From 16100127de193b47b0dd086c901b4888231f235f Mon Sep 17 00:00:00 2001
From: Boopathi Rajaa <me@boopathi.in>
Date: Thu, 29 Sep 2016 12:02:47 +0200
Subject: [PATCH] Fix var hoisting bug in DCE

Fix bug where var declarations after compleition statements are
deadcode eliminated - vars are hoisted in JS and need to be preserved
like function declarations

(Close #167)
---
 .../__tests__/dead-code-elimination-test.js   | 35 +++++++++++++++++++
 .../src/index.js                              | 10 ++++--
 2 files changed, 43 insertions(+), 2 deletions(-)

diff --git a/packages/babel-plugin-minify-dead-code-elimination/__tests__/dead-code-elimination-test.js b/packages/babel-plugin-minify-dead-code-elimination/__tests__/dead-code-elimination-test.js
index 6f02deaed..95c87fe3b 100644
--- a/packages/babel-plugin-minify-dead-code-elimination/__tests__/dead-code-elimination-test.js
+++ b/packages/babel-plugin-minify-dead-code-elimination/__tests__/dead-code-elimination-test.js
@@ -1999,4 +1999,39 @@ describe("dce-plugin", () => {
     const expected = source;
     expect(transform(source)).toBe(expected);
   });
+
+  it("should preserve variabledeclarations(var) after completion statements", () => {
+    const source = unpad(`
+      function foo() {
+        a = 1;
+        return a;
+        var a;
+      }
+    `);
+
+    const expected = source;
+
+    expect(transform(source)).toBe(expected);
+  });
+
+  it("should NOT preserve variabledeclarations(let) after completion statements", () => {
+    const source = unpad(`
+      function foo() {
+        a = 1;
+        b = 2;
+        return a + b;
+        let a, b;
+      }
+    `);
+
+    const expected = unpad(`
+      function foo() {
+        a = 1;
+        b = 2;
+        return a + b;
+      }
+    `);
+
+    expect(transform(source)).toBe(expected);
+  });
 });
diff --git a/packages/babel-plugin-minify-dead-code-elimination/src/index.js b/packages/babel-plugin-minify-dead-code-elimination/src/index.js
index 003dbb7c1..11308c034 100644
--- a/packages/babel-plugin-minify-dead-code-elimination/src/index.js
+++ b/packages/babel-plugin-minify-dead-code-elimination/src/index.js
@@ -283,7 +283,7 @@ module.exports = ({ types: t, traverse }) => {
           continue;
         }
 
-        if (purge && !p.isFunctionDeclaration()) {
+        if (purge && !canExistAfterCompletion(p)) {
           removeOrVoid(p);
         }
       }
@@ -299,7 +299,7 @@ module.exports = ({ types: t, traverse }) => {
 
       // Not last in it's block? (See BlockStatement visitor)
       if (path.container.length - 1 !== path.key &&
-          !path.getSibling(path.key + 1).isFunctionDeclaration() &&
+          !canExistAfterCompletion(path.getSibling(path.key + 1)) &&
           path.parentPath.isBlockStatement()
       ) {
         // This is probably a new oppurtinity by some other transform
@@ -900,4 +900,10 @@ module.exports = ({ types: t, traverse }) => {
       };
     }
   }
+
+  // things that are hoisted
+  function canExistAfterCompletion(path) {
+    return path.isFunctionDeclaration()
+      || path.isVariableDeclaration({ kind: "var" });
+  }
 };