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node_modules/eslint/lib/rules/no-unsafe-negation.js

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+/**
+ * @fileoverview Rule to disallow negating the left operand of relational operators
+ * @author Toru Nagashima
+ */
+
+"use strict";
+
+//------------------------------------------------------------------------------
+// Requirements
+//------------------------------------------------------------------------------
+
+const astUtils = require("./utils/ast-utils");
+
+//------------------------------------------------------------------------------
+// Helpers
+//------------------------------------------------------------------------------
+
+/**
+ * Checks whether the given operator is a relational operator or not.
+ *
+ * @param {string} op - The operator type to check.
+ * @returns {boolean} `true` if the operator is a relational operator.
+ */
+function isRelationalOperator(op) {
+    return op === "in" || op === "instanceof";
+}
+
+/**
+ * Checks whether the given node is a logical negation expression or not.
+ *
+ * @param {ASTNode} node - The node to check.
+ * @returns {boolean} `true` if the node is a logical negation expression.
+ */
+function isNegation(node) {
+    return node.type === "UnaryExpression" && node.operator === "!";
+}
+
+//------------------------------------------------------------------------------
+// Rule Definition
+//------------------------------------------------------------------------------
+
+module.exports = {
+    meta: {
+        type: "problem",
+
+        docs: {
+            description: "disallow negating the left operand of relational operators",
+            category: "Possible Errors",
+            recommended: true,
+            url: "https://eslint.org/docs/rules/no-unsafe-negation"
+        },
+
+        schema: [],
+        fixable: null,
+        messages: {
+            unexpected: "Unexpected negating the left operand of '{{operator}}' operator."
+        }
+    },
+
+    create(context) {
+        const sourceCode = context.getSourceCode();
+
+        return {
+            BinaryExpression(node) {
+                if (isRelationalOperator(node.operator) &&
+                    isNegation(node.left) &&
+                    !astUtils.isParenthesised(sourceCode, node.left)
+                ) {
+                    context.report({
+                        node,
+                        loc: node.left.loc,
+                        messageId: "unexpected",
+                        data: { operator: node.operator }
+                    });
+                }
+            }
+        };
+    }
+};