//打印字符串
console.log('Start app.');

> node app.js

//打印字符串
console.log('Start app.');

//导入node自带modules，fs文件操作
const fs = require('fs');
//os系统操作
const os = require('os');
//获取系统名字
var user = os.userInfo();

//添加字符串到回调函数中，并执行回调函数。出错就会报错，不出错就会打印出'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