transform.ts

import type { ASTNode, FieldNode, OperationDefinitionNode } from "graphql";
import { Kind, visit } from "graphql";

const TYPENAME_FIELD: FieldNode = {
  kind: Kind.FIELD,
  name: {
    kind: Kind.NAME,
    value: "__typename",
  },
};

/**
 * Adds `__typename` to all selection sets in the document. The operation
 * definition's selection set remains unchanged.
 *
 * @param doc - The `ASTNode` to add `__typename` to
 *
 * @example
 *
 * ```ts
 * const document = gql`
 *   # ...
 * `;
 *
 * const withTypename = addTypenameToDocument(document);
 * ```
 */
export const addTypenameToDocument = Object.assign(
  function <TNode extends ASTNode>(doc: TNode): TNode {
    return visit(doc, {
      SelectionSet: {
        enter(node, _key, parent) {
          // Don't add __typename to OperationDefinitions.
          if (
            parent &&
            (parent as OperationDefinitionNode).kind ===
              Kind.OPERATION_DEFINITION
          ) {
            return;
          }

          // No changes if no selections.
          const { selections } = node;
          if (!selections) {
            return;
          }

          // If selections already have a __typename, or are part of an
          // introspection query, do nothing.
          const skip = selections.some((selection) => {
            return (
              selection.kind === Kind.FIELD &&
              (selection.name.value === "__typename" ||
                selection.name.value.lastIndexOf("__", 0) === 0)
            );
          });
          if (skip) {
            return;
          }

          // If this SelectionSet is @export-ed as an input variable, it should
          // not have a __typename field (see issue #4691).
          const field = parent as FieldNode;
          if (
            field.kind === Kind.FIELD &&
            field.directives &&
            field.directives.some((d) => d.name.value === "export")
          ) {
            return;
          }

          // Create and return a new SelectionSet with a __typename Field.
          return {
            ...node,
            selections: [...selections, TYPENAME_FIELD],
          };
        },
      },
    });
  },
  {
    added(field: FieldNode): boolean {
      return field === TYPENAME_FIELD;
    },
  }
);