test.js

// import {plainToClass} from "class-transformer";
require("@babel/core").transform("code", {
	plugins: ["@babel/plugin-proposal-decorators"]
});

/*
{
	"id": 1,
	"firstName": "Johny",
	"lastName": "Cage",
	"age": 27,
	"nested" : {
		"key" : "value"
	}
}

*/

const plainToClass = require("class-transformer").plainToClass;
const Type = require("class-transformer").Type
const userJson = require("./test.json");
var reflectMetadata = require('reflect-metadata');

class Nested {
	constructor(data) {
		this.key = null;
	}
}

class User {
	constructor() {
		this.id = null;
		this.firstName = null;
		this.lastName = null;

		// decoratorsBeforeExport: true
		@Type(() => Nested)
		this.nested = null
	}

	getId() {
		return this.id
	}
}

let users = plainToClass(User, userJson);
// console.log("users are ", users.getNested())