class Storage {
 #value = {};
 
 initialize () {
  //read file content, assign to #value
  //this.#value = JSON.parse(someReadFn());
 }

 save () {
  //write #value back to file
    //someSaveMethod(JSON.stringify(this.#value));
 }
 
 set value (extraValue) {
  this.#value = Object.assign(this.#value, extraValue);
 }

 get value () {
  return this.#value;
 }

}

const storage = new Storage();
storage.initialize();

console.log(storage.value);
storage.value = { a: 1, b: 2 };
console.log(storage.value);
storage.value = { b: 3, d: 4 };
console.log(storage.value);

storage.save();