From https://github.com/koajs/etag/blob/master/index.js#L24-L30

index.js

function etag(options) {
  return function etag(ctx, next) {
    return next()
      // `getResponseEntity` checks to see if the response has a body, and
      // stringifies it depending on what kind of body it has (JSON, HTML, 
      // image, etc)
      .then(() => getResponseEntity(ctx))
    
      // `setEtag` calculates the ETag from the response body if it exists,
      // then sets that ETag as a header
      .then(entity => setEtag(ctx, entity, options));
  };
}