Add missing parentheses to let

changelog_unreleased/javascript/14000.md

@@ -0,0 +1,19 @@
+#### Fix missing parentheses when assigning to property of variable named `let` (#14000 by @fisker)
+
+<!-- prettier-ignore -->
+```jsx
+// Input
+(let[0] = 2);
+
+// Prettier stable
+let[0] = 2;
+
+// Prettier stable (second format)
+SyntaxError: Unexpected token (1:5)
+> 1 | let[0] = 2;
+    |     ^
+  2 |
+
+// Prettier main
+(let)[0] = 2;
+```

package.json

@@ -28,7 +28,7 @@
     "@glimmer/syntax": "0.84.2",
     "@iarna/toml": "2.2.5",
     "@typescript-eslint/typescript-estree": "5.45.0",
-    "acorn": "8.8.0",
+    "acorn": "8.8.1",
     "acorn-jsx": "5.3.2",
     "angular-estree-parser": "2.5.1",
     "angular-html-parser": "1.8.0",
@@ -44,7 +44,7 @@
     "editorconfig": "0.15.3",
     "editorconfig-to-prettier": "0.2.0",
     "escape-string-regexp": "5.0.0",
-    "espree": "9.4.0",
+    "espree": "9.4.1",
     "esutils": "2.0.3",
     "fast-glob": "3.2.11",
     "fast-json-stable-stringify": "2.1.0",

src/language-js/needs-parens.js

@@ -77,6 +77,17 @@ function needsParens(path, options) {
       return true;
     }
 
+    // `(let)[a] = 1`
+    if (
+      name === "object" &&
+      node.name === "let" &&
+      parent.type === "MemberExpression" &&
+      parent.computed &&
+      !parent.optional
+    ) {
+      return path.callParent(isAssignmentExpressionLeft);
+    }
+
     return false;
   }
 
@@ -963,4 +974,16 @@ function shouldWrapFunctionForExportDefault(path, options) {
   );
 }
 
+function isAssignmentExpressionLeft(path) {
+  const name = path.getName();
+  const parent = path.getParentNode();
+
+  // "OptionalMemberExpression" can't be here
+  if (name === "object" && parent.type === "MemberExpression") {
+    return path.callParent(isAssignmentExpressionLeft);
+  }
+
+  return name === "left" && parent.type === "AssignmentExpression";
+}
+
 module.exports = needsParens;

tests/config/format-test.js

@@ -41,6 +41,10 @@ const unstableTests = new Map(
     "js/comments/html-like/comment.js",
     "js/for/continue-and-break-comment-without-blocks.js",
     "typescript/satisfies-operators/comments-unstable.ts",
+    [
+      "js/identifier/parentheses/let-in-assignment.js",
+      (options) => options.semi === false,
+    ],
   ].map((fixture) => {
     const [file, isUnstable = () => true] = Array.isArray(fixture)
       ? fixture