fix: Duplicate declaration in transformed for...of

packages/babel-plugin-transform-for-of/src/index.ts

@@ -167,21 +167,29 @@ export default declare((api, options: Options) => {
     }) as t.For;
 
     t.inherits(loop, node);
-    t.ensureBlock(loop);
 
     const iterationValue = t.memberExpression(
       t.cloneNode(right),
       t.cloneNode(iterationKey),
       true,
     );
 
+    if (
+      t.isBlockStatement(loop.body) &&
+      Object.keys(path.getBindingIdentifiers()).some(id =>
+        path.get("body").scope.hasOwnBinding(id),
+      )
+    ) {
+      loop.body = t.blockStatement([loop.body]);
+    } else {
+      loop.body = t.toBlock(loop.body);
+    }
+
     const left = node.left;
     if (t.isVariableDeclaration(left)) {
       left.declarations[0].init = iterationValue;
-      // @ts-expect-error todo(flow->ts):
       loop.body.body.unshift(left);
     } else {
-      // @ts-expect-error todo(flow->ts):
       loop.body.body.unshift(
         t.expressionStatement(
           t.assignmentExpression("=", left, iterationValue),
@@ -234,10 +242,19 @@ export default declare((api, options: Options) => {
           );
         }
 
-        // ensure that it's a block so we can take all its statements
-        path.ensureBlock();
-
-        (node.body as t.BlockStatement).body.unshift(declar);
+        let blockBody;
+        const body = path.get("body");
+        if (
+          body.isBlockStatement() &&
+          Object.keys(path.getBindingIdentifiers()).some(id =>
+            body.scope.hasOwnBinding(id),
+          )
+        ) {
+          blockBody = t.blockStatement([declar, body.node]);
+        } else {
+          blockBody = t.toBlock(body.node);
+          blockBody.body.unshift(declar);
+        }
 
         const nodes = builder.build({
           CREATE_ITERATOR_HELPER: state.addHelper(builder.helper),
@@ -247,7 +264,7 @@ export default declare((api, options: Options) => {
             : null,
           STEP_KEY: t.identifier(stepKey),
           OBJECT: node.right,
-          BODY: node.body,
+          BODY: blockBody,
         });
         const container = builder.getContainer(nodes);
 

packages/babel-plugin-transform-for-of/test/fixtures/for-of-as-array/for-of-redeclaration-in-body/input.js

@@ -1,3 +1,7 @@
 for (const elm of array) {
   const elm = 2;
 }
+
+for (let x of []) {
+  let x = 1;
+}

packages/babel-plugin-transform-for-of/test/fixtures/for-of-as-array/for-of-redeclaration-in-body/output.js

@@ -4,3 +4,10 @@ for (let _i = 0, _array = array; _i < _array.length; _i++) {
     const elm = 2;
   }
 }
+
+for (let _i2 = 0, _ref = []; _i2 < _ref.length; _i2++) {
+  let x = _ref[_i2];
+  {
+    let x = 1;
+  }
+}

packages/babel-plugin-transform-for-of/test/fixtures/loose/for-of-redeclaration-in-body/input.js

@@ -0,0 +1,9 @@
+for (let x of y) {
+  let x = 1;
+  let y = 2;
+}
+
+for (let x of []) {
+  let x = 1;
+  let y = 2;
+}

packages/babel-plugin-transform-for-of/test/fixtures/loose/for-of-redeclaration-in-body/output.js

@@ -0,0 +1,15 @@
+for (var _iterator = babelHelpers.createForOfIteratorHelperLoose(y), _step; !(_step = _iterator()).done;) {
+  let x = _step.value;
+  {
+    let x = 1;
+    let y = 2;
+  }
+}
+
+for (var _i = 0, _arr = []; _i < _arr.length; _i++) {
+  let x = _arr[_i];
+  {
+    let x = 1;
+    let y = 2;
+  }
+}