flow like the river

BACK_BACK/node_modules/graphql/validation/rules/NoUnusedFragmentsRule.js.flow

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+// @flow strict
+import { GraphQLError } from '../../error/GraphQLError';
+
+import type { ASTVisitor } from '../../language/visitor';
+
+import type { ASTValidationContext } from '../ValidationContext';
+
+/**
+ * No unused fragments
+ *
+ * A GraphQL document is only valid if all fragment definitions are spread
+ * within operations, or spread within other fragments spread within operations.
+ */
+export function NoUnusedFragmentsRule(
+  context: ASTValidationContext,
+): ASTVisitor {
+  const operationDefs = [];
+  const fragmentDefs = [];
+
+  return {
+    OperationDefinition(node) {
+      operationDefs.push(node);
+      return false;
+    },
+    FragmentDefinition(node) {
+      fragmentDefs.push(node);
+      return false;
+    },
+    Document: {
+      leave() {
+        const fragmentNameUsed = Object.create(null);
+        for (const operation of operationDefs) {
+          for (const fragment of context.getRecursivelyReferencedFragments(
+            operation,
+          )) {
+            fragmentNameUsed[fragment.name.value] = true;
+          }
+        }
+
+        for (const fragmentDef of fragmentDefs) {
+          const fragName = fragmentDef.name.value;
+          if (fragmentNameUsed[fragName] !== true) {
+            context.reportError(
+              new GraphQLError(
+                `Fragment "${fragName}" is never used.`,
+                fragmentDef,
+              ),
+            );
+          }
+        }
+      },
+    },
+  };
+}