在學(xué)習(xí)ES6的generate，下面例子為什么不能打印，還會報錯？如何修改？我是應(yīng)該執(zhí)行g(shù)en()還是gen.next()var fs = require('fs');var readFile = function (fileName) {  return new Promise(function (resolve, reject) {    fs.readFile(fileName, function(error, data) {      if (error) reject(error);      resolve(data);    });  });};var gen = function* () {  var f1 = yield readFile('../test/1.json');  var f2 = yield readFile('../test/2.json');  console.log(JSON.stringify(f1));  console.log(JSON.stringify(f1));};//gen()console.log(gen.next())