[babel-plugin-transform-es2015-parameters] more oportunities for optimisations

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

@@ -52,19 +52,34 @@ let memberExpressionOptimisationVisitor = {
     if (state.noOptimise) {
       state.deopted = true;
     } else {
-      if (path.parentPath.isMemberExpression({ computed: true, object: node })) {
+      let {parentPath} = path;
-        // if we know that this member expression is referencing a number then we can safely
+
-        // optimise it
+      // ex: args[0]
-        let prop = path.parentPath.get("property");
+      if (parentPath.isMemberExpression({ computed: true, object: node })) {
+        // if we know that this member expression is referencing a number then
+        // we can safely optimise it
+        let prop = parentPath.get("property");
         if (prop.isBaseType("number")) {
           state.candidates.push(path);
           return;
         }
       }
 
+      // ex: args.length
+      if (parentPath.isMemberExpression({ computed: false, object: node })) {
+        let prop = parentPath.get("property");
+        if (prop.node.name === "length") {
+          state.candidates.push(path);
+          return;
+        }
+      }
+
+      // we can only do these optimizations if the rest variable would match
+      // the arguments exactly
       // optimise single spread args in calls
-      if (path.parentPath.isSpreadElement() && state.offset === 0) {
+      // ex: fn(...args)
-        let call = path.parentPath.parentPath;
+      if (state.offset === 0 && parentPath.isSpreadElement()) {
+        let call = parentPath.parentPath;
         if (call.isCallExpression() && call.node.arguments.length === 1) {
           state.candidates.push(path);
           return;

packages/babel-plugin-transform-es2015-parameters/test/fixtures/parameters/rest-member-expression-deoptimisation/actual.js

@@ -28,3 +28,7 @@ var b = function (foo, ...bar) {
   var join = "join";
   return bar[join];
 };
+
+var b = function (...bar) {
+  return bar.len;
+};

packages/babel-plugin-transform-es2015-parameters/test/fixtures/parameters/rest-member-expression-deoptimisation/expected.js

@@ -53,3 +53,11 @@ var b = function (foo) {
 
   return bar[join];
 };
+
+var b = function () {
+  for (var _len7 = arguments.length, bar = Array(_len7), _key7 = 0; _key7 < _len7; _key7++) {
+    bar[_key7] = arguments[_key7];
+  }
+
+  return bar.len;
+};

packages/babel-plugin-transform-es2015-parameters/test/fixtures/parameters/rest-member-expression-optimisation/actual.js

@@ -1,9 +1,17 @@
 var t = function (...items) {
-    var x = items[0];
+  var x = items[0];
-    var y = items[1];
+  var y = items[1];
 }
 
 function t(...items) {
-    var x = items[0];
+  var x = items[0];
-    var y = items[1];
+  var y = items[1];
 }
+
+function t(...items) {
+  var a = [];
+  for (var i = 0; i < items.length; i++) {
+    a.push(i);
+  }
+  return a;
+}

packages/babel-plugin-transform-es2015-parameters/test/fixtures/parameters/rest-member-expression-optimisation/expected.js

@@ -1,9 +1,17 @@
 var t = function () {
-    var x = arguments.length <= 0 || arguments[0] === undefined ? undefined : arguments[0];
+  var x = arguments.length <= 0 || arguments[0] === undefined ? undefined : arguments[0];
-    var y = arguments.length <= 1 || arguments[1] === undefined ? undefined : arguments[1];
+  var y = arguments.length <= 1 || arguments[1] === undefined ? undefined : arguments[1];
 };
 
 function t() {
-    var x = arguments.length <= 0 || arguments[0] === undefined ? undefined : arguments[0];
+  var x = arguments.length <= 0 || arguments[0] === undefined ? undefined : arguments[0];
-    var y = arguments.length <= 1 || arguments[1] === undefined ? undefined : arguments[1];
+  var y = arguments.length <= 1 || arguments[1] === undefined ? undefined : arguments[1];
+}
+
+function t() {
+  var a = [];
+  for (var i = 0; i < arguments.length; i++) {
+    a.push(i);
+  }
+  return a;
 }