code: function classMapDirective(classInfo) {
    return function(part) {
      if (!isAttributePart(part) || part.name !== 'class') {
        throw Error('The `classMap` directive can only be used in the `class` attribute');
      }
  
      let value = '';
  
      for (const key in classInfo) {
        if (classInfo[key]) {
          value += `${value.length ? ' ' : ''}${key}`;
        }
      }
  
      part.setValue(value);
    };
  }
docs: Applies CSS classes, where'classInfo' keys are added as class names if values are truthy.
  Only applies to 'class' attribute.
  
  @param { object } classInfo
  @returns { (part: AttributePart) => void }