关于变量赋值问题

let a = {"name": "张三", "age": 20}
let b = a
b.name = "李四"
b.age = 25
let c = "xxx"
let d = c
d = "yyy"
console.log(a) // 为什么是 {"name": "李四", "age": 25}
console.log(c) // 为什么是 "xxx"
js新手求教，望各位大大解惑，不胜感激