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)
babel · Sep 29, 2016 · 1610012 · 1610012
1 parent 32c809b
commit 1610012
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Showing 2 changed files with 43 additions and 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
@@ -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
@@ -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" });
+  }
 };