Fix: prefer-const false positive at non-blocked if (fixes #5610)

docs/rules/prefer-const.md

@@ -26,6 +26,11 @@ for (let i in [1,2,3]) {
 for (let a of [1,2,3]) {
     console.log(a);
 }
+
+// the initializer is separated.
+let a;
+a = 0;
+console.log(a);
 ```
 
 The following patterns are not considered problems:
@@ -52,6 +57,13 @@ for (let i = 0, end = 10; i < end; ++i) {
     console.log(a);
 }
 
+// the initializer is located at another block.
+let a;
+if (true) {
+    a = 0;
+}
+console.log(a);
+
 // suggest to use `no-var` rule.
 var b = 3;
 console.log(b);

lib/rules/prefer-const.js

@@ -11,6 +11,7 @@
 //------------------------------------------------------------------------------
 
 var PATTERN_TYPE = /^(?:.+?Pattern|RestElement|Property)$/;
+var DECLARATION_HOST_TYPE = /^(?:Program|BlockStatement|SwitchCase)$/;
 
 /**
  * Adds multiple items to the tail of an array.
@@ -47,7 +48,8 @@ function canBecomeVariableDeclaration(identifier) {
         node.type === "VariableDeclarator" ||
         (
             node.type === "AssignmentExpression" &&
-            node.parent.type === "ExpressionStatement"
+            node.parent.type === "ExpressionStatement" &&
+            DECLARATION_HOST_TYPE.test(node.parent.parent.type)
         )
     );
 }

tests/lib/rules/prefer-const.js

@@ -63,6 +63,7 @@ ruleTester.run("prefer-const", rule, {
             parserOptions: { ecmaVersion: 6 }
         },
         { code: "/*exported a*/ let a; function init() { a = foo(); }", parserOptions: { ecmaVersion: 6 } },
+        { code: "let a; if (true) a = 0; foo(a);", parserOptions: { ecmaVersion: 6 } },
 
         // The assignment is located in a different scope.
         // Those are warned by prefer-smaller-scope.
@@ -147,6 +148,11 @@ ruleTester.run("prefer-const", rule, {
             parserOptions: { ecmaVersion: 6 },
             errors: [{ message: "'x' is never reassigned, use 'const' instead.", type: "Identifier"}]
         },
+        {
+            code: "switch (a) { case 0: let x; x = 0; }",
+            parserOptions: { ecmaVersion: 6 },
+            errors: [{ message: "'x' is never reassigned, use 'const' instead.", type: "Identifier"}]
+        },
         {
             code: "(function() { let x; x = 1; })();",
             parserOptions: { ecmaVersion: 6 },