--- /dev/null
+/**
+ * @fileoverview Rule to flag for-in loops without if statements inside
+ * @author Nicholas C. Zakas
+ */
+
+"use strict";
+
+//------------------------------------------------------------------------------
+// Rule Definition
+//------------------------------------------------------------------------------
+
+module.exports = {
+ meta: {
+ type: "suggestion",
+
+ docs: {
+ description: "require `for-in` loops to include an `if` statement",
+ category: "Best Practices",
+ recommended: false,
+ url: "https://eslint.org/docs/rules/guard-for-in"
+ },
+
+ schema: [],
+ messages: {
+ wrap: "The body of a for-in should be wrapped in an if statement to filter unwanted properties from the prototype."
+ }
+ },
+
+ create(context) {
+
+ return {
+
+ ForInStatement(node) {
+ const body = node.body;
+
+ // empty statement
+ if (body.type === "EmptyStatement") {
+ return;
+ }
+
+ // if statement
+ if (body.type === "IfStatement") {
+ return;
+ }
+
+ // empty block
+ if (body.type === "BlockStatement" && body.body.length === 0) {
+ return;
+ }
+
+ // block with just if statement
+ if (body.type === "BlockStatement" && body.body.length === 1 && body.body[0].type === "IfStatement") {
+ return;
+ }
+
+ // block that starts with if statement
+ if (body.type === "BlockStatement" && body.body.length >= 1 && body.body[0].type === "IfStatement") {
+ const i = body.body[0];
+
+ // ... whose consequent is a continue
+ if (i.consequent.type === "ContinueStatement") {
+ return;
+ }
+
+ // ... whose consequent is a block that contains only a continue
+ if (i.consequent.type === "BlockStatement" && i.consequent.body.length === 1 && i.consequent.body[0].type === "ContinueStatement") {
+ return;
+ }
+ }
+
+ context.report({ node, messageId: "wrap" });
+ }
+ };
+
+ }
+};