better arguments aliasing for arrow functions, add it to block binding - fixes #52

lib/6to5/transformers/arrow-functions.js

@@ -19,8 +19,7 @@ exports.ArrowFunctionExpression = function (node) {
 
 exports.FunctionDeclaration =
 exports.FunctionExpression = function (node, parent, opts, generateUid) {
-  var hasArguments = false;
-  var id;
+  var argumentsId;
 
   // traverse the function and find all arrow functions
   traverse(node, function (node) {
@@ -28,22 +27,15 @@ exports.FunctionExpression = function (node, parent, opts, generateUid) {
 
     // traverse all child nodes of this arrow function and find a sole arguments
     // identifier
-    traverse(node, function (node, parent) {
-      if (node.type === "Identifier" && node.name === "arguments" &&
-          parent.type !== "MemberExpression") {
-        hasArguments = true;
-        id = id || b.identifier(generateUid("arguments"));
-        return id;
-      }
-    }, traverse.FUNCTION_TYPES);
+    argumentsId = util.aliasArguments(generateUid, node, argumentsId);
 
     return false;
   }, ["FunctionDeclaration", "FunctionExpression"]);
 
-  if (hasArguments) {
+  if (argumentsId) {
     util.ensureBlock(node);
     node.body.body.unshift(b.variableDeclaration("var", [
-      b.variableDeclarator(id, b.identifier("arguments"))
+      b.variableDeclarator(argumentsId, b.identifier("arguments"))
     ]));
   }
 };

lib/6to5/transformers/block-binding.js

@@ -23,8 +23,8 @@ var hasLet = function (nodes) {
   return has;
 };
 
-exports.Program = function (node) {
-   if (hasLet(node.body)) node.body = buildNode(node.body).node;
+exports.Program = function (node, parent, opts, generateUid) {
+   if (hasLet(node.body)) node.body = buildNode(node.body, generateUid).node;
 };
 
 exports.BlockStatement = function (node, parent, opts, generateUid) {
@@ -35,7 +35,7 @@ exports.BlockStatement = function (node, parent, opts, generateUid) {
 
   var body = node.body;
 
-  var built = buildNode(node.body, true);
+  var built = buildNode(node.body, generateUid);
   node.body = built.node;
 
   traverse(built.body, function (node) {
@@ -62,13 +62,13 @@ exports.BlockStatement = function (node, parent, opts, generateUid) {
 };
 
 var buildForStatement = function (key) {
-  return function (node, parent) {
+  return function (node, parent, opts, generateUid) {
     if (isLet(node[key])) {
       if (parent.type === "LabeledStatement") {
         throw util.errorWithNode(parent, "Label statements not supported with block binding yet.");
       }
 
-      return buildNode(node).node;
+      return buildNode(node, generateUid).node;
     }
   };
 };
@@ -76,7 +76,7 @@ var buildForStatement = function (key) {
 exports.ForOfStatement = exports.ForInStatement = buildForStatement("left");
 exports.ForStatement = buildForStatement("init");
 
-var buildNode = function (node) {
+var buildNode = function (node, generateUid) {
   var nodes = [];
 
   // hoist normal variable declarations
@@ -113,17 +113,27 @@ var buildNode = function (node) {
 
   //
 
+  var argumentsId = util.aliasArguments(generateUid, node);
+
+  if (argumentsId) {
+    nodes.push(b.variableDeclaration("var", [
+      b.variableDeclarator(argumentsId, b.identifier("arguments"))
+    ]));
+  }
+
+  //
+
   var block = b.blockStatement([]);
   block.body = node;
 
   var func = b.functionExpression(null, [], block, false);
 
+  //
+
   var templateName = "function-call";
   if (traverse.hasType(node, "ThisExpression")) templateName += "-this";
   if (traverse.hasType(node, "ReturnStatement", traverse.FUNCTION_TYPES)) templateName += "-return";
 
-  //
-
   nodes.push(util.template(templateName, {
     FUNCTION: func
   }, true));

lib/6to5/util.js

@@ -212,6 +212,28 @@ exports.parse = function (opts, code, callback) {
   }
 };
 
+exports.aliasArguments = function (generateUid, node, id) {
+  var isArgumentIdentifier = function (node) {
+    return node.type === "Identifier" && node.name === "arguments";
+  };
+
+  var getId = function () {
+    return id = id || b.identifier(generateUid("arguments"));
+  };
+
+  traverse(node, function (node, parent) {
+    if (isArgumentIdentifier(node) && parent.type !== "MemberExpression") {
+      return getId();
+    } else if (node.type === "MemberExpression" && isArgumentIdentifier(node.object)) {
+      node.object = getId();
+    } else {
+      return;
+    }
+  }, traverse.FUNCTION_TYPES);
+
+  return id;
+};
+
 try {
   exports.templates = require("../../templates.json");
 } catch (err) {

test/fixtures/syntax/arrow-functions/arguments/actual.js

@@ -1,10 +1,10 @@
-function outer() {
+function one() {
   var inner = () => arguments;
   return [].slice.call(inner());
 }
-console.log(outer(1, 2));
+one(1, 2);
 
-function outer() {
+function two() {
   var inner = () => arguments;
 
   var another = function () {
@@ -13,4 +13,22 @@ function outer() {
 
   return [].slice.call(inner());
 }
-console.log(outer(1, 2));
+two(1, 2);
+
+function three() {
+  var fn = () => arguments[0] + "bar";
+  return fn();
+}
+three("foo");
+
+function four() {
+  var fn = () => arguments[0].foo + "bar";
+  return fn();
+}
+four({ foo: "foo" });
+
+function five(obj) {
+  var fn = () => obj.arguments[0].foo + "bar";
+  return fn();
+}
+five({ arguments: ["foo"] });

test/fixtures/syntax/arrow-functions/arguments/expected.js

@@ -1,11 +1,11 @@
-function outer() {
+function one() {
   var _arguments = arguments;
   var inner = function () { return _arguments; };
   return [].slice.call(inner());
 }
-console.log(outer(1, 2));
+one(1, 2);
 
-function outer() {
+function two() {
   var _arguments2 = arguments;
   var inner = function () { return _arguments2; };
 
@@ -16,4 +16,30 @@ function outer() {
 
   return [].slice.call(inner());
 }
-console.log(outer(1, 2));
+two(1, 2);
+
+function three() {
+  var _arguments4 = arguments;
+  var fn = function () {
+    return _arguments4[0] + "bar";
+  };
+  return fn();
+}
+three("foo");
+
+function four() {
+  var _arguments5 = arguments;
+  var fn = function () {
+    return _arguments5[0].foo + "bar";
+  };
+  return fn();
+}
+four({ foo: "foo" });
+
+function five(obj) {
+  var fn = function () {
+    return obj.arguments[0].foo + "bar";
+  };
+  return fn();
+}
+five({ arguments: ["foo"] });

test/fixtures/syntax/block-binding/arguments/actual.js

@@ -0,0 +1,6 @@
+(function () {
+  if (true) {
+    let a = arguments[0];
+    console.log(a);
+  }
+})(1);

test/fixtures/syntax/block-binding/arguments/expected.js

@@ -0,0 +1,9 @@
+(function () {
+  if (true) {
+    var _arguments = arguments;
+    (function () {
+      var a = _arguments[0];
+      console.log(a);
+    })();
+  }
+})(1);