update default parameters transformer to use var a = arguments[0] === undefined ? 1 : arguments[0]; instead of if (a === undefined) a = 1;

lib/6to5/transformation/templates/default-parameter.js

@@ -0,0 +1 @@
+var VARIABLE_NAME = ARGUMENTS[ARGUMENT_KEY] === undefined ? DEFAULT_VALUE : ARGUMENTS[ARGUMENT_KEY];

lib/6to5/transformation/templates/if-undefined-set-to.js

@@ -1 +0,0 @@
-if (VARIABLE === undefined) VARIABLE = DEFAULT;

lib/6to5/transformation/transformers/es6-default-parameters.js

@@ -49,16 +49,29 @@ exports.Function = function (node, parent, file, scope) {
 
   var body = [];
 
+  var argsIdentifier = t.identifier("arguments");
+  argsIdentifier._ignoreAliasFunctions = true;
+
+  var lastNonDefaultParam = 0;
+
   for (i in node.defaults) {
     def = node.defaults[i];
-    if (!def) continue;
+    if (!def) {
+      lastNonDefaultParam = +i + 1;
+      continue;
+    }
 
-    body.push(util.template("if-undefined-set-to", {
+    body.push(util.template("default-parameter", {
-      VARIABLE: node.params[i],
+      VARIABLE_NAME: node.params[i],
-      DEFAULT:  def
+      DEFAULT_VALUE: def,
+      ARGUMENT_KEY:  t.literal(+i),
+      ARGUMENTS:     argsIdentifier
     }, true));
   }
 
+  // we need to cut off all trailing default parameters
+  node.params = node.params.slice(0, lastNonDefaultParam);
+
   if (iife) {
     var container = t.functionExpression(null, [], node.body, node.generator);
     container._aliasFunction = true;

test/fixtures/transformation/es6-arrow-functions/default-parameters/expected.js

@@ -1,7 +1,7 @@
 "use strict";
 
-var some = function (count) {
+var some = function () {
-  if (count === undefined) count = "30";
+  var count = arguments[0] === undefined ? "30" : arguments[0];
   console.log("count", count);
 };
 

test/fixtures/transformation/es6-default-parameters/multiple/expected.js

@@ -1,12 +1,12 @@
 "use strict";
 
-var t = function (t, f) {
+var t = function () {
-  if (t === undefined) t = "foo";
+  var t = arguments[0] === undefined ? "foo" : arguments[0];
-  if (f === undefined) f = 5;
+  var f = arguments[1] === undefined ? 5 : arguments[1];
   return t + " bar " + f;
 };
 
-var a = function (t, f) {
+var a = function (t) {
-  if (f === undefined) f = 5;
+  var f = arguments[1] === undefined ? 5 : arguments[1];
   return t + " bar " + f;
 };

test/fixtures/transformation/es6-default-parameters/single/expected.js

@@ -1,6 +1,6 @@
 "use strict";
 
-var t = function (t) {
+var t = function () {
-  if (t === undefined) t = "foo";
+  var t = arguments[0] === undefined ? "foo" : arguments[0];
   return t + " bar";
 };