Flip default parameter template (#4515)

* Flip default parameter template

YMMV, I saved ~10b on a 2kb library. Not noticeable at the small scale, by why not do it anyway?

I've (unscientifically) found that flipping the default parameter conditional yields better gzip results. I think this is due to the slightly longer string it can now repeatedly match:

```js
// old
var param = arguments.length <= 0 || void 0 === arguments[0] ? null : arguments[0]
--------------------------------------------------------------^

// new
var param = arguments.length > 1 && void 0 !== arguments[1] ? arguments[1] : null
------------------------------------------------------------------------^
```

Though it's entirely likely gzip will also choose up to the index of the arguments if you many default parameters at different indexes.

* Update tests

packages/babel-core/test/fixtures/transformation/misc/regression-1168/expected.js

@@ -1,5 +1,5 @@
 function test() {
-  var x = arguments.length <= 0 || arguments[0] === undefined ? "hi" : arguments[0];
+  var x = arguments.length > 0 && arguments[0] !== undefined ? arguments[0] : "hi";
 
   return x;
-}
+}

packages/babel-plugin-transform-es2015-arrow-functions/test/fixtures/arrow-functions/default-parameters/expected.js

@@ -1,12 +1,12 @@
 var some = function () {
-  let count = arguments.length <= 0 || arguments[0] === undefined ? "30" : arguments[0];
+  let count = arguments.length > 0 && arguments[0] !== undefined ? arguments[0] : "30";
 
   console.log("count", count);
 };
 
 var collect = function () {
-  let since = arguments.length <= 0 || arguments[0] === undefined ? 0 : arguments[0];
+  let since = arguments.length > 0 && arguments[0] !== undefined ? arguments[0] : 0;
   let userid = arguments[1];
 
   console.log(userid);
-};
+};

packages/babel-plugin-transform-es2015-destructuring/test/fixtures/destructuring/default-precedence/expected.js

@@ -1,6 +1,6 @@
 var f0 = function (a) {
-  var b = arguments.length <= 1 || arguments[1] === undefined ? a : arguments[1];
-  var c = arguments.length <= 2 || arguments[2] === undefined ? b : arguments[2];
+  var b = arguments.length > 1 && arguments[1] !== undefined ? arguments[1] : a;
+  var c = arguments.length > 2 && arguments[2] !== undefined ? arguments[2] : b;
 
   return [a, b, c];
 };
@@ -9,8 +9,8 @@ assert.deepEqual(f0(1), [1, 1, 1]);
 
 var f1 = function (_ref) {
   var a = _ref.a;
-  var b = arguments.length <= 1 || arguments[1] === undefined ? a : arguments[1];
-  var c = arguments.length <= 2 || arguments[2] === undefined ? b : arguments[2];
+  var b = arguments.length > 1 && arguments[1] !== undefined ? arguments[1] : a;
+  var c = arguments.length > 2 && arguments[2] !== undefined ? arguments[2] : b;
 
   return [a, b, c];
 };
@@ -19,10 +19,10 @@ assert.deepEqual(f1({ a: 1 }), [1, 1, 1]);
 
 var f2 = function (_ref2) {
   var a = _ref2.a;
-  var b = arguments.length <= 1 || arguments[1] === undefined ? a : arguments[1];
-  var c = arguments.length <= 2 || arguments[2] === undefined ? a : arguments[2];
+  var b = arguments.length > 1 && arguments[1] !== undefined ? arguments[1] : a;
+  var c = arguments.length > 2 && arguments[2] !== undefined ? arguments[2] : a;
 
   return [a, b, c];
 };
 
-assert.deepEqual(f2({ a: 1 }), [1, 1, 1]);
+assert.deepEqual(f2({ a: 1 }), [1, 1, 1]);

packages/babel-plugin-transform-es2015-modules-commonjs/test/fixtures/regression/T7160/expected.js

