feat(makeDefaultExport): add deprecation message for generated default exports

If a module does not define a default export, loader.js assigns `exports.default = exports` for compatibility with modules that don't use `_interopRequireDefault` from babel6. Mutating exports is unexpected behavior and causes difficult-to-diagnose [bugs][1]. We should remove `makeDefaultExport` in version 5.0. This update

* adds a deprecation warning in preparation for removing `makeDefaultExport`.
* adds a debug build with the deprecation enabled.

Addresses ember-cli#114

[1]:mike-north/ember-lodash#104

Author: Lenny Burdette

build.js

@@ -7,18 +7,54 @@ var mkdirp = require('mkdirp').sync;
 
 mkdirp('./dist/loader');
 var source = fs.readFileSync('./lib/loader/loader.js', 'utf8');
+
 var instrumented = transform(source, {
-  plugins: ['transform-es2015-destructuring']
+  plugins: [
+    'transform-es2015-destructuring',
+    ['babel-plugin-debug-macros', {
+      envFlags: {
+        source: 'env-flags',
+        flags: { DEBUG: true }
+      },
+      debugTools: {
+        source: 'debug-tools'
+      }
+    }]
+  ]
+}).code;
+
+var debug = transform(source, {
+  plugins: [
+    'transform-es2015-destructuring',
+    ['babel-plugin-debug-macros', {
+      envFlags: {
+        source: 'env-flags',
+        flags: { DEBUG: true }
+      },
+      debugTools: {
+        source: 'debug-tools'
+      }
+    }]
+  ]
 }).code;
 
 var stripped = transform(source, {
   // strip-heimdall *must* come before transpiling destructuring
   plugins: [
     'babel6-plugin-strip-heimdall',
-    'transform-es2015-destructuring'
+    'transform-es2015-destructuring',
+    ['babel-plugin-debug-macros', {
+      envFlags: {
+        source: 'env-flags',
+        flags: { DEBUG: false }
+      },
+      debugTools: {
+        source: 'debug-tools'
+      }
+    }]
   ],
 }).code;
 
 fs.writeFileSync('./dist/loader/loader.instrument.js', instrumented);
+fs.writeFileSync('./dist/loader/loader.debug.js', debug);
 fs.writeFileSync('./dist/loader/loader.js', stripped);
-

lib/loader/loader.js

@@ -1,3 +1,5 @@
+import { DEBUG } from 'env-flags';
+
 var loader, define, requireModule, require, requirejs;
 
 (function(global) {
@@ -110,12 +112,45 @@ var loader, define, requireModule, require, requirejs;
 
   }
 
+  if (DEBUG) {
+    var deprecationLogger = function () {
+      console.warn.apply(console, arguments);
+    };
+
+    var constructMakeDefaultExportDeprecation = function (id) {
+      return 'The `' + id + '` module does not define a default export, but loader.js ' +
+        'generated one anyway. This behavior is deprecated and will be removed in v5.0.0.';
+    };
+
+    Object.defineProperty(requirejs, 'deprecationLogger', {
+      enumerable: false,
+      get() {
+        return deprecationLogger;
+      },
+      set(newLogger) {
+        deprecationLogger = newLogger;
+      }
+    });
+  }
+
   Module.prototype.makeDefaultExport = function() {
     var exports = this.module.exports;
     if (exports !== null &&
         (typeof exports === 'object' || typeof exports === 'function') &&
           exports['default'] === undefined && Object.isExtensible(exports)) {
-      exports['default'] = exports;
+
+      if (DEBUG) {
+        var id = this.id;
+
+        Object.defineProperty(exports, 'default', {
+          get: function () {
+            deprecationLogger(constructMakeDefaultExportDeprecation(id));
+            return exports;
+          }
+        });
+      } else {
+        exports['default'] = exports;
+      }
     }
   };
 

package.json

@@ -10,6 +10,7 @@
   "devDependencies": {
     "ara": "0.0.3",
     "babel-core": "^6.25.0",
+    "babel-plugin-debug-macros": "^0.1.10",
     "babel-plugin-transform-es2015-destructuring": "^6.23.0",
     "babel6-plugin-strip-heimdall": "^6.0.1",
     "heimdalljs": "^0.3.2",

tests/all.js

@@ -640,6 +640,11 @@ test('if factory returns a value it is used as export', function() {
 });
 
 test('if a module has no default property assume the return is the default', function() {
+  var deprecationMessage;
+  require.deprecationLogger = function(message) {
+    deprecationMessage = message;
+  };
+
   define('foo', [], function() {
     return {
       bar: 'bar'
@@ -664,10 +669,17 @@ test('if a module has no default property assume the return is the default', fun
   });
 
