text: /*!
   * heading-level.js | MIT (c) Shinnosuke Watanabe
   * https://github.com/shinnn/heading-level.js
  */
  window.headingLevel = function headingLevel(tagName) {
    'use strict';
  
    if (typeof tagName !== 'string') {
      throw new TypeError(
        String(tagName) +
        ' is not a string. Expected a header tag name, e.g. "h1", "H2".'
      );
    }
  
    if (/^h\d/mi.test(tagName)) {
      var level = Number(tagName.charAt(1));
      if (1 <= level && level <= 6) {
        return level;
      }
    }
  
    return null;
  };
  