flow like the river

VISUALIZACION/node_modules/graphql/validation/rules/UniqueVariableNamesRule.js.flow

@@ -0,0 +1,36 @@
+// @flow strict
+import { GraphQLError } from '../../error/GraphQLError';
+
+import type { ASTVisitor } from '../../language/visitor';
+import type { VariableDefinitionNode } from '../../language/ast';
+
+import type { ASTValidationContext } from '../ValidationContext';
+
+/**
+ * Unique variable names
+ *
+ * A GraphQL operation is only valid if all its variables are uniquely named.
+ */
+export function UniqueVariableNamesRule(
+  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(
+            `There can be only one variable named "$${variableName}".`,
+            [knownVariableNames[variableName], node.variable.name],
+          ),
+        );
+      } else {
+        knownVariableNames[variableName] = node.variable.name;
+      }
+    },
+  };
+}