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node_modules/graphql/validation/rules/UniqueArgumentNamesRule.js.flow

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+// @flow strict
+import { GraphQLError } from '../../error/GraphQLError';
+import type { ASTVisitor } from '../../language/visitor';
+
+import type { ASTValidationContext } from '../ValidationContext';
+
+/**
+ * Unique argument names
+ *
+ * A GraphQL field or directive is only valid if all supplied arguments are
+ * uniquely named.
+ */
+export function UniqueArgumentNamesRule(
+  context: ASTValidationContext,
+): ASTVisitor {
+  let knownArgNames = Object.create(null);
+  return {
+    Field() {
+      knownArgNames = Object.create(null);
+    },
+    Directive() {
+      knownArgNames = Object.create(null);
+    },
+    Argument(node) {
+      const argName = node.name.value;
+      if (knownArgNames[argName]) {
+        context.reportError(
+          new GraphQLError(
+            `There can be only one argument named "${argName}".`,
+            [knownArgNames[argName], node.name],
+          ),
+        );
+      } else {
+        knownArgNames[argName] = node.name;
+      }
+      return false;
+    },
+  };
+}