ref;
+}();
+
+var X =
+/*#__PURE__*/
+function () {
+  "use strict";
+
+  function X() {
+    var x = arguments.length > 0 && arguments[0] !== undefined ? arguments[0] : foo;
+    babelHelpers.classCallCheck(this, X);
+    this.x = x;
+  }
+
+  return X;
+}();
diff --git a/packages/babel-plugin-transform-parameters/test/fixtures/parameters/rest-arrow-functions/output.js b/packages/babel-plugin-transform-parameters/test/fixtures/parameters/rest-arrow-functions/output.js
index d2856c566e..83b41f4d72 100644
--- a/packages/babel-plugin-transform-parameters/test/fixtures/parameters/rest-arrow-functions/output.js
+++ b/packages/babel-plugin-transform-parameters/test/fixtures/parameters/rest-arrow-functions/output.js
@@ -51,6 +51,8 @@ var innerclassproperties = function () {
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   return _temp = _class = function _class() {
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diff --git a/packages/babel-plugin-transform-parameters/test/fixtures/parameters/rest-nested-iife/output.js b/packages/babel-plugin-transform-parameters/test/fixtures/parameters/rest-nested-iife/output.js
index 7975fc6d85..d11588d86b 100644
--- a/packages/babel-plugin-transform-parameters/test/fixtures/parameters/rest-nested-iife/output.js
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@@ -3,6 +3,8 @@ function broken(x) {
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     /*#__PURE__*/
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diff --git a/packages/babel-plugin-transform-react-jsx/test/fixtures/react/optimisation.react.constant-elements/output.js b/packages/babel-plugin-transform-react-jsx/test/fixtures/react/optimisation.react.constant-elements/output.js
index 985568ba1d..1af954e468 100644
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@@ -9,6 +9,8 @@ var _ref =
 let App =
 /*#__PURE__*/
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diff --git a/packages/babel-plugin-transform-runtime/test/fixtures/runtime/class/output.js b/packages/babel-plugin-transform-runtime/test/fixtures/runtime/class/output.js
index a273aa1824..60d3e2fb72 100644
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+++ b/packages/babel-plugin-transform-runtime/test/fixtures/runtime/class/output.js
@@ -1,5 +1,7 @@
 var _classCallCheck = require("@babel/runtime/helpers/classCallCheck");
 
 let Foo = function Foo() {
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+
   _classCallCheck(this, Foo);
 };
diff --git a/packages/babel-plugin-transform-runtime/test/fixtures/runtime/no-helpers/output.js b/packages/babel-plugin-transform-runtime/test/fixtures/runtime/no-helpers/output.js
index 0040583f47..b8dfb31983 100644
--- a/packages/babel-plugin-transform-runtime/test/fixtures/runtime/no-helpers/output.js
+++ b/packages/babel-plugin-transform-runtime/test/fixtures/runtime/no-helpers/output.js
@@ -1,5 +1,7 @@
 function _classCallCheck(instance, Constructor) { if (!(instance instanceof Constructor)) { throw new TypeError("Cannot call a class as a function"); } }
 
 let Foo = function Foo() {
+  "use strict";
+
   _classCallCheck(this, Foo);
 };
diff --git a/packages/babel-plugin-transform-runtime/test/fixtures/use-options/useBuiltIns-useESModules/output.js b/packages/babel-plugin-transform-runtime/test/fixtures/use-options/useBuiltIns-useESModules/output.js
index e6a57dbeb5..d5ad90a1c2 100644
--- a/packages/babel-plugin-transform-runtime/test/fixtures/use-options/useBuiltIns-useESModules/output.js
+++ b/packages/babel-plugin-transform-runtime/test/fixtures/use-options/useBuiltIns-useESModules/output.js
@@ -9,6 +9,8 @@ var _inherits = require("@babel/runtime/helpers/builtin/es6/inherits");
 let Foo =
 /*#__PURE__*/
 function (_Bar) {
+  "use strict";
+
   _inherits(Foo, _Bar);
 
   function Foo() {
diff --git a/packages/babel-plugin-transform-runtime/test/fixtures/use-options/useBuiltIns/output.js b/packages/babel-plugin-transform-runtime/test/fixtures/use-options/useBuiltIns/output.js
index a9cfc25a85..0dcb1a6dc1 100644
--- a/packages/babel-plugin-transform-runtime/test/fixtures/use-options/useBuiltIns/output.js
+++ b/packages/babel-plugin-transform-runtime/test/fixtures/use-options/useBuiltIns/output.js
@@ -9,6 +9,8 @@ var _inherits = require("@babel/runtime/helpers/builtin/inherits");
 let Foo =
 /*#__PURE__*/
 function (_Bar) {
+  "use strict";
+
   _inherits(Foo, _Bar);
 
   function Foo() {
diff --git a/packages/babel-plugin-transform-runtime/test/fixtures/use-options/useESModules/output.js b/packages/babel-plugin-transform-runtime/test/fixtures/use-options/useESModules/output.js
index 19375e1d05..9d1599d1f2 100644
--- a/packages/babel-plugin-transform-runtime/test/fixtures/use-options/useESModules/output.js
+++ b/packages/babel-plugin-transform-runtime/test/fixtures/use-options/useESModules/output.js
@@ -9,6 +9,8 @@ var _inherits = require("@babel/runtime/helpers/es6/inherits");
 let Foo =
 /*#__PURE__*/
 function (_Bar) {
+  "use strict";
+
   _inherits(Foo, _Bar);
 
   function Foo() {
diff --git a/packages/babel-standalone/test/babel.js b/packages/babel-standalone/test/babel.js
index 0bc617450e..46f9187288 100644
--- a/packages/babel-standalone/test/babel.js
+++ b/packages/babel-standalone/test/babel.js
@@ -19,7 +19,7 @@
         sourceType: "script",
         presets: ["es2015-loose"],
       }).code;
-      expect(output).toBe("var A = function A() {};");
+      expect(output).toBe('var A = function A() {\n  "use strict";\n};');
     });
     it("handles the typescript preset", () => {
       const output = Babel.transform("var a: string;", {