Its ES6 and Typescript era. Nowadays you are working with classes and constructor objects more than ever. Class-transformer allows you to transform plain object to some instance of class and versa. Also it allows to serialize / deserialize object based on criteria. This tool is super useful on both frontend and backend.

Example how to use with angular 2 in plunker. Source code is available here.

• What is class-transformer
• Installation
  • Node.js
  • Browser
• Methods
  • plainToClass
  • plainToClassFromExist
  • classToPlain
  • classToClass
  • serialize
  • deserialize and deserializeArray
• Enforcing type-safe instance
• Working with nested objects
  • Providing more than one type option
• Exposing getters and method return values
• Exposing properties with different names
• Skipping specific properties
• Skipping depend of operation
• Skipping all properties of the class
• Skipping private properties, or some prefixed properties
• Using groups to control excluded properties
• Using versioning to control exposed and excluded properties
• Сonverting date strings into Date objects
• Working with arrays
• Additional data transformation
  • Basic usage
  • Advanced usage
• Other decorators
• Working with generics
• Implicit type conversion
• How does it handle circular references?
• Example with Angular2
• Samples
• Release notes

In JavaScript there are two types of objects:

• plain (literal) objects
• class (constructor) objects

Plain objects are objects that are instances of Object class. Sometimes they are called literal objects, when created via {} notation. Class objects are instances of classes with own defined constructor, properties and methods. Usually you define them via class notation.

So, what is the problem?

Sometimes you want to transform plain javascript object to the ES6 classes you have. For example, if you are loading a json from your backend, some api or from a json file, and after you JSON.parse it you have a plain javascript object, not instance of class you have.

For example you have a list of users in your users.json that you are loading:

[
  {
    "id": 1,
    "firstName": "Johny",
    "lastName": "Cage",
    "age": 27
  },
  {
    "id": 2,
    "firstName": "Ismoil",
    "lastName": "Somoni",
    "age": 50
  },
  {
    "id": 3,
    "firstName": "Luke",
    "lastName": "Dacascos",
    "age": 12
  }
]

And you have a User class:

export class User {
  id: number;
  firstName: string;
  lastName: string;
  age: number;

  getName() {
    return this.firstName + ' ' + this.lastName;
  }

  isAdult() {
    return this.age > 36 && this.age < 60;
  }
}

You are assuming that you are downloading users of type User from users.json file and may want to write following code:

fetch('users.json').then((users: User[]) => {
  // you can use users here, and type hinting also will be available to you,
  //  but users are not actually instances of User class
  // this means that you can't use methods of User class
});

In this code you can use users[0].id, you can also use users[0].firstName and users[0].lastName. However you cannot use users[0].getName() or users[0].isAdult() because "users" actually is array of plain javascript objects, not instances of User object. You actually lied to compiler when you said that its users: User[].

So what to do? How to make a users array of instances of User objects instead of plain javascript objects? Solution is to create new instances of User object and manually copy all properties to new objects. But things may go wrong very fast once you have a more complex object hierarchy.

Alternatives? Yes, you can use class-transformer. Purpose of this library is to help you to map your plain javascript objects to the instances of classes you have.

This library also great for models exposed in your APIs, because it provides a great tooling to control what your models are exposing in your API. Here is an example how it will look like:

fetch('users.json').then((users: Object[]) => {
  const realUsers = plainToClass(User, users);
  // now each user in realUsers is an instance of User class
});

Now you can use users[0].getName() and users[0].isAdult() methods.

1. Install module:

   npm install class-transformer --save

2. reflect-metadata shim is required, install it too:

   npm install reflect-metadata --save

   and make sure to import it in a global place, like app.ts:

   import 'reflect-metadata';

3. ES6 features are used, if you are using old version of node.js you may need to install es6-shim:

   npm install es6-shim --save

   and import it in a global place like app.ts:

   import 'es6-shim';

1. Install module:

   npm install class-transformer --save

2. reflect-metadata shim is required, install it too:

   npm install reflect-metadata --save

   add <script> to reflect-metadata in the head of your index.html:

   <html>
     <head>
       <!-- ... -->
       <script src="node_modules/reflect-metadata/Reflect.js"></script>
     </head>
     <!-- ... -->
   </html>

   If you are using angular 2 you should already have this shim installed.