@@ -4,9 +4,9 @@ Object.defineProperty(exports, "__esModule", {
   value: true
 });
 var foo = exports.foo = function foo(gen) {
-  var ctx = arguments.length <= 1 || arguments[1] === undefined ? null : arguments[1];
+  var ctx = arguments.length > 1 && arguments[1] !== undefined ? arguments[1] : null;
 };
 
 var bar = exports.bar = function bar(gen) {
-  var ctx = arguments.length <= 1 || arguments[1] === undefined ? null : arguments[1];
-};
+  var ctx = arguments.length > 1 && arguments[1] !== undefined ? arguments[1] : null;
+};

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

@@ -7,10 +7,10 @@ import * as t from "babel-types";
 
 let buildDefaultParam = template(`
   let VARIABLE_NAME =
-    ARGUMENTS.length <= ARGUMENT_KEY || ARGUMENTS[ARGUMENT_KEY] === undefined ?
-      DEFAULT_VALUE
+    ARGUMENTS.length > ARGUMENT_KEY && ARGUMENTS[ARGUMENT_KEY] !== undefined ?
+      ARGUMENTS[ARGUMENT_KEY]
     :
-      ARGUMENTS[ARGUMENT_KEY];
+      DEFAULT_VALUE;
 `);
 
 let buildCutOff = template(`

packages/babel-plugin-transform-es2015-parameters/test/fixtures/parameters/default-before-last/expected.js

@@ -1,4 +1,4 @@
 function foo() {
-  var a = arguments.length <= 0 || arguments[0] === undefined ? "foo" : arguments[0];
+  var a = arguments.length > 0 && arguments[0] !== undefined ? arguments[0] : "foo";
   var b = arguments[1];
-}
+}

packages/babel-plugin-transform-es2015-parameters/test/fixtures/parameters/default-eval/expected.js

@@ -1,11 +1,11 @@
 var x = "outside";
 function outer() {
-  var a = arguments.length <= 0 || arguments[0] === undefined ? function () {
+  var a = arguments.length > 0 && arguments[0] !== undefined ? arguments[0] : function () {
     return eval("x");
-  } : arguments[0];
+  };
   return function () {
     var x = "inside";
     return a();
   }();
 }
-outer();
+outer();

packages/babel-plugin-transform-es2015-parameters/test/fixtures/parameters/default-multiple/expected.js

@@ -1,12 +1,12 @@
 var t = function () {
-  var e = arguments.length <= 0 || arguments[0] === undefined ? "foo" : arguments[0];
-  var f = arguments.length <= 1 || arguments[1] === undefined ? 5 : arguments[1];
+  var e = arguments.length > 0 && arguments[0] !== undefined ? arguments[0] : "foo";
+  var f = arguments.length > 1 && arguments[1] !== undefined ? arguments[1] : 5;
 
   return e + " bar " + f;
 };
 
 var a = function (e) {
-  var f = arguments.length <= 1 || arguments[1] === undefined ? 5 : arguments[1];
+  var f = arguments.length > 1 && arguments[1] !== undefined ? arguments[1] : 5;
 
   return e + " bar " + f;
-};
+};

packages/babel-plugin-transform-es2015-parameters/test/fixtures/parameters/default-single/expected.js

@@ -1,5 +1,5 @@
 var t = function () {
-  var f = arguments.length <= 0 || arguments[0] === undefined ? "foo" : arguments[0];
+  var f = arguments.length > 0 && arguments[0] !== undefined ? arguments[0] : "foo";
 
   return f + " bar";
-};
+};

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

@@ -1,6 +1,6 @@
 // #3861
 function t() {
-  var x = arguments.length <= 0 || arguments[0] === undefined ? "default" : arguments[0];
+  var x = arguments.length > 0 && arguments[0] !== undefined ? arguments[0] : "default";
   var _ref = arguments[1];
   var a = _ref.a;
   var b = _ref.b;
@@ -10,4 +10,4 @@ function t() {
   }
 
   console.log(x, a, b, args);
-}
+}