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

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+// @flow strict
+
+import { GraphQLError } from '../../error/GraphQLError';
+
+import { type ASTVisitor } from '../../language/visitor';
+import { type VariableDefinitionNode } from '../../language/ast';
+
+import { type ASTValidationContext } from '../ValidationContext';
+
+export function duplicateVariableMessage(variableName: string): string {
+  return `There can be only one variable named "${variableName}".`;
+}
+
+/**
+ * Unique variable names
+ *
+ * A GraphQL operation is only valid if all its variables are uniquely named.
+ */
+export function UniqueVariableNames(context: ASTValidationContext): ASTVisitor {
+  let knownVariableNames = Object.create(null);
+  return {
+    OperationDefinition() {
+      knownVariableNames = Object.create(null);
+    },
+    VariableDefinition(node: VariableDefinitionNode) {
+      const variableName = node.variable.name.value;
+      if (knownVariableNames[variableName]) {
+        context.reportError(
+          new GraphQLError(duplicateVariableMessage(variableName), [
+            knownVariableNames[variableName],
+            node.variable.name,
+          ]),
+        );
+      } else {
+        knownVariableNames[variableName] = node.variable.name;
+      }
+    },
+  };
+}