3. If you are using system.js you may want to add this into map and package config:

   {
     "map": {
       "class-transformer": "node_modules/class-transformer"
     },
     "packages": {
       "class-transformer": { "main": "index.js", "defaultExtension": "js" }
     }
   }

This method transforms a plain javascript object to instance of specific class.

import { plainToClass } from 'class-transformer';

let users = plainToClass(User, userJson); // to convert user plain object a single user. also supports arrays

This method transforms a plain object into an instance using an already filled Object which is an instance of the target class.

const defaultUser = new User();
defaultUser.role = 'user';

let mixedUser = plainToClassFromExist(defaultUser, user); // mixed user should have the value role = user when no value is set otherwise.

This method transforms your class object back to plain javascript object, that can be JSON.stringify later.

import { classToPlain } from 'class-transformer';
let photo = classToPlain(photo);

This method transforms your class object into a new instance of the class object. This may be treated as deep clone of your objects.

import { classToClass } from 'class-transformer';
let photo = classToClass(photo);

You can also use an ignoreDecorators option in transformation options to ignore all decorators you classes is using.

You can serialize your model right to json using serialize method:

import { serialize } from 'class-transformer';
let photo = serialize(photo);

serialize works with both arrays and non-arrays.

You can deserialize your model from json using the deserialize method:

import { deserialize } from 'class-transformer';
let photo = deserialize(Photo, photo);

To make deserialization work with arrays, use the deserializeArray method:

import { deserializeArray } from 'class-transformer';
let photos = deserializeArray(Photo, photos);

The default behaviour of the plainToClass method is to set all properties from the plain object, even those which are not specified in the class.

import { plainToClass } from 'class-transformer';

class User {
  id: number;
  firstName: string;
  lastName: string;
}

const fromPlainUser = {
  unkownProp: 'hello there',
  firstName: 'Umed',
  lastName: 'Khudoiberdiev',
};

console.log(plainToClass(User, fromPlainUser));

// User {
//   unkownProp: 'hello there',
//   firstName: 'Umed',
//   lastName: 'Khudoiberdiev',
// }

If this behaviour does not suit your needs, you can use the excludeExtraneousValues option in the plainToClass method while exposing all your class properties as a requirement.

import { Expose, plainToClass } from 'class-transformer';

class User {
  @Expose() id: number;
  @Expose() firstName: string;
  @Expose() lastName: string;
}

const fromPlainUser = {
  unkownProp: 'hello there',
  firstName: 'Umed',
  lastName: 'Khudoiberdiev',
};

console.log(plainToClass(User, fromPlainUser, { excludeExtraneousValues: true }));

// User {
//   id: undefined,
//   firstName: 'Umed',
//   lastName: 'Khudoiberdiev'
// }

When you are trying to transform objects that have nested objects, it's required to known what type of object you are trying to transform. Since Typescript does not have good reflection abilities yet, we should implicitly specify what type of object each property contain. This is done using @Type decorator.

Lets say we have an album with photos. And we are trying to convert album plain object to class object:

import { Type, plainToClass } from 'class-transformer';

export class Album {
  id: number;

  name: string;

  @Type(() => Photo)
  photos: Photo[];
}

export class Photo {
  id: number;
  filename: string;
}

let album = plainToClass(Album, albumJson);
// now album is Album object with Photo objects inside

In case the nested object can be of different types, you can provide an additional options object, that specifies a discriminator. The discriminator option must define a property that holds the subtype name for the object and the possible subTypes that the nested object can converted to. A sub type has a value, that holds the constructor of the Type and the name, that can match with the property of the discriminator.

Lets say we have an album that has a top photo. But this photo can be of certain different types. And we are trying to convert album plain object to class object. The plain object input has to define the additional property __type. This property is removed during transformation by default:

JSON input:

{
  "id": 1,
  "name": "foo",
  "topPhoto": {
    "id": 9,
    "filename": "cool_wale.jpg",
    "depth": 1245,
    "__type": "underwater"
  }
}

import { Type, plainToClass } from 'class-transformer';

export abstract class Photo {
  id: number;
  filename: string;
}

export class Landscape extends Photo {
  panorama: boolean;
}

export class Portrait extends Photo {
  person: Person;
}

export class UnderWater extends Photo {
  depth: number;
}

export class Album {
  id: number;
  name: string;

  @Type(() => Photo, {
    discriminator: {
      property: '__type',
      subTypes: [
        { value: Landscape, name: 'landscape' },
        { value: Portrait, name: 'portrait' },
        { value: UnderWater, name: 'underwater' },
      ],
    },
  })
  topPhoto: Landscape | Portrait | UnderWater;
}

let album = plainToClass(Album, albumJson);
// now album is Album object with a UnderWater object without `__type` property.

