From aefbb1380eeb2f3ca5f5dcf9b419be4371612d20 Mon Sep 17 00:00:00 2001
From: Anatoli Papirovski <apapirovski@mac.com>
Date: Thu, 9 Nov 2017 14:51:56 -0500
Subject: [PATCH] No unneeded empty arrays in transform spread (#6763)

* No unneeded empty arrays in transform spread

Since Array.prototype.concat creates a new array from inputs, there's
no need to call it from a new empty array ([].concat()).

* [fixup] simplify detection of new array
---
 .../destructuring/array-unpack-optimisation/expected.js   | 2 +-
 packages/babel-plugin-transform-spread/README.md          | 2 +-
 packages/babel-plugin-transform-spread/src/index.js       | 8 ++++----
 .../test/fixtures/spread/arguments-array/expected.js      | 2 +-
 .../test/fixtures/spread/array-literal-first/expected.js  | 2 +-
 .../test/fixtures/spread/known-rest/expected.js           | 2 +-
 .../test/fixtures/spread/single/expected.js               | 2 +-
 7 files changed, 10 insertions(+), 10 deletions(-)

diff --git a/packages/babel-plugin-transform-destructuring/test/fixtures/destructuring/array-unpack-optimisation/expected.js b/packages/babel-plugin-transform-destructuring/test/fixtures/destructuring/array-unpack-optimisation/expected.js
index 32816cdc73..7743f54e7c 100644
--- a/packages/babel-plugin-transform-destructuring/test/fixtures/destructuring/array-unpack-optimisation/expected.js
+++ b/packages/babel-plugin-transform-destructuring/test/fixtures/destructuring/array-unpack-optimisation/expected.js
@@ -21,7 +21,7 @@ var _ref4 = [a[1], a[0]];
 a[0] = _ref4[0];
 a[1] = _ref4[1];
 
-var _ref5 = [].concat(babelHelpers.toConsumableArray(foo), [bar]),
+var _ref5 = babelHelpers.toConsumableArray(foo).concat([bar]),
     a = _ref5[0],
     b = _ref5[1];
 
diff --git a/packages/babel-plugin-transform-spread/README.md b/packages/babel-plugin-transform-spread/README.md
index 5f7ca6de78..42aaf881d7 100644
--- a/packages/babel-plugin-transform-spread/README.md
+++ b/packages/babel-plugin-transform-spread/README.md
@@ -15,7 +15,7 @@ var b = [...a, 'foo'];
 
 ```js
 var a = [ 'a', 'b', 'c' ];
-var b = [].concat(a, [ 'foo' ]);
+var b = a.concat([ 'foo' ]);
 ```
 
 ## Installation
diff --git a/packages/babel-plugin-transform-spread/src/index.js b/packages/babel-plugin-transform-spread/src/index.js
index 8a2a77ed61..6c23a02595 100644
--- a/packages/babel-plugin-transform-spread/src/index.js
+++ b/packages/babel-plugin-transform-spread/src/index.js
@@ -52,11 +52,11 @@ export default function(api, options) {
         if (!hasSpread(elements)) return;
 
         const nodes = build(elements, scope, state);
-        let first = nodes.shift();
+        const first = nodes.shift();
 
-        if (!t.isArrayExpression(first)) {
-          nodes.unshift(first);
-          first = t.arrayExpression([]);
+        if (nodes.length === 0 && first !== elements[0].argument) {
+          path.replaceWith(first);
+          return;
         }
 
         path.replaceWith(
diff --git a/packages/babel-plugin-transform-spread/test/fixtures/spread/arguments-array/expected.js b/packages/babel-plugin-transform-spread/test/fixtures/spread/arguments-array/expected.js
index 09da80138e..5cae73788a 100644
--- a/packages/babel-plugin-transform-spread/test/fixtures/spread/arguments-array/expected.js
+++ b/packages/babel-plugin-transform-spread/test/fixtures/spread/arguments-array/expected.js
@@ -1,5 +1,5 @@
 function foo() {
-  return bar([].concat(Array.prototype.slice.call(arguments)));
+  return bar(Array.prototype.slice.call(arguments));
 }
 
 function bar(one, two, three) {
diff --git a/packages/babel-plugin-transform-spread/test/fixtures/spread/array-literal-first/expected.js b/packages/babel-plugin-transform-spread/test/fixtures/spread/array-literal-first/expected.js
index 1da8c64e83..fbe8da8f82 100644
--- a/packages/babel-plugin-transform-spread/test/fixtures/spread/array-literal-first/expected.js
+++ b/packages/babel-plugin-transform-spread/test/fixtures/spread/array-literal-first/expected.js
@@ -1 +1 @@
-var lyrics = [].concat(babelHelpers.toConsumableArray(parts), ["head", "and", "toes"]);
+var lyrics = babelHelpers.toConsumableArray(parts).concat(["head", "and", "toes"]);
diff --git a/packages/babel-plugin-transform-spread/test/fixtures/spread/known-rest/expected.js b/packages/babel-plugin-transform-spread/test/fixtures/spread/known-rest/expected.js
index 4cd381b324..64c67e4ca8 100644
--- a/packages/babel-plugin-transform-spread/test/fixtures/spread/known-rest/expected.js
+++ b/packages/babel-plugin-transform-spread/test/fixtures/spread/known-rest/expected.js
@@ -3,5 +3,5 @@ function foo() {
     bar[_key] = arguments[_key];
   }
 
-  return [].concat(bar);
+  return bar.concat();
 }
diff --git a/packages/babel-plugin-transform-spread/test/fixtures/spread/single/expected.js b/packages/babel-plugin-transform-spread/test/fixtures/spread/single/expected.js
index 9286e2ae99..c9808b4b97 100644
--- a/packages/babel-plugin-transform-spread/test/fixtures/spread/single/expected.js
+++ b/packages/babel-plugin-transform-spread/test/fixtures/spread/single/expected.js
@@ -1 +1 @@
-[].concat(babelHelpers.toConsumableArray(foo));
+babelHelpers.toConsumableArray(foo);