flow like the river

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

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+// @flow strict
+import inspect from '../../jsutils/inspect';
+
+import { GraphQLError } from '../../error/GraphQLError';
+
+import type { FieldNode } from '../../language/ast';
+import type { ASTVisitor } from '../../language/visitor';
+
+import { getNamedType, isLeafType } from '../../type/definition';
+
+import type { ValidationContext } from '../ValidationContext';
+
+/**
+ * Scalar leafs
+ *
+ * A GraphQL document is valid only if all leaf fields (fields without
+ * sub selections) are of scalar or enum types.
+ */
+export function ScalarLeafsRule(context: ValidationContext): ASTVisitor {
+  return {
+    Field(node: FieldNode) {
+      const type = context.getType();
+      const selectionSet = node.selectionSet;
+      if (type) {
+        if (isLeafType(getNamedType(type))) {
+          if (selectionSet) {
+            const fieldName = node.name.value;
+            const typeStr = inspect(type);
+            context.reportError(
+              new GraphQLError(
+                `Field "${fieldName}" must not have a selection since type "${typeStr}" has no subfields.`,
+                selectionSet,
+              ),
+            );
+          }
+        } else if (!selectionSet) {
+          const fieldName = node.name.value;
+          const typeStr = inspect(type);
+          context.reportError(
+            new GraphQLError(
+              `Field "${fieldName}" of type "${typeStr}" must have a selection of subfields. Did you mean "${fieldName} { ... }"?`,
+              node,
+            ),
+          );
+        }
+      }
+    },
+  };
+}