Hint: The same applies for arrays with different sub types. Moreover you can specify keepDiscriminatorProperty: true in the options to keep the discriminator property also inside your resulting class.

You can expose what your getter or method return by setting an @Expose() decorator to those getters or methods:

import { Expose } from 'class-transformer';

export class User {
  id: number;
  firstName: string;
  lastName: string;
  password: string;

  @Expose()
  get name() {
    return this.firstName + ' ' + this.lastName;
  }

  @Expose()
  getFullName() {
    return this.firstName + ' ' + this.lastName;
  }
}

If you want to expose some of the properties with a different name, you can do that by specifying a name option to @Expose decorator:

import { Expose } from 'class-transformer';

export class User {
  @Expose({ name: 'uid' })
  id: number;

  firstName: string;

  lastName: string;

  @Expose({ name: 'secretKey' })
  password: string;

  @Expose({ name: 'fullName' })
  getFullName() {
    return this.firstName + ' ' + this.lastName;
  }
}

Sometimes you want to skip some properties during transformation. This can be done using @Exclude decorator:

import { Exclude } from 'class-transformer';

export class User {
  id: number;

  email: string;

  @Exclude()
  password: string;
}

Now when you transform a User, the password property will be skipped and not be included in the transformed result.

You can control on what operation you will exclude a property. Use toClassOnly or toPlainOnly options:

import { Exclude } from 'class-transformer';

export class User {
  id: number;

  email: string;

  @Exclude({ toPlainOnly: true })
  password: string;
}

Now password property will be excluded only during classToPlain operation. Vice versa, use the toClassOnly option.

You can skip all properties of the class, and expose only those are needed explicitly:

import { Exclude, Expose } from 'class-transformer';

@Exclude()
export class User {
  @Expose()
  id: number;

  @Expose()
  email: string;

  password: string;
}

Now id and email will be exposed, and password will be excluded during transformation. Alternatively, you can set exclusion strategy during transformation:

import { classToPlain } from 'class-transformer';
let photo = classToPlain(photo, { strategy: 'excludeAll' });

In this case you don't need to @Exclude() a whole class.

If you name your private properties with a prefix, lets say with _, then you can exclude such properties from transformation too:

import { classToPlain } from 'class-transformer';
let photo = classToPlain(photo, { excludePrefixes: ['_'] });

This will skip all properties that start with _ prefix. You can pass any number of prefixes and all properties that begin with these prefixes will be ignored. For example:

import { Expose, classToPlain } from 'class-transformer';

export class User {
  id: number;
  private _firstName: string;
  private _lastName: string;
  _password: string;

  setName(firstName: string, lastName: string) {
    this._firstName = firstName;
    this._lastName = lastName;
  }

  @Expose()
  get name() {
    return this._firstName + ' ' + this._lastName;
  }
}

const user = new User();
user.id = 1;
user.setName('Johny', 'Cage');
user._password = '123';

const plainUser = classToPlain(user, { excludePrefixes: ['_'] });
// here plainUser will be equal to
// { id: 1, name: "Johny Cage" }

You can use groups to control what data will be exposed and what will not be:

import { Exclude, Expose, classToPlain } from 'class-transformer';

export class User {
  id: number;

  name: string;

  @Expose({ groups: ['user', 'admin'] }) // this means that this data will be exposed only to users and admins
  email: string;

  @Expose({ groups: ['user'] }) // this means that this data will be exposed only to users
  password: string;
}

let user1 = classToPlain(user, { groups: ['user'] }); // will contain id, name, email and password
let user2 = classToPlain(user, { groups: ['admin'] }); // will contain id, name and email

If you are building an API that has different versions, class-transformer has extremely useful tools for that. You can control which properties of your model should be exposed or excluded in what version. Example:

import { Exclude, Expose, classToPlain } from 'class-transformer';

export class User {
  id: number;

  name: string;

  @Expose({ since: 0.7, until: 1 }) // this means that this property will be exposed for version starting from 0.7 until 1
  email: string;

  @Expose({ since: 2.1 }) // this means that this property will be exposed for version starting from 2.1
  password: string;
}

let user1 = classToPlain(user, { version: 0.5 }); // will contain id and name
let user2 = classToPlain(user, { version: 0.7 }); // will contain id, name and email
let user3 = classToPlain(user, { version: 1 }); // will contain id and name
let user4 = classToPlain(user, { version: 2 }); // will contain id and name
let user5 = classToPlain(user, { version: 2.1 }); // will contain id, name and password

Sometimes you have a Date in your plain javascript object received in a string format. And you want to create a real javascript Date object from it. You can do it simply by passing a Date object to the @Type decorator:

import { Type } from 'class-transformer';

export class User {
  id: number;

  email: string;

  password: string;

  @Type(() => Date)
  registrationDate: Date;
}

Same technique can be used with Number, String, Boolean primitive types when you want to convert your values into these types.

