code: function serializeKey( name ) {
     const identifier = /^[A-Za-z$_][A-Za-z0-9$_]*$/.test( name );
     const keyword = [
        'if', 'else',
        'switch', 'case', 'default',
        'try', 'catch', 'finally',
        'function', 'return',
        'var', 'let', 'const'
     ].indexOf( name ) >= 0;
  
     return ( identifier && !keyword ) ? name : `"${name}"`;
  }
docs: Serialize an object key.
  Wrap the key in quotes if it is either not a valid identifier or if it
  is a problematic keyword.
  @private
  @param {String} name the key to serialize
  @return {String} the serialized key