# JSON methods, toJSON

## JSON.stringify

* `JSON.stringify` to convert objects into JSON.
* `JSON.parse` to convert JSON back into an object.

```javascript
let student = {
  name: 'John',
  age: 30,
  isAdmin: false,
  courses: ['html', 'css', 'js'],
  wife: null
};

let json = JSON.stringify(student);

alert(typeof json); // we've got a string!

alert(json);
/* JSON-encoded object:
{
  "name": "John",
  "age": 30,
  "isAdmin": false,
  "courses": ["html", "css", "js"],
  "wife": null
}
*/
```

* 字串使用 ""  `'John'` 變成 `"John"。`
* 屬性名稱也是"" `age:30` 變成 `"age":30。`

支援 JSON 的類型

* Objects `{ ... }`
* Arrays `[ ... ]`
* Primitives:
  * strings,
  * numbers,
  * boolean values `true/false`,
  * `null`.

```javascript
// a number in JSON is just a number
alert( JSON.stringify(1) ) // 1

// a string in JSON is still a string, but double-quoted
alert( JSON.stringify('test') ) // "test"

alert( JSON.stringify(true) ); // true

alert( JSON.stringify([1, 2, 3]) ); // [1,2,3]
```

JSON.stringify 不支援

* Function properties (methods).
* Symbolic properties.
* Properties that store `undefined`.

```javascript
let user = {
  sayHi() { // ignored
    alert("Hello");
  },
  [Symbol("id")]: 123, // ignored
  something: undefined // ignored
};

alert( JSON.stringify(user) ); // {} (empty object)
```

```javascript
// 巢狀的物件也可以轉換
let meetup = {
  title: "Conference",
  room: {
    number: 123,
    participants: ["john", "ann"]
  }
};

alert( JSON.stringify(meetup) );
/* The whole structure is stringified:
{
  "title":"Conference",
  "room":{"number":23,"participants":["john","ann"]},
}
*/

// 不能用在雙重指向
let room = {
  number: 23
};

let meetup = {
  title: "Conference",
  participants: ["john", "ann"]
};

meetup.place = room;       // meetup references room
room.occupiedBy = meetup; // room references meetup

JSON.stringify(meetup); // Error: Converting circular structure to JSON
```

![](/files/-Lh_pBVPt9IVu-tFH6CW)

## Excluding and transforming: replacer

```javascript
let json = JSON.stringify(value[, replacer, space])

// 用 replacer 選擇要 JSON 的屬性
let room = {
  number: 23
};
let meetup = {
  title: "Conference",
  participants: [{name: "John"}, {name: "Alice"}],
  place: room // meetup references room
};
room.occupiedBy = meetup; // room references meetup
alert( JSON.stringify(meetup, ['title', 'participants']) );
// {"title":"Conference","participants":[{},{}]}

// JSON 全部屬性
let room = {
  number: 23
};

let meetup = {
  title: "Conference",
  participants: [{name: "John"}, {name: "Alice"}],
  place: room // meetup references room
};

room.occupiedBy = meetup; // room references meetup

alert( JSON.stringify(meetup, ['title', 'participants', 'place', 'name', 'number']) );
/*
{
  "title":"Conference",
  "participants":[{"name":"John"},{"name":"Alice"}],
  "place":{"number":23}
}
*/

// 用函式 replacer
let room = {
  number: 23
};

let meetup = {
  title: "Conference",
  participants: [{name: "John"}, {name: "Alice"}],
  place: room // meetup references room
};

room.occupiedBy = meetup; // room references meetup

alert( JSON.stringify(meetup, function replacer(key, value) {
  alert(`${key}: ${value}`); // to see what replacer gets
  return (key == 'occupiedBy') ? undefined : value;
}));

/* key:value pairs that come to replacer:
:             [object Object]
title:        Conference
participants: [object Object],[object Object]
0:            [object Object]
name:         John
1:            [object Object]
name:         Alice
place:        [object Object]
number:       23
*/
```

## Formatting: spacer

```javascript
let user = {
  name: "John",
  age: 25,
  roles: {
    isAdmin: false,
    isEditor: true
  }
};

alert(JSON.stringify(user, null, 2));
/* two-space indents:
{
  "name": "John",
  "age": 25,
  "roles": {
    "isAdmin": false,
    "isEditor": true
  }
}
*/

/* for JSON.stringify(user, null, 4) the result would be more indented:
{
    "name": "John",
    "age": 25,
    "roles": {
        "isAdmin": false,
        "isEditor": true
    }
}
*/
```

## Custom “toJSON”

就像物件使用 toString 轉換成 string，物件也可以使用 toJSON 轉換成 JSON。

```javascript
let room = {
  number: 23
};

let meetup = {
  title: "Conference",
  date: new Date(Date.UTC(2017, 0, 1)),
  room
};

alert( JSON.stringify(meetup) );
/*
  {
    "title":"Conference",
    "date":"2017-01-01T00:00:00.000Z",  // Date 有內建轉換 JSON
    "room": {"number":23}               
  }
*/

// 對 room 加 toJSON
let room = {
  number: 23,
  toJSON() {
    return this.number;
  }
};

let meetup = {
  title: "Conference",
  room
};

alert( JSON.stringify(room) ); // 23

alert( JSON.stringify(meetup) );
/*
  {
    "title":"Conference",
    "room": 23
  }
*/
```

## JSON.parse

```javascript
let value = JSON.parse(str[, reviver]);

// stringified array
let numbers = "[0, 1, 2, 3]";
numbers = JSON.parse(numbers);
alert( numbers[1] ); // 1

// nested object
let user = '{ "name": "John", "age": 35, "isAdmin": false, "friends": [0,1,2,3] }';
user = JSON.parse(user);
alert( user.friends[1] ); // 1

// 手寫的 JSON 可能有很多錯誤
let json = `{
  name: "John",                     // mistake: property name without quotes
  "surname": 'Smith',               // mistake: single quotes in value (must be double)
  'isAdmin': false                  // mistake: single quotes in key (must be double)
  "birthday": new Date(2000, 2, 3), // mistake: no "new" is allowed, only bare values
  "friends": [0,1,2,3]              // here all fine
}`;
// JSON 沒有註解
```

## Using reviver

```javascript
// title: (meetup title), date: (meetup date)
let str = '{"title":"Conference","date":"2017-11-30T12:00:00.000Z"}';

// meetup.date 不是 Date object
let str = '{"title":"Conference","date":"2017-11-30T12:00:00.000Z"}';
let meetup = JSON.parse(str);
alert( meetup.date.getDate() ); // Error!

// 加上 reviving 函式，判斷 date 返回 Date object
let str = '{"title":"Conference","date":"2017-11-30T12:00:00.000Z"}';
let meetup = JSON.parse(str, function(key, value) {
  if (key == 'date') return new Date(value);
  return value;
});
alert( meetup.date.getDate() ); // now works!

// 也可以用在巢狀物件
let schedule = `{
  "meetups": [
    {"title":"Conference","date":"2017-11-30T12:00:00.000Z"},
    {"title":"Birthday","date":"2017-04-18T12:00:00.000Z"}
  ]
}`;

schedule = JSON.parse(schedule, function(key, value) {
  if (key == 'date') return new Date(value);
  return value;
});

alert( schedule.meetups[1].date.getDate() ); // works!
```


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