When you are using arrays you must provide a type of the object that array contains. This type, you specify in a @Type() decorator:

import { Type } from 'class-transformer';

export class Photo {
  id: number;

  name: string;

  @Type(() => Album)
  albums: Album[];
}

You can also use custom array types:

import { Type } from 'class-transformer';

export class AlbumCollection extends Array<Album> {
  // custom array functions ...
}

export class Photo {
  id: number;

  name: string;

  @Type(() => Album)
  albums: AlbumCollection;
}

Library will handle proper transformation automatically.

ES6 collections Set and Map also require the @Type decorator:

export class Skill {
  name: string;
}

export class Weapon {
  name: string;
  range: number;
}

export class Player {
  name: string;

  @Type(() => Skill)
  skills: Set<Skill>;

  @Type(() => Weapon)
  weapons: Map<string, Weapon>;
}

You can perform additional data transformation using @Transform decorator. For example, you want to make your Date object to be a moment object when you are transforming object from plain to class:

import { Transform } from 'class-transformer';
import * as moment from 'moment';
import { Moment } from 'moment';

export class Photo {
  id: number;

  @Type(() => Date)
  @Transform(({ value }) => moment(value), { toClassOnly: true })
  date: Moment;
}

Now when you call plainToClass and send a plain representation of the Photo object, it will convert a date value in your photo object to moment date. @Transform decorator also supports groups and versioning.

The @Transform decorator is given more arguments to let you configure how you want the transformation to be done.

@Transform(({ value, key, obj, type }) => value)

The above decorators accept one optional argument: ClassTransformOptions - The transform options like groups, version, name

An example:

@Exclude()
class User {
  id: number;

  @Expose()
  firstName: string;

  @Expose()
  lastName: string;

  @Expose({ groups: ['user.email'] })
  email: string;

  password: string;
}

class UserController {
  @TransformClassToPlain({ groups: ['user.email'] })
  getUser() {
    const user = new User();
    user.firstName = 'Snir';
    user.lastName = 'Segal';
    user.password = 'imnosuperman';

    return user;
  }
}

const controller = new UserController();
const user = controller.getUser();

the user variable will contain only firstName,lastName, email properties because they are the exposed variables. email property is also exposed because we metioned the group "user.email".

Generics are not supported because TypeScript does not have good reflection abilities yet. Once TypeScript team provide us better runtime type reflection tools, generics will be implemented. There are some tweaks however you can use, that maybe can solve your problem. Checkout this example.

NOTE If you use class-validator together with class-transformer you propably DON'T want to enable this function.

Enables automatic conversion between built-in types based on type information provided by Typescript. Disabled by default.

import { IsString } from 'class-validator';

class MyPayload {
  @IsString()
  prop: string;
}

const result1 = plainToClass(MyPayload, { prop: 1234 }, { enableImplicitConversion: true });
const result2 = plainToClass(MyPayload, { prop: 1234 }, { enableImplicitConversion: false });

/**
 *  result1 will be `{ prop: "1234" }` - notice how the prop value has been converted to string.
 *  result2 will be `{ prop: 1234 }` - default behaviour
 */

Circular references are ignored. For example, if you are transforming class User that contains property photos with type of Photo, and Photo contains link user to its parent User, then user will be ignored during transformation. Circular references are not ignored only during classToClass operation.

Lets say you want to download users and want them automatically to be mapped to the instances of User class.

import { plainToClass } from 'class-transformer';

this.http
  .get('users.json')
  .map(res => res.json())
  .map(res => plainToClass(User, res as Object[]))
  .subscribe(users => {
    // now "users" is type of User[] and each user has getName() and isAdult() methods available
    console.log(users);
  });

You can also inject a class ClassTransformer as a service in providers, and use its methods.

Example how to use with angular 2 in plunker. Source code is here.

Take a look on samples in ./sample for more examples of usages.

See information about breaking changes and release notes here.