String.prototype.toUpperCase()

const sentence = "The quick brown fox jumps over the lazy dog.";
console.log(sentence.toUpperCase());
// 予想される結果: "THE QUICK BROWN FOX JUMPS OVER THE LAZY DOG."

toUpperCase()

console.log("alphabet".toUpperCase()); // 'ALPHABET'

const a = String.prototype.toUpperCase.call({
  toString() {
    return "abcdef";
  },
});
const b = String.prototype.toUpperCase.call(true);
// 'ABCDEF TRUE' と表示
console.log(a, b);