拆分和加入（ES6）[关闭]

Question

我试图编写代码，使名字和姓氏的首字母大写，其余字母小写，还使用 split 和 join 进行间距。

let [firstName, lastName]=prompt("What is your name?").split(" ");
firstName=firstName.toUpperCase(firstName).slice(0,1);
lastName=lastName.toUpperCase(lastName).slice(0,1);
firstName=firstName.toLowerCase(firstName).slice(1,firstName.length);
lastName=lastName.toLowerCase(lastName).slice(1,lastName.length);
const newName=[firstName,lastName].join(" ");
console.log(newName);

显示的是“amnik idhu”而不是“Ramnik Sidhu”