[
    {
        "username": "唐三",
        "age": 18
    },
    {
        "username": "唐昊",
        "age": 45
    },
    {
        "username": "唐银",
        "age": 40
    }
]

js代码:
var obj = {username: '唐三', age: 18};
// JSON对象转json字符串
var str = JSON.stringify(obj);
typeof str; // string

// str2是个符合JSON数据格式的字符串
var str2 = '{"username":"唐三","age":18}';
// JSON字符串转JSON对象
var obj2 = JSON.parse(str2);
typeof obj2;  // object

3.JSON应用

js代码:
<script>
        var obj = [
            {
                "username":"唐三",
                "age":18
            },
            {
                "username":"唐昊",
                "age":45
            },
            {
                "username":"唐银",
                "age":40
            }
        ]
        //对象转json字符串
        var str = JSON.stringify(obj);
        //string
        console.log(typeof str);
        
        //将符合json格式的字符串存入缓存
        var person = localStorage.setItem('person',str);

        //将存入数据取出来
        var str1 = localStorage.getItem('person');
        //将取出来的字符串转成JSON对象
        var person1 = JSON.parse(str1);
        console.log(person1);
    </script>