Serializing an ES6 class object as JSON

Question

class MyClass {
  constructor() {
    this.foo = 3
  }
}

var myClass = new MyClass()

I'd like to serialize myClass object to json.

One easy way I can think of is, since every member is actually javascript object (array, etc..) I guess I can maintain a variable to hold the member variables.

this.prop.foo = this.foo and so on.

I expected to find a toJSON/fromJSON library for class objects since I used them with other languages such as swift/java, but couldn't find one for javascript.

Maybe class construct is too new, or what I'm asking can be somehow easily achieved without a library.

Answer 1

As with any other object you want to stringify in JS, you can use JSON.stringify:

JSON.stringify(yourObject);

---

class MyClass {
  constructor() {
    this.foo = 3
  }
}

var myClass = new MyClass()

console.log(JSON.stringify(myClass));

Also worth noting is that you can customize how stringify serializes your object by giving it a toJSON method. The value used to represent your object in the resulting JSON string will be the result of calling the toJSON method on that object.

Answer 2

I know this question is old but I've been clawing my eyes out until I wrote a compact real, "safe", solution.

Deserialization returns objects that still have working methods attached to them.

The only thing you need to do is register the classes you want to use in the constructor of the serializer.

---

class Serializer{
    constructor(types){this.types = types;}
    serialize(object) {
        let idx = this.types.findIndex((e)=> {return e.name == object.constructor.name});
        if (idx == -1) throw "type  '" + object.constructor.name + "' not initialized";
        return JSON.stringify([idx, Object.entries(object)]);
    }
    deserialize(jstring) {
        let array = JSON.parse(jstring);
        let object = new this.types[array[0]]();
        array[1].map(e=>{object[e[0]] = e[1];});
        return object;
    }
}

class MyClass {
    constructor(foo) {this.foo = foo;}
    getFoo(){return this.foo;}
}

var serializer = new Serializer([MyClass]);

console.log(serializer.serialize(new MyClass(42)));
//[0,[["foo",42]]]

console.log(serializer.deserialize('[0,[["foo",42]]]').getFoo());
//42

The above should be enough to get you going, but more details and minified version can be found here.

Answer 3

I've came across this library which does both serialization and deserialization of complex objects (including nested objects and arrays):

https://github.com/typestack/class-transformer

It has at least two methods:

plainToClass() -> json obj to class
classToPlain() -> class to json obj

Answer 4

I made a module esserializer to solve this issue. It is a utility to serialize JavaScript class instance, and deserialize the "serialized-text" into an instance object, with all Class/Property/Method etc. retained.

To serialize an instance, just invoke the serialize() method:

const ESSerializer = require('esserializer');
let serializedString = ESSerializer.serialize(anObject);

The internal mechanism of serialize() is: save the instance' property and its class name information into string, recursively.

To deserialize from string, just invoke the deserialize() method, passing all involved classes as parameter:

const ESSerializer = require('esserializer');
const ClassA = require('./ClassA');
const ClassB = require('./ClassB');
const ClassC = require('./ClassC');

let deserializedObj = ESSerializer.deserialize(serializedString, [ClassA, ClassB, ClassC]);

The internal mechanism of deserialize() is: manually compose the object with its prototype information, recursively.

Answer 5

It's easy if you don't mind passing the class definition into decode.

// the code
const encode = (object) => JSON.stringify(Object.entries(object))

const decode = (string, T) => {
  const object = new T()
  JSON.parse(string).map(([key, value]) => (object[key] = value))
  return object
}

// test the code
class A {
  constructor(n) {
    this.n = n
  }

  inc(n) {
    this.n += n
  }
}

const a = new A(1)
const encoded = encode(a)
const decoded = decode(encoded, A)
decoded.inc(2)
console.log(decoded)

Answer 6

I also need class serialization, so I made a library.

https://github.com/denostack/superserial

I think the toJSON/fromJSON function you expect can be implemented through toSerialize/toDeserialize.

import { Serializer, toDeserialize, toSerialize } from "superserial";

class User {
  static [toDeserialize](data: { serializedBirth: number }) {
    return new User(data.serializedBirth);
  }

  #birth: number;

  constructor(
    birth: number,
  ) {
    this.#birth = birth;
  }

  get age() {
    return new Date().getFullYear() - this.#birth;
  }

  [toSerialize]() {
    return {
      serializedBirth: this.#birth,
    };
  }
}

const serializer = new Serializer({
  classes: {
    User, // Define the class to use for deserialization here
  },
});

then, serialize,

const serialized = serializer.serialize(new User(2002));

serialized string:

MyClass{"name":"wan2land","serializedBirth":2000}

deserialize,

const user = serializer.deserialize<User>(serialized);

Of course, toSerialize and toDeserialize can be omitted. :-)

Answer 7

You need to be able to reinitialize objects recursively. Having a parameterless constructor is not essential, you can get away without having one.

