encoding-negotiator/index.mjs

a negotiator for the accept-encoding header

export function negotiateEncoding(header, supportedEncodings) {
  if (!header) {
    return undefined;
  }
  const supportedEncodingMap = createMap(supportedEncodings);
  const acceptedEncodings = parse(header)
    .sort((a, b) => comparator(a, b, supportedEncodingMap))
    .filter(isNonZeroQuality);
  return determinePreffered(acceptedEncodings, supportedEncodingMap);
}

function determinePreffered(acceptedEncodings, supportedEncodings) {
  for (const encoding of acceptedEncodings) {
    const selected = supportedEncodings[encoding.name];
    if (selected) {
      return selected.encoding;
    }
  }
  return null;
}

function createMap(supported) {
  const supportedEncodings = {};
  let priority = 0;
  if (supported.length > 0) {
    supportedEncodings["*"] = { encoding: supported[0], priority };
    priority++;
  }
  for (const encoding of supported) {
    supportedEncodings[encoding] = { encoding, priority };
    priority++;
  }
  return supportedEncodings;
}

function parse(header) {
  const split = header.split(",");
  return split.map(parseEncoding);
}

function isNonZeroQuality(encoding) {
  return encoding.quality !== 0;
}

function parseEncoding(encoding) {
  const [name, second] = encoding.trim().split(";");
  const quality = getQuality(second);
  return {
    name,
    quality,
  };
}

function getQuality(second) {
  if (!second) {
    return 1;
  }
  const [, quality] = second.trim().split("=");
  return parseFloat(quality);
}

function comparator(a, b, supportedEncodingMap) {
  if (a.quality === b.quality) {
    if (
      supportedEncodingMap[a.name] &&
      supportedEncodingMap[b.name] &&
      supportedEncodingMap[a.name].priority <
        supportedEncodingMap[b.name].priority
    ) {
      return -1;
    } else {
      return 1;
    }
  }
  return b.quality - a.quality;
}