import type { RequestHeaders } from '../types';

export default function getEnvelopeBody(
  method: string,
  headers: Record<string, string> = {},
  data: unknown,
): string {
  const mappedHeaders: RequestHeaders = Object.entries(headers).reduce((acc, [key, val]) => {
    if (acc[key]) {
      acc[key].push(val);
    } else {
      acc[key] = [val];
    }
    return acc;
  }, {} as RequestHeaders);

  // As per the Json type, `data` could be null. We don't want to send 'null' as the value of body
  // JSON.stringify will preserve that, whereas with 'undefined` the field is dropped during
  // serialization
  if (data) {
    return JSON.stringify({
      method,
      headers: mappedHeaders,
      body: data,
    });
  }

  return JSON.stringify({
    method,
    headers: mappedHeaders,
  });
}