我可以在运行时向对象属性添加属性吗？

const obj = {};
obj.newProperty = "value";

obj = {}
obj["new_property"] = "value"

import java.util.HashMap;

public class Main {
    public static void main(String[] args) {
        HashMap<String, String> obj = new HashMap<>();
        obj.put("newProperty", "value");
    }
}

using System.Collections.Generic;

class MainClass {
    public static void Main (string[] args) {
        Dictionary<string, string> obj = new Dictionary<string, string>();
        obj.Add("newProperty", "value");
    }
}