Javascript Object To ES6 Classes

I have a javascript plugin written in vanilla javascript in Object Notation

example

start = {
  config: {
    a : 1
  },
  core: {
    engine_part1:function() {

    },
    engine_part2: function() {
    } 
  }
  init: function(){

  }
}

In this Object Notation the functions can be declare inside core ie, engine_part1() and engine_part2()

Is there any workarounds to achieve the same with Javascript Classes?

In Javascript classes the class have datas and functions but I couldn't write functions inside an object like core in object notation.

Thanks in advance

Simply use the class statement to declare your class, then add its properties in the constructor and its (static) methods in the class' body.

class Start {
  constructor() {
    this.config = {
      a: 1
    };
    
    this.core = {
      engine_part1: () => (console.log('engine_part1')),
      engine_part2: () => (console.log('engine_part2')),
    }
  }
  
  init() {
    console.log('init');
  }
}

const start = new Start;

console.log(start.config);

start.core.engine_part1();
start.init();