fix: check constant violation inside loops

packages/babel-plugin-transform-block-scoping/src/index.ts

@@ -1,5 +1,5 @@
 import { declare } from "@babel/helper-plugin-utils";
-import type { NodePath, Visitor, Scope } from "@babel/traverse";
+import type { NodePath, Visitor, Scope, Binding } from "@babel/traverse";
 import { visitor as tdzVisitor } from "./tdz";
 import type { TDZVisitorState } from "./tdz";
 import { traverse, template, types as t } from "@babel/core";
@@ -487,13 +487,21 @@ class BlockScoping {
   }
 
   checkConstants() {
-    const scope = this.scope;
-    const state = this.state;
+    const constBindings = new Map<string, Binding>();
+
+    // In some cases, there are two different scopes: for example,
+    // for (const x of y) {} has a scope for the loop head and one
+    // for the body.
+    for (const scope of new Set([this.scope, this.blockPath.scope])) {
+      for (const name of Object.keys(scope.bindings)) {
+        const binding = scope.bindings[name];
+        if (binding.kind === "const") constBindings.set(name, binding);
+      }
+    }
 
-    for (const name of Object.keys(scope.bindings)) {
-      const binding = scope.bindings[name];
-      if (binding.kind !== "const") continue;
+    const { state } = this;
 
+    for (const [name, binding] of constBindings) {
       for (const violation of binding.constantViolations) {
         const readOnlyError = state.addHelper("readOnlyError");
         const throwNode = t.callExpression(readOnlyError, [

packages/babel-plugin-transform-block-scoping/test/fixtures/const-violations/assignment-in-for-loop/exec.js

@@ -0,0 +1,11 @@
+expect(function () {
+    for (const element = 1; element++;) break;
+}).toThrow('"element" is read-only');
+
+expect(function () {
+    for (;;) {
+        const from = 50;
+        from--;
+        break;
+    }
+}).toThrow('"from" is read-only');

packages/babel-plugin-transform-block-scoping/test/fixtures/const-violations/assignment-in-for-loop/input.js

@@ -0,0 +1,4 @@
+for (const element = 1; element++;) {
+  const from = 50;
+  from--;
+}

packages/babel-plugin-transform-block-scoping/test/fixtures/const-violations/assignment-in-for-loop/output.js

@@ -0,0 +1,4 @@
+for (var element = 1; +element, babelHelpers.readOnlyError("element");) {
+  var from = 50;
+  +from, babelHelpers.readOnlyError("from");
+}

packages/babel-plugin-transform-block-scoping/test/fixtures/const-violations/assignment-in-for-of-loop/exec.js

@@ -0,0 +1,7 @@
+expect(function () {
+    const array = [1, 2, 3];
+    for (const element of array) {
+        const from = 50;
+        from--;
+    }
+}).toThrow('"from" is read-only');

packages/babel-plugin-transform-block-scoping/test/fixtures/const-violations/assignment-in-for-of-loop/input.js

@@ -0,0 +1,4 @@
+for (const element of []) {
+  const from = 50;
+  from--;
+}

packages/babel-plugin-transform-block-scoping/test/fixtures/const-violations/assignment-in-for-of-loop/output.js

@@ -0,0 +1,4 @@
+for (var element of []) {
+  var from = 50;
+  +from, babelHelpers.readOnlyError("from");
+}