import { Middleware } from 'koa';

/**
 * Returns 304 response for cacheable requests if etag matches
 */
export function createEtagCacheMiddleware(): Middleware {
  return async function etagCacheMiddleware(ctx, next) {
    await next();

    if (!ctx.body || ctx.status === 304) {
      return;
    }

    if (!['GET', 'HEAD'].includes(ctx.method)) {
      return;
    }

    if (ctx.status < 200 || ctx.status >= 300) {
      return;
    }

    // let koa check if the respone is still fresh this means
    // the etags of the request and response match, so the browser
    // still has a fresh copy so it doesn't need to re-download it
    if (ctx.fresh) {
      ctx.status = 304;
      ctx.response.remove('content-type');
      ctx.response.remove('content-length');
    }

    // don't send last-modified since it doesn't do microseconds, we already
    // generate an etag based on the timestamp from the filesystem
    ctx.response.remove('last-modified');
  };
}
