code: function parseSourceElement() {
          var token = lookahead();
  
          if (token.type === Token.Keyword) {
              switch (token.value) {
              case 'const':
              case 'let':
                  return parseConstLetDeclaration(token.value);
              case 'function':
                  return parseFunctionDeclaration();
              default:
                  return parseStatement();
              }
          }
  
          if (token.type !== Token.EOF) {
              return parseStatement();
          }
      }
docs: 14 Program