Fix: no-extra-parens false positives for variables called "let" (#8808)

"let" is unusual because it's sometimes parsed as a variable declaration keyword, and sometimes as an identifier for a variable. This commit fixes some bugs in the `no-extra-parens` rule where parentheses are unnecessary for most variable names, but are necessary when the variable is called "let".

lib/rules/no-extra-parens.js

@@ -426,7 +426,7 @@ module.exports = {
                     secondToken.type === "Keyword" && (
                         secondToken.value === "function" ||
                         secondToken.value === "class" ||
-                        secondToken.value === "let" && astUtils.isOpeningBracketToken(sourceCode.getTokenAfter(secondToken))
+                        secondToken.value === "let" && astUtils.isOpeningBracketToken(sourceCode.getTokenAfter(secondToken, astUtils.isNotClosingParenToken))
                     )
                 )
             ) {
@@ -512,16 +512,27 @@ module.exports = {
             ExportDefaultDeclaration: node => checkExpressionOrExportStatement(node.declaration),
             ExpressionStatement: node => checkExpressionOrExportStatement(node.expression),
 
-            ForInStatement(node) {
-                if (hasExcessParens(node.right)) {
-                    report(node.right);
-                }
-                if (hasExcessParens(node.left)) {
-                    report(node.left);
-                }
-            },
+            "ForInStatement, ForOfStatement"(node) {
+                if (node.left.type !== "VariableDeclarator") {
+                    const firstLeftToken = sourceCode.getFirstToken(node.left, astUtils.isNotOpeningParenToken);
+
+                    if (
+                        firstLeftToken.value === "let" && (
+
+                            // If `let` is the only thing on the left side of the loop, it's the loop variable: `for ((let) of foo);`
+                            // Removing it will cause a syntax error, because it will be parsed as the start of a VariableDeclarator.
+                            firstLeftToken.range[1] === node.left.range[1] ||
 
-            ForOfStatement(node) {
+                            // If `let` is followed by a `[` token, it's a property access on the `let` value: `for ((let[foo]) of bar);`
+                            // Removing it will cause the property access to be parsed as a destructuring declaration of `foo` instead.
+                            astUtils.isOpeningBracketToken(
+                                sourceCode.getTokenAfter(firstLeftToken, astUtils.isNotClosingParenToken)
+                            )
+                        )
+                    ) {
+                        tokensToIgnore.add(firstLeftToken);
+                    }
+                }
                 if (hasExcessParens(node.right)) {
                     report(node.right);
                 }

tests/lib/rules/no-extra-parens.js

@@ -450,7 +450,12 @@ ruleTester.run("no-extra-parens", rule, {
         {
             code: "((function(){}).foo)();",
             options: ["functions"]
-        }
+        },
+        "(let)[foo]",
+        "for ((let) in foo);",
+        "for ((let[foo]) in bar);",
+        "for ((let)[foo] in bar);",
+        "for ((let[foo].bar) in baz);"
     ],
 
     invalid: [
@@ -1052,6 +1057,24 @@ ruleTester.run("no-extra-parens", rule, {
             "MemberExpression",
             1,
             { parserOptions: { ecmaVersion: 2015 } }
+        ),
+        invalid(
+            "(let).foo",
+            "let.foo",
+            "Identifier",
+            1
+        ),
+        invalid(
+            "for ((let.foo) in bar);",
+            "for (let.foo in bar);",
+            "MemberExpression",
+            1
+        ),
+        invalid(
+            "for ((let).foo.bar in baz);",
+            "for (let.foo.bar in baz);",
+            "Identifier",
+            1
         )
     ]
 });