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node_modules/eslint/lib/rules/no-duplicate-case.js

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+/**
+ * @fileoverview Rule to disallow a duplicate case label.
+ * @author Dieter Oberkofler
+ * @author Burak Yigit Kaya
+ */
+
+"use strict";
+
+//------------------------------------------------------------------------------
+// Rule Definition
+//------------------------------------------------------------------------------
+
+module.exports = {
+    meta: {
+        type: "problem",
+
+        docs: {
+            description: "disallow duplicate case labels",
+            category: "Possible Errors",
+            recommended: true,
+            url: "https://eslint.org/docs/rules/no-duplicate-case"
+        },
+
+        schema: [],
+
+        messages: {
+            unexpected: "Duplicate case label."
+        }
+    },
+
+    create(context) {
+        const sourceCode = context.getSourceCode();
+
+        return {
+            SwitchStatement(node) {
+                const previousKeys = new Set();
+
+                for (const switchCase of node.cases) {
+                    if (switchCase.test) {
+                        const key = sourceCode.getText(switchCase.test);
+
+                        if (previousKeys.has(key)) {
+                            context.report({ node: switchCase, messageId: "unexpected" });
+                        } else {
+                            previousKeys.add(key);
+                        }
+                    }
+                }
+            }
+        };
+    }
+};