Add missing parentheses to let (prettier#14000)

Co-authored-by: Georgii Dolzhykov <thorn.mailbox@gmail.com>

changelog_unreleased/javascript/14000.md

@@ -0,0 +1,19 @@
+#### Fix missing parentheses when an expression statement starts with `let[` (#14000 by @fisker, @thorn0)
+
+<!-- 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 +77,39 @@ function needsParens(path, options) {
       return true;
     }
 
+    // `(let)[a] = 1`
+    if (
+      name === "object" &&
+      node.name === "let" &&
+      parent.type === "MemberExpression" &&
+      parent.computed &&
+      !parent.optional
+    ) {
+      const statement = path.findAncestor(
+        (node) =>
+          node.type === "ExpressionStatement" ||
+          node.type === "ForStatement" ||
+          node.type === "ForInStatement" ||
+          node.type === "ForOfStatement"
+      );
+      const expression = !statement
+        ? undefined
+        : statement.type === "ExpressionStatement"
+        ? statement.expression
+        : statement.type === "ForStatement"
+        ? statement.init
+        : statement.left;
+      if (
+        expression &&
+        startsWithNoLookaheadToken(
+          expression,
+          (leftmostNode) => leftmostNode === node
+        )
+      ) {
+        return true;
+      }
+    }
+
     return false;
   }
 
@@ -157,7 +190,11 @@ function needsParens(path, options) {
       if (
         startsWithNoLookaheadToken(
           node,
-          /* forbidFunctionClassAndDoExpr */ true
+          (node) =>
+            node.type === "ObjectExpression" ||
+            node.type === "FunctionExpression" ||
+            node.type === "ClassExpression" ||
+            node.type === "DoExpression"
         )
       ) {
         return true;
@@ -170,7 +207,7 @@ function needsParens(path, options) {
         node.type !== "SequenceExpression" && // these have parens added anyway
         startsWithNoLookaheadToken(
           node,
-          /* forbidFunctionClassAndDoExpr */ false
+          (node) => node.type === "ObjectExpression"
         )
       ) {
         return true;

src/language-js/print/function.js

@@ -373,7 +373,10 @@ function printArrowFunction(path, options, print, args) {
   // a <= a ? a : a
   const shouldAddParens =
     node.body.type === "ConditionalExpression" &&
-    !startsWithNoLookaheadToken(node.body, /* forbidFunctionAndClass */ false);
+    !startsWithNoLookaheadToken(
+      node.body,
+      (node) => node.type === "ObjectExpression"
+    );
 
   return group([
     ...parts,

src/language-js/utils/index.js

@@ -940,74 +940,59 @@ function shouldPrintComma(options, level = "es5") {
 }
 
 /**
- * Tests if an expression starts with `{`, or (if forbidFunctionClassAndDoExpr
- * holds) `function`, `class`, or `do {}`. Will be overzealous if there's
- * already necessary grouping parentheses.
+ * Tests if the leftmost node of the expression matches the predicate. E.g.,
+ * used to check whether an expression statement needs to be wrapped in extra
+ * parentheses because it starts with:
+ *
+ * - `{`
+ * - `function`, `class`, or `do {}`
+ * - `let[`
+ *
+ * Will be overzealous if there already are necessary grouping parentheses.
  *
  * @param {Node} node
- * @param {boolean} forbidFunctionClassAndDoExpr
+ * @param {(leftmostNode: Node) => boolean} predicate
  * @returns {boolean}
  */
-function startsWithNoLookaheadToken(node, forbidFunctionClassAndDoExpr) {
-  node = getLeftMost(node);
+function startsWithNoLookaheadToken(node, predicate) {
   switch (node.type) {
-    case "FunctionExpression":
-    case "ClassExpression":
-    case "DoExpression":
-      return forbidFunctionClassAndDoExpr;
-    case "ObjectExpression":
-      return true;
+    case "BinaryExpression":
+    case "LogicalExpression":
+    case "AssignmentExpression":
+    case "NGPipeExpression":
+      return startsWithNoLookaheadToken(node.left, predicate);
     case "MemberExpression":
     case "OptionalMemberExpression":
-      return startsWithNoLookaheadToken(
-        node.object,
-        forbidFunctionClassAndDoExpr
-      );
+      return startsWithNoLookaheadToken(node.object, predicate);
     case "TaggedTemplateExpression":
       if (node.tag.type === "FunctionExpression") {
         // IIFEs are always already parenthesized
         return false;
       }
-      return startsWithNoLookaheadToken(node.tag, forbidFunctionClassAndDoExpr);
+      return startsWithNoLookaheadToken(node.tag, predicate);
     case "CallExpression":
     case "OptionalCallExpression":
       if (node.callee.type === "FunctionExpression") {
         // IIFEs are always already parenthesized
         return false;
       }
-      return startsWithNoLookaheadToken(
-        node.callee,
-        forbidFunctionClassAndDoExpr
-      );
+      return startsWithNoLookaheadToken(node.callee, predicate);
     case "ConditionalExpression":
-      return startsWithNoLookaheadToken(
-        node.test,
-        forbidFunctionClassAndDoExpr
-      );
+      return startsWithNoLookaheadToken(node.test, predicate);
     case "UpdateExpression":
       return (
-        !node.prefix &&
-        startsWithNoLookaheadToken(node.argument, forbidFunctionClassAndDoExpr)
+        !node.prefix && startsWithNoLookaheadToken(node.argument, predicate)
       );
     case "BindExpression":
-      return (
-        node.object &&
-        startsWithNoLookaheadToken(node.object, forbidFunctionClassAndDoExpr)
-      );
+      return node.object && startsWithNoLookaheadToken(node.object, predicate);
     case "SequenceExpression":
-      return startsWithNoLookaheadToken(
-        node.expressions[0],
-        forbidFunctionClassAndDoExpr
-      );
+      return startsWithNoLookaheadToken(node.expressions[0], predicate);
     case "TSSatisfiesExpression":
     case "TSAsExpression":
     case "TSNonNullExpression":
-      return startsWithNoLookaheadToken(
-        node.expression,
-        forbidFunctionClassAndDoExpr
-      );
+      return startsWithNoLookaheadToken(node.expression, predicate);
     default:
-      return false;
+      return predicate(node);
   }
 }
 
@@ -1093,13 +1078,6 @@ function getPrecedence(operator) {
   return PRECEDENCE.get(operator);
 }
 
-function getLeftMost(node) {
-  while (node.left) {
-    node = node.left;
-  }
-  return node;
-}
-
 function isBitwiseOperator(operator) {
   return (
     Boolean(bitshiftOperators[operator]) ||

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