fixes

packages/babel-plugin-transform-es2015-parameters/src/index.js

@@ -16,7 +16,7 @@ export default function() {
         }
 
         const convertedRest = convertFunctionRest(path);
-        const convertedParams = convertFunctionParams(path);
+        const convertedParams = convertFunctionParams(path, this.opts.loose);
 
         if (convertedRest || convertedParams) {
           // Manually reprocess this scope to ensure that the moved params are updated.

packages/babel-plugin-transform-es2015-parameters/src/params.js

@@ -10,6 +10,16 @@ const buildDefaultParam = template(`
       DEFAULT_VALUE;
 `);
 
+const buildLooseDefaultParam = template(`
+  if (ASSIGNMENT_IDENTIFIER === UNDEFINED) {
+    ASSIGNMENT_IDENTIFIER = DEFAULT_VALUE;
+  }
+`);
+
+const buildLooseDestructuredDefaultParam = template(`
+  let ASSIGNMENT_IDENTIFIER = PARAMETER_NAME === UNDEFINED ? DEFAULT_VALUE : PARAMETER_NAME ;
+`);
+
 const buildArgumentsAccess = template(`
   let $0 = arguments[$1];
 `);
@@ -35,7 +45,7 @@ const iifeVisitor = {
   },
 };
 
-export default function convertFunctionParams(path) {
+export default function convertFunctionParams(path, loose) {
   const { node, scope } = path;
 
   const state = {
@@ -49,6 +59,44 @@ export default function convertFunctionParams(path) {
 
   //
   const params = path.get("params");
+
+  if (loose) {
+    const body = [];
+    for (let i = 0; i < params.length; ++i) {
+      const param = params[i];
+      if (param.isAssignmentPattern()) {
+        const left = param.get("left");
+        const right = param.get("right");
+
+        const undefinedNode = scope.buildUndefinedNode();
+
+        if (left.isIdentifier()) {
+          body.push(
+            buildLooseDefaultParam({
+              ASSIGNMENT_IDENTIFIER: left.node,
+              DEFAULT_VALUE: right.node,
+              UNDEFINED: undefinedNode,
+            }),
+          );
+          param.replaceWith(left.node);
+        } else if (left.isObjectPattern() || left.isArrayPattern()) {
+          const paramName = scope.generateUidIdentifier();
+          body.push(
+            buildLooseDestructuredDefaultParam({
+              ASSIGNMENT_IDENTIFIER: left.node,
+              DEFAULT_VALUE: right.node,
+              PARAMETER_NAME: paramName,
+              UNDEFINED: undefinedNode,
+            }),
+          );
+          param.replaceWith(paramName);
+        }
+      }
+    }
+    path.get("body").unshiftContainer("body", body);
+    return;
+  }
+
   for (let i = 0; i < params.length; i++) {
     const param = params[i];
 

packages/babel-plugin-transform-es2015-parameters/test/fixtures/parameters/rest-nested-5656/expected.js

@@ -28,7 +28,7 @@ function d(thing) {
   };
 
   {
-    var _args = thing;
+    var args = thing;
     foo(thing);
   }
 }

packages/babel-plugin-transform-object-rest-spread/test/fixtures/object-rest/parameters/expected.js

@@ -17,15 +17,15 @@ function a3(_ref3) {
       c2 = babelHelpers.objectWithoutProperties(_ref3, ["a2", "b2"]);
 }
 
-function a4(_ref4, _ref5) {
-  let {
-    a5
-  } = _ref5,
-      c5 = babelHelpers.objectWithoutProperties(_ref5, ["a5"]);
+function a4(_ref5, _ref4) {
   let {
     a3
+  } = _ref5,
+      c3 = babelHelpers.objectWithoutProperties(_ref5, ["a3"]);
+  let {
+    a5
   } = _ref4,
-      c3 = babelHelpers.objectWithoutProperties(_ref4, ["a3"]);
+      c5 = babelHelpers.objectWithoutProperties(_ref4, ["a5"]);
 }
 
 function a5(_ref6) {