JavaScript中的凯撒密码-为什么这里的'A‘变成了'[’？

// 凯撒密码加密
function caesarCipherEncrypt(str, shift) {
  let result = "";
  for (let i = 0; i < str.length; i++) {
    let char = str[i];
    if (char >= "A" && char <= "Z") {
      let code = ((char.charCodeAt(0) - 65 + shift) % 26) + 65;
      char = String.fromCharCode(code);
    }
    result += char;
  }
  return result;
}

// 凯撒密码解密
function caesarCipherDecrypt(str, shift) {
  let result = "";
  for (let i = 0; i < str.length; i++) {
    let char = str[i];
    if (char >= "A" && char <= "Z") {
      let code = ((char.charCodeAt(0) - 65 - shift + 26) % 26) + 65;
      char = String.fromCharCode(code);
    }
    result += char;
  }
  return result;
}

// 示例
let plaintext = "HELLO";
let shift = 1;
let ciphertext = caesarCipherEncrypt(plaintext, shift);
console.log(ciphertext);  // "IFMMP"
console.log(caesarCipherDecrