src/language-graphql/printer-graphql.js/printInterfaces
function printInterfaces(path, options, print) {
  const node = path.getNode();
  const parts = [];
  const { interfaces } = node;
  const printed = path.map((node) => print(node), "interfaces");

  for (let index = 0; index < interfaces.length; index++) {
    const interfaceNode = interfaces[index];
    parts.push(printed[index]);
    const nextInterfaceNode = interfaces[index + 1];
    if (nextInterfaceNode) {
      const textBetween = options.originalText.slice(
        interfaceNode.loc.end,
        nextInterfaceNode.loc.start
      );
      const hasComment = textBetween.includes("#");
      const separator = textBetween.replace(/#.*/g, "").trim();

      parts.push(separator === "," ? "," : " &", hasComment ? line : " ");
    }
  }

  return parts;
}