Here's how I perform deep copy:

class Serializer
{
  constructor(types){
    this.types = types;
  }

  markRecursive(object)
  {
    // anoint each object with a type index
    let idx = this.types.findIndex(t => {
      return t.name === object.constructor.name;
    });
    if (idx !== -1)
    {
      object['typeIndex'] = idx;

      for (let key in object)
      {
        if (object.hasOwnProperty(key) && object[key] != null)
          this.markRecursive(object[key]);
      }
    }
  }

  cleanUp(object)
  {
    if (object.hasOwnProperty('typeIndex')) {
      delete object.typeIndex;
      for (let key in object) {
        if (object.hasOwnProperty(key) && object[key] != null) {
          console.log(key);
          this.cleanUp(object[key]);
        }
      }
    }
  }

  reconstructRecursive(object)
  {
    if (object.hasOwnProperty('typeIndex'))
    {
      let type = this.types[object.typeIndex];
      let obj = new type();
      for (let key in object)
      {
        if (object.hasOwnProperty(key) && object[key] != null) {
          obj[key] = this.reconstructRecursive(object[key]);
        }
      }
      delete obj.typeIndex;
      return obj;
    }
    return object;
  }

  clone(object)
  {
    this.markRecursive(object);
    let copy = JSON.parse(JSON.stringify(object));
    this.cleanUp(object);
    return this.reconstructRecursive(copy);
  }
}

The idea is simple: when serializing, a member of every known type (a type that's in this.types) is anointed with a member called typeIndex. After deserialization, we recursively initialize every substructure that has a typeIndex, then get rid of it to avoid polluting the structure.

Answer 8

Not a new topic, but there is a new solution : the modern approach (in December 2021) is to use @badcafe/jsonizer : https://badcafe.github.io/jsonizer

• Unlike other solutions, it doesn't pollute you data with injected class names,
• and it reifies the expected data hierarchy.
• below are some examples in Typescript, but it works as well in JS

Before showing an example with a class, let's start with a simple data structure :

const person = {
    name: 'Bob',
    birthDate: new Date('1998-10-21'),
    hobbies: [
        {   hobby: 'programming',
            startDate: new Date('2021-01-01'),
        },
        {   hobby: 'cooking',
            startDate: new Date('2020-12-31'),
        },
    ]
}
const personJson = JSON.stringify(person);
// {
//     "name": "Bob",
//     "birthDate": "1998-10-21T00:00:00.000Z",
//     "hobbies": [
//         {
//             "hobby": "programming",
//             "startDate": "2021-01-01T00:00:00.000Z"
//         },
//         {
//             "hobby": "cooking",
//             "startDate": "2020-12-31T00:00:00.000Z"
//         }
//     ]
// }
// store or send the data

Notice that dates are serialized to strings, and if you parse that JSON, dates won't be Date instances, they will be Strings

Now, let's use Jsonizer

// in Jsonizer, a reviver is made of field mappers :
const personReviver = Jsonizer.reviver<typeof person>({
    birthDate: Date,
    hobbies: {
        '*': {
            startDate: Date
        }
    }
});
const personFromJson = JSON.parse(personJson, personReviver);

Every dates string in the JSON text have been mapped to Date objects in the parsed result.

Jsonizer can indifferently revive JSON data structures (arrays, objects) or class instances with recursively nested custom classes, third-party classes, built-in classes, or sub JSON structures (arrays, objects).

Now, let's use a class instead :

// in Jsonizer, a class reviver is made of field mappers + an instance builder :
@Reviver<Person>({ //   bind the reviver to the class
    '.': ({name, birthDate, hobbies}) => new Person(name, birthDate, hobbies), //   instance builder
    birthDate: Date,
    hobbies: {
        '*': {
            startDate: Date
        }
    }
})
class Person {
    constructor( // all fields are passed as arguments to the constructor
        public name: string,
        public birthDate: Date
        public hobbies: Hobby[]
    ) {}
}
interface Hobby {
    hobby: string,
    startDate: Date
}

const person = new Person(
    'Bob',
    new Date('1998-10-21'),
    [
        {   hobby: 'programming',
            startDate: new Date('2021-01-01'),
        },
        {   hobby: 'cooking',
            startDate: new Date('2020-12-31'),
        },
    ]
);
const personJson = JSON.stringify(person);

const personReviver = Reviver.get(Person); //   extract the reviver from the class
const personFromJson = JSON.parse(personJson, personReviver);

Finally, let's use 2 classes :

@Reviver<Hobby>({
    '.': ({hobby, startDate}) => new Hobby(hobby, startDate), //   instance builder
    startDate: Date
})
class Hobby {
    constructor (
        public hobby: string,
        public startDate: Date
    ) {}
}

@Reviver<Person>({
    '.': ({name, birthDate, hobbies}) => new Person(name, birthDate, hobbies), //   instance builder
    birthDate: Date,
    hobbies: {
        '*': Hobby  //   we can refer a class decorated with @Reviver
    }
})
class Person {
    constructor(
        public name: string,
        public birthDate: Date,
        public hobbies: Hobby[]
    ) {}
}

const person = new Person(
    'Bob',
    new Date('1998-10-21'),
    [
        new Hobby('programming', new Date('2021-01-01')),
        new Hobby('cooking', new Date('2020-12-31')
    ]
);
const personJson = JSON.stringify(person);

const personReviver = Reviver.get(Person); //   extract the reviver from the class
const personFromJson = JSON.parse(personJson, personReviver);