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node_modules/eslint/lib/rules/no-negated-in-lhs.js

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+/**
+ * @fileoverview A rule to disallow negated left operands of the `in` operator
+ * @author Michael Ficarra
+ * @deprecated in ESLint v3.3.0
+ */
+
+"use strict";
+
+//------------------------------------------------------------------------------
+// Rule Definition
+//------------------------------------------------------------------------------
+
+module.exports = {
+    meta: {
+        type: "problem",
+
+        docs: {
+            description: "disallow negating the left operand in `in` expressions",
+            category: "Possible Errors",
+            recommended: false,
+            url: "https://eslint.org/docs/rules/no-negated-in-lhs"
+        },
+
+        replacedBy: ["no-unsafe-negation"],
+
+        deprecated: true,
+        schema: []
+    },
+
+    create(context) {
+
+        return {
+
+            BinaryExpression(node) {
+                if (node.operator === "in" && node.left.type === "UnaryExpression" && node.left.operator === "!") {
+                    context.report({ node, message: "The 'in' expression's left operand is negated." });
+                }
+            }
+        };
+
+    }
+};