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