JavaScript method that returns the file extension of the given path, from the last occurrence of the period.

/**
 * Returns the extension of the path, from the last occurrence of the `.`
 * (period) character to end of string in the last portion of the path. If there
 * is no `.` in the last portion of the path, or if there are no `.` characters
 * other than the first character of the basename of path, an empty string is
 * returned.
 *
 * Clone of https://nodejs.org/api/path.html#path_path_extname_path
 *
 * @param {string} path
 * @return {string}
 */
const getFileExtension = path => {
  if (typeof path !== "string") path = `${path}`;
  let startDot = -1;
  let startPart = 0;
  let end = -1;
  let matchedSlash = true;
  // Track the state of characters (if any) we see before our first dot and
  // after any path separator we find
  let preDotState = 0;
  for (let i = path.length - 1; i >= 0; --i) {
    const code = path.charCodeAt(i);
    if (code === 47 /*/*/) {
      // If we reached a path separator that was not part of a set of path
      // separators at the end of the string, stop now
      if (!matchedSlash) {
        startPart = i + 1;
        break;
      }
      continue;
    }
    if (end === -1) {
      // We saw the first non-path separator, mark this as the end of our
      // extension
      matchedSlash = false;
      end = i + 1;
    }
    if (code === 46 /*.*/) {
      // If this is our first dot, mark it as the start of our extension
      if (startDot === -1) startDot = i;
      else if (preDotState !== 1) preDotState = 1;
    } else if (startDot !== -1) {
      // We saw a non-dot and non-path separator before our dot, so we should
      // have a good chance at having a non-empty extension
      preDotState = -1;
    }
  }

  if (
    startDot === -1 ||
    end === -1 ||
    // We saw a non-dot character immediately before the dot
    preDotState === 0 ||
    // The (right-most) trimmed path component is exactly '..'
    (preDotState === 1 && startDot === end - 1 && startDot === startPart + 1)
  ) {
    return "";
  }

  return path.slice(startDot, end);
};

//
// Examples:

getFileExtension("file"); // ""
getFileExtension(null); // ""
getFileExtension(""); // ""
getFileExtension("--"); // ""
getFileExtension("file.ext"); // ".ext"
getFileExtension("/root/path/file.ext"); // ".ext"
getFileExtension("C:/file.ext"); // ".ext"
getFileExtension("<path>:/file.ext"); // ".ext"
getFileExtension("https://example.com/file.ext"); // ".ext"
getFileExtension("https://example.com/file.ext?param=value"); // ".ext?param=value"