为什么我不能在javascript中为字符串对象添加属性？

let str = "Hello, World!";
str.myProperty = "My Value";
console.log(str.myProperty); // 输出 undefined

let myString = {
  value: "Hello, World!",
  myProperty: "My Value"
};
console.log(myString.value); // 输出 "Hello, World!"
console.log(myString.myProperty); // 输出 "My Value"