Serializing & Deserializing

This document shows how to serialize class instances into plain objects and, conversely, how to deserialize plain objects into class instances. It is based on the class-transformer library.

Serialization is particularly interesting if you need to transform HTTP request bodies into model instances or, inversely, convert model instances into plain objects to be returned in HTTP responses.

The class-transformer library

npm install class-transformer

The class-transformer has two main functions to transform objects: plainToClass and classToPlain. Some examples of their use are given below.

Other functions also exist and can be found in the README of the library repository.

plainToClass

import { plainToClass } from 'class-transformer';

class User {
  firstName: string;
  lastName: string;

  getFullName() {
    return firstName +  ' ' + lastName;
  }
}

const user = {
  firstName: 'John',
  lastName: 'Doe'
}

const user2 = plainToClass(User, user);
// user2 is an instance of User
console.log(user2.getFullName());
// John Doe

classToPlain

import { classToPlain, Exclude } from 'class-transformer';
 
export class User {
  id: number;
  email: string;
  
  @Exclude()
  password: string;
}

const user = new User();
user.id = 1;
user.email = 'jane.doe@foalts.org';
user.password = 'xxx';

const serializedUser = classToPlain(user);
console.log(serializedUser instanceof User);
// false
console.log(serializedUser);
// {
//   id: 1,
//   email: 'jane.doe@foalts.org'
// }

Additional options can be provided to the classToPlain or plainToClass functions. class-transformer also offers other interesting features (nested objects, property renaming, etc) that can be found here.

Caution: These functions do not validate data. They do not guarantee that all declared properties are assigned and that no additional properties are assigned to the object. They behave more or less like a call to Object.assign. Please refer to the validation page if you need to validate data.

Usage with a Hook

npm install class-transformer @foal/typestack

If you want to use class-transformer within a hook to transform request bodies, you can install the package @foal/typestack for this. It provides a @UnserializeBody hook that transforms the request body into an instance of a given class.

product.entity.ts

import { BaseEntity, Column, Entity, PrimaryGeneratedColumn } from 'typeorm';

@Entity()
// BaseEntity adds the method "save" to the class.
export class Product extends BaseEntity {
  @PrimaryGeneratedColumn()
  id: number;

  @Column()
  name: string;
}

api.controller.ts

import { Context, HttpResponseCreated, Post, ValidateBody } from '@foal/core';
import { UnserializeBody } from '@foal/typestack';
import { Product } from '../entities';

export class ApiController {

  @Post('/products')
  @ValidateBody({
    additionalProperties: false,
    properties: {
      name: { type: 'string' }
    },
    required: [ 'name' ],
    type: 'object',
  })
  @UnserializeBody(Product)
  async createProduct(ctx: Context) {
    // ctx.request.body is an instance of Product
    const product = ctx.request.body;
    await product.save();
    return new HttpResponseCreated();
  }

}

The hook takes also an optional parameter to specify the options of the class-transformer library.