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/*! * Based on https://github.com/jshttp/fresh/blob/master/index.js * Copyright(c) 2012 TJ Holowaychuk * Copyright(c) 2016-2017 Douglas Christopher Wilson * Copyright(c) 2020 Christian Norrman * MIT Licensed */
/**  * Regex to check if no-cache exists in a Cache-Control header.  * @constant  * @private  */const CACHE_CONTROL_NO_CACHE_REGEXP = /(?:^|,)\s*?no-cache\s*?(?:,|$)/;
/** * Regex to sanitize ETags within a If-None-Match header. * @constant * @private */const NONE_MATCH_SANITIZE_REGEXP = /(W\/| )/g;
/** * Check freshness of the response using request and response headers. * @param {Headers} reqHeaders Server request headers object * @param {Headers} resHeaders Server response headers object * * @returns {boolean} */export default function fresh(  reqHeaders: Headers,  resHeaders: Headers,): boolean {  const modifiedSince = reqHeaders.get("If-Modified-Since");  const noneMatch = reqHeaders.get("If-None-Match");
  // unconditional request  if (!modifiedSince && !noneMatch) {    return false;  }
  // If-None-Match  if (noneMatch && noneMatch !== "*") {    let etag = resHeaders.get("ETag");    if (!etag) {      return false;    } else if (etag.startsWith("W/")) {      etag = etag.substring(2);    }
    const etags = noneMatch      .replace(NONE_MATCH_SANITIZE_REGEXP, "")      .split(",");
    if (!etags.includes(etag)) {      return false;    }  }
  // If-Modified-Since  if (modifiedSince) {    const lastModified = resHeaders.get("Last-Modified");    if (      !lastModified || !(Date.parse(lastModified) <= Date.parse(modifiedSince))    ) {      return false;    }  }
  // Always return stale when Cache-Control: no-cache  // to support end-to-end reload requests  // https://tools.ietf.org/html/rfc2616#section-14.9.4  const cacheControl = reqHeaders.get("Cache-Control");  if (cacheControl && CACHE_CONTROL_NO_CACHE_REGEXP.test(cacheControl)) {    return false;  }
  return true;}