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

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+// @flow strict
+import { GraphQLError } from '../../error/GraphQLError';
+
+import type { ASTVisitor } from '../../language/visitor';
+
+import type { ValidationContext } from '../ValidationContext';
+
+/**
+ * No undefined variables
+ *
+ * A GraphQL operation is only valid if all variables encountered, both directly
+ * and via fragment spreads, are defined by that operation.
+ */
+export function NoUndefinedVariablesRule(
+  context: ValidationContext,
+): ASTVisitor {
+  let variableNameDefined = Object.create(null);
+
+  return {
+    OperationDefinition: {
+      enter() {
+        variableNameDefined = Object.create(null);
+      },
+      leave(operation) {
+        const usages = context.getRecursiveVariableUsages(operation);
+
+        for (const { node } of usages) {
+          const varName = node.name.value;
+          if (variableNameDefined[varName] !== true) {
+            context.reportError(
+              new GraphQLError(
+                operation.name
+                  ? `Variable "$${varName}" is not defined by operation "${operation.name.value}".`
+                  : `Variable "$${varName}" is not defined.`,
+                [node, operation],
+              ),
+            );
+          }
+        }
+      },
+    },
+    VariableDefinition(node) {
+      variableNameDefined[node.variable.name.value] = true;
+    },
+  };
+}