sitespeed.io/lib/support/helpers/httpErrors.js

sitespeed.io is an open-source tool for comprehensive web performance analysis, enabling you to test, monitor, and optimize your website’s speed using real browsers in various environments.

export function httpErrors(httpCodes) {
  let data = '';
  for (let code of Object.keys(httpCodes)) {
    if (Number(code) > 399) {
      data += `${code}: ${httpCodes[code]} `;
    }
  }
  return data === '' ? '0' : data;
}