TypeScript: Working with JSON

EDITS:


So you have a User type in your code.

interface User {
  name:    string;
  age:     number;
  created: Date;
}

At some point you’re going to want to encode this as JSON. This works as you’d expect.

 > JSON.stringify({ name: "bob", age: 34, created: new Date() });
'{"name":"bob","age":34,"created":"2016-03-19T18:15:12.710Z"}'

The problem is that the created field is no longer a Date when you parse it back.

> JSON.parse('{"name":"bob","age":34,"created":"2016-03-19T18:15:12.710Z"}')
{ name: 'bob', age: 34, created: '2016-03-19T18:15:12.710Z' }

The way I went about fixing this is by introducing a UserJSON interface.
Since it only contains primitives, it can be converter to and from JSON without altering it.

interface UserJSON {
  name:    string;
  age:     number;
  created: string;
}

Then I convert from User -> UserJSON before ‘stringifying’ to JSON and convert from UserJSON -> User after parsing from JSON. Here’s an example of some client code doing this.

function getUsers(): Promise<User[]> {
  return ajax.get<UserJSON[]>('/users').then(data => {
    return data.data.map(decodeUser);
  });
}

function updateUser(id: number|string, user: User): Promise<{}> {
  return ajax.put<{}>(`/users/${id}`, encodeUser(user));
}

Here are the conversion functions.

function encodeUser(user: User): UserJSON {
  return {
    name:    user.name,
    age:     user.age,
    created: user.created.toString()
  };
}

function decodeUser(json: UserJSON): User {
  return {
    name:    json.name,
    age:     json.age,
    created: new Date(json.created)
  };
}

This works, but it’s a contrived example. In real cases, there will be a lot more properties and this quickly turns into a huge pain in the ass. Let’s use Object.assign to clean it up a bit.

function encodeUser(user: User): UserJSON {
  return Object.assign({}, user, {
    created: user.created.toString()
  });
}

function decodeUser(json: UserJSON): User {
  return Object.assign({}, json, {
    created: new Date(json.created)
  });
}

So far so good, but what happens when User is a class?

class User {

  private created: Date;

  constructor(
    private name: string,
    private age:  string
  ) {
    this.created = new Date();
  }

  getName(): string {
    return this.name;
  }
}

For this to work, I use Object.create to make a new instance of User without using the constructor. Then assign the properties to that. The encoding function doesn’t change.

function decodeUser(json: UserJSON): User {
  let user = Object.create(User.prototype);
  return Object.assign(user, json, {
    created: new Date(json.created)
  });
}

Finally, the encode and decode functions can just be methods on the User class.

class User {

  private created: Date;

  constructor(
    private name: string,
    private age:  string
  ) {
    this.created = new Date();
  }

  getName(): string {
    return this.name;
  }

  encode(): UserJSON {
    return Object.assign({}, this, {
      created: this.created.toString()
    });
  }

  static decode(json: UserJSON): User {
    let user = Object.create(User.prototype);
    return Object.assign(user, json, {
      created: new Date(json.created)
    });
  }
}

When JSON.stringify is invoked on an object, it checks for a method called toJSON to convert the data before ‘stringifying’ it. In light of this, let’s rename encode and decode to toJSON and fromJSON.

class User {

  /* ... */

  toJSON(): UserJSON {
    return Object.assign({}, this, {
      created: this.created.toString()
    });
  }

  static fromJSON(json: UserJSON): User {
    let user = Object.create(User.prototype);
    return Object.assign(user, json, {
      created: new Date(json.created)
    });
  }
}

We don’t need to call user.encode() explicitly anymore!

let data = JSON.stringify(new User("Steve", 39));
let user = User.fromJSON(JSON.parse(data));

This is good, but we can do better. JSON.parse accepts a second parameter called reviver which is a function that gets called with every key/value pair in the object as it’s being parsed. The root object is passed to reviver with an empty string as the key. Let’s add a reviver function to our User class.

class User {

  /* ... */

  static reviver(key: string, value: any): any {
    return key === "" ? User.fromJSON(value) : value;
  }
}

Now we can write:

let user = JSON.parse(data, User.reviver);

Not too shabby…

The nice thing about using this pattern is that it composes very well.
Say the user had an account property which contained an instance of Account.

class User {

  private account: Account;

  /* ... */
  
  static fromJSON(json: UserJSON): User {
    let user = Object.create(User.prototype);
    return Object.assign(user, json, {
      created: new Date(json.created),
      account: Account.fromJSON(json.account)
    });
  }
}

And here’s the full commented User class.

class User {

  private created: Date;

  constructor(
    private name: string,
    private age:  string
  ) {
    this.created = new Date();
  }

  getName(): string {
    return this.name;
  }

  // toJSON is automatically used by JSON.stringify
  toJSON(): UserJSON {
    // copy all fields from `this` to an empty object and return in
    return Object.assign({}, this, {
      // convert fields that need converting
      created: this.created.toString()
    });
  }

  // fromJSON is used to convert an serialized version
  // of the User to an instance of the class
  static fromJSON(json: UserJSON|string): User {
    if (typeof json === 'string') {
      // if it's a string, parse it first
      return JSON.parse(json, User.reviver);
    } else {
      // create an instance of the User class
      let user = Object.create(User.prototype);
      // copy all the fields from the json object
      return Object.assign(user, json, {
        // convert fields that need converting
        created: new Date(json.created),
      });
    }
  }

  // reviver can be passed as the second parameter to JSON.parse
  // to automatically call User.fromJSON on the resulting value.
  static reviver(key: string, value: any): any {
    return key === "" ? User.fromJSON(value) : value;
  }
}

// A representation of User's data that can be converted to
// and from JSON without being altered.
interface UserJSON {
  name:    string;
  age:     number;
  created: string;
}

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