string.toUpperCase() : 문자열을 대문자로 변환하기
var string = "I want to earn money a lot";
var upperString = string.toUpperCase();
console.log(upperString); // I WANT TO EARN MONEY A LOT
string.toLowerCase() : 문자열을 소문자로 변환하기
var string = "I WANT TO EARN MONEY A LOT";
var lowerString = string.toLowerCase();
console.log(lowerString); // I want to earn money a lot