   equal(foo.bar, 'bar');
+
+  equal(deprecationMessage, 'The `foo` module does not define a default export, but loader.js generated one anyway. This behavior is deprecated and will be removed in v5.0.0.');
 });
 
 
 test('if a CJS style module has no default export assume module.exports is the default', function() {
+  var deprecationMessage;
+  require.deprecationLogger = function(message) {
+    deprecationMessage = message;
+  };
+
   define('Foo', ['require', 'exports', 'module'], function(require, exports, module) {
     module.exports = function Foo() {
       this.bar = 'bar';
@@ -692,10 +704,17 @@ test('if a CJS style module has no default export assume module.exports is the d
     resolveRelative: 0,
     pendingQueueLength: 1
   });
+
+  equal(deprecationMessage, 'The `Foo` module does not define a default export, but loader.js generated one anyway. This behavior is deprecated and will be removed in v5.0.0.');
 });
 
 
 test('if a module has no default property assume its export is default (function)', function() {
+  var deprecationMessage;
+  require.deprecationLogger = function(message) {
+    deprecationMessage = message;
+  };
+
   var theFunction = function theFunction() {};
   define('foo', ['require', 'exports', 'module'], function() {
     return theFunction;
@@ -718,9 +737,16 @@ test('if a module has no default property assume its export is default (function
     resolveRelative: 0,
     pendingQueueLength: 1
   });
+
+  equal(deprecationMessage, 'The `foo` module does not define a default export, but loader.js generated one anyway. This behavior is deprecated and will be removed in v5.0.0.');
 });
 
 test('if a module has no default property assume its export is default (object)', function() {
+  var deprecationMessage;
+  require.deprecationLogger = function(message) {
+    deprecationMessage = message;
+  };
+
   var theObject = {};
   define('foo', ['require', 'exports', 'module'], function() {
     return theObject;
@@ -743,9 +769,16 @@ test('if a module has no default property assume its export is default (object)'
     resolveRelative: 0,
     pendingQueueLength: 1
   });
+
+  equal(deprecationMessage, 'The `foo` module does not define a default export, but loader.js generated one anyway. This behavior is deprecated and will be removed in v5.0.0.');
 });
 
 test('does not add default if export is frozen', function() {
+  var deprecationMessage;
+  require.deprecationLogger = function(message) {
+    deprecationMessage = message;
+  };
+
   var theObject = Object.freeze({});
   define('foo', ['require', 'exports', 'module'], function() {
     return theObject;
@@ -768,9 +801,16 @@ test('does not add default if export is frozen', function() {
     resolveRelative: 0,
     pendingQueueLength: 1
   });
+
+  equal(deprecationMessage, undefined);
 });
 
 test('does not add default if export is sealed', function() {
+  var deprecationMessage;
+  require.deprecationLogger = function(message) {
+    deprecationMessage = message;
+  };
+
   var theObject = Object.seal({ derp: {} });
   define('foo', ['require', 'exports', 'module'], function() {
     return theObject;
@@ -793,6 +833,8 @@ test('does not add default if export is sealed', function() {
     resolveRelative: 0,
     pendingQueueLength: 1
   });
+
+  equal(deprecationMessage, undefined);
 });
 
 test('has good error message for missing module', function() {

yarn.lock

@@ -230,6 +230,12 @@ babel-plugin-dead-code-elimination@^1.0.2:
   version "1.0.2"
   resolved "https://registry.yarnpkg.com/babel-plugin-dead-code-elimination/-/babel-plugin-dead-code-elimination-1.0.2.tgz#5f7c451274dcd7cccdbfbb3e0b85dd28121f0f65"
 
+babel-plugin-debug-macros@^0.1.10:
+  version "0.1.10"
+  resolved "https://npm.vip.global.square/babel-plugin-debug-macros/-/babel-plugin-debug-macros-0.1.10.tgz#dd077ad6e1cc0a8f9bbc6405c561392ebfc9a01c"
+  dependencies:
+    semver "^5.3.0"
+
 babel-plugin-eval@^1.0.1:
   version "1.0.1"
   resolved "https://registry.yarnpkg.com/babel-plugin-eval/-/babel-plugin-eval-1.0.1.tgz#a2faed25ce6be69ade4bfec263f70169195950da"
@@ -1962,7 +1968,7 @@ rsvp@~3.2.1:
   version "3.2.1"
   resolved "https://registry.yarnpkg.com/rsvp/-/rsvp-3.2.1.tgz#07cb4a5df25add9e826ebc67dcc9fd89db27d84a"
 
-semver@^5.1.0:
+semver@^5.1.0, semver@^5.3.0:
   version "5.3.0"
   resolved "https://registry.yarnpkg.com/semver/-/semver-5.3.0.tgz#9b2ce5d3de02d17c6012ad326aa6b4d0cf54f94f"