Uppercase and Lowercase in JavaScript

In JavaScript, strings have built-in methods to change their case:

s.toUpperCase()   // converts all characters to uppercase
s.toLowerCase()   // converts all characters to lowercase

Here are some examples:

let word = "Hello World";
console.log(word.toUpperCase());  // "HELLO WORLD"
console.log(word.toLowerCase());  // "hello world"

let name = "ALICE";
console.log(name.toLowerCase());  // "alice"

These methods return a new string — the original is not modified.