class-transformer

Its ES6 and Typescript era. Nowadays you are working with classes and constructor objects more then 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

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 you 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 example how it will look like:

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

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

Installation

Node.js

Install module:

npm install class-transformer --save
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";
```
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";
```

Browser

Install module:

npm install class-transformer --save
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.

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" }
    }
}

Methods

plainToClass

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

classToPlain

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);

classToClass

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

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

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

serialize

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

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

serialize works with both arrays and non-arrays.

deserialize and deserializeArray

You can deserialize your model to from a json using deserialize method:

import {deserialize} from "class-transformer";
let photo = deserialize(photo);

To make deserialization to work with arrays use deserializeArray method:

import {deserializeArray} from "class-transformer";
let photos = deserializeArray(photos);

Working with nested objects

When you are trying to transform objects that have nested objects, its 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

Exposing getters and method return values

You can expose what your getter or method return by setting a @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;
    }
}

Exposing properties with different names

If you want to expose some of properties with a different name, you can do it 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;
    }
}

Skipping specific properties

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, password property will be skipped and not be included in the transformed result.

Skipping depend of operation

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. Oppositely, use toClassOnly option.

Skipping all properties of the class

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.

Skipping private properties, or some prefixed properties

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} 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" }

Using groups to control excluded properties

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

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

@Exclude()
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;
}

import {classToPlain} from "class-transformer";
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

Using versioning to control exposed and excluded properties

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} from "class-transformer";

@Exclude()
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;
}

import {classToPlain} from "class-transformer";
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 nad password

Сonverting date strings into Date objects

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;
}

Note, that dates will be converted to strings when you'll try to convert class object to plain object.

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

Working with arrays

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 {ArrayType} from "class-transformer";

export class Photo {

    id: number;

    name: string;

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

You can also use custom array types:

import {ArrayType} 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.

Additional data transformation

Basic usage

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.

Advanced usage

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

@Transform((value, obj, type) => value)

Argument	Description
`value`	The property value before the transformation.
`obj`	The transformation source object.
`type`	The transformation type.

Other decorators

Signature	Example	Description
`@TransformClassToPlain`	`@TransformClassToPlain({ groups: ["user"] })`	Transform the method return with classToPlain and expose the properties on the class.
`@TransformClassToClass`	`@TransformClassToClass({ groups: ["user"] })`	Transform the method return with classToClass and expose the properties on the class.

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 becuase they are the exposed variables. email property is also exposed becuase we metioned the group "user.email".

Working with generics

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

How does it handle circular references?

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.

Example with Angular2

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.

Samples

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

Release notes

See information about breaking changes and release notes here.

wux5/class-transformer