如何模拟内部单例对象的字段？

function createSingleton() {
  var privateField = "This is a private field";

  return {
    getPrivateField: function() {
      return privateField;
    },
    setPrivateField: function(value) {
      privateField = value;
    }
  };
}

var singleton = createSingleton();
console.log(singleton.getPrivateField()); // 输出 "This is a private field"
singleton.setPrivateField("Updated private field");
console.log(singleton.getPrivateField()); // 输出 "Updated private field"

var singleton = (function() {
  var privateField = "This is a private field";

  return {
    getPrivateField: function() {
      return privateField;
    },
    setPrivateField: function(value) {
      privateField = value;
    }
  };
})();

console.log(singleton.getPrivateField()); // 输出 "This is a private field"
singleton.setPrivateField("Updated private field");
console.log(singleton.getPrivateField()); // 输出 "Updated private field"