//列印字串
console.log('Start app.');

> node app.js

//列印字串
console.log('Start app.');

//匯入node自帶modules，fs檔案操作
const fs = require
 
('fs');
//os系統操作
const os = require('os');
//獲取系統名字
var user = os.userInfo();

//新增字串到回撥函式中，並執行回撥函式。出錯就會報錯，不出錯就會打印出'The "data to append" was appended to file!'
fs.appendFile('greetings.txt',`Hello ${user.username}`,(err) => {
  if (err) throw err;
  console.log('The "data to append" was appended to file!'
 
);
}
);

> node app.js