Utilities

/**
 * Type representing various header initialization formats
 */
type HeadersInit = ConstructorParameters<typeof Headers>[0];

/**
 * Converts potential Headers objects to plain headers objects
 * @param headers - Headers in any compatible format (Headers, array of pairs, or object)
 * @returns Plain object with string key-value pairs
 */
function headersToClassicHeaders<T extends Record<string, string>>(
  headers: HeadersInit
): T;

import { headersToClassicHeaders, HeadersInit } from "googleapis-common";

// Convert Headers object to plain object
const webHeaders = new Headers({
  'Content-Type': 'application/json',
  'Authorization': 'Bearer token123'
});

const plainHeaders = headersToClassicHeaders(webHeaders);
console.log(plainHeaders);
// Output: { 'Content-Type': 'application/json', 'Authorization': 'Bearer token123' }

// Convert array format to plain object
const arrayHeaders: [string, string][] = [
  ['Content-Type', 'application/xml'],
  ['Accept', 'application/json']
];

const convertedHeaders = headersToClassicHeaders(arrayHeaders);
console.log(convertedHeaders);
// Output: { 'Content-Type': 'application/xml', 'Accept': 'application/json' }

// Plain object passthrough (no conversion needed)
const objectHeaders = {
  'User-Agent': 'my-app/1.0',
  'X-API-Key': 'secret-key'
};

const passedHeaders = headersToClassicHeaders(objectHeaders);
console.log(passedHeaders === objectHeaders); // true (same reference)

/**
 * Marshals a GaxiosResponse into a library-friendly type with HTTP/2 compatibility
 * @param res - The Gaxios response to marshall
 * @returns GaxiosResponse with HTTP/2-ready/compatible headers
 */
function marshallGaxiosResponse<T extends GaxiosResponse>(
  res?: T
): GaxiosResponseWithHTTP2;

import { 
  marshallGaxiosResponse, 
  GaxiosResponse, 
  GaxiosResponseWithHTTP2 
} from "googleapis-common";

// Marshall a standard Gaxios response
async function makeRequest(): Promise<GaxiosResponseWithHTTP2> {
  // Assume this returns a standard GaxiosResponse
  const rawResponse: GaxiosResponse = await someGaxiosRequest();
  
  // Convert to HTTP/2-compatible format
  const marshalled = marshallGaxiosResponse(rawResponse);
  
  // Now guaranteed to have plain object headers
  console.log(typeof marshalled.headers); // 'object'
  console.log(marshalled.headers['content-type']); // Accessible as plain object
  
  return marshalled;
}

// Handle undefined responses gracefully
const emptyResponse = marshallGaxiosResponse(undefined);
console.log(emptyResponse.headers); // {}

// Type safety with generics
interface ApiData {
  items: string[];
  count: number;
}

const typedResponse = marshallGaxiosResponse<GaxiosResponse<ApiData>>(apiResponse);
console.log(typedResponse.data.items); // Type-safe access to response data

/**
 * Type for various header input formats supported by headersToClassicHeaders
 */
type HeadersInit = 
  | Headers 
  | Array<[string, string]> 
  | Record<string, string>
  | undefined
  | null;

/**
 * Generic constraint for header objects
 */
type HeaderRecord = Record<string, string>;