//字符串
var personString = '{"name":"Andrew","age":25}';
console.log( typeof personString);//string
//转换为对象
var person = JSON.parse(personString);

console.log(typeof person);//object
console.log(person);//{ name: 'Andrew', age: 25 }

const fs=  require('fs');

//对象
var originalNote = {
  title:'some title',
  body:'some body'
};
//转换为字符串
var originalNoteString = JSON.stringify(originalNote);
//字符串写入到文件中
fs.writeFileSync('notes.json',originalNoteString);
//读取文件
var noteString = fs.readFileSync('notes.json');
//转换为对象
var note = JSON.parse(noteString);
console.log(typeof note);//object
console.log(note.title);//some title

> node json.js

string
object
{ name: 'Andrew', age: 25 }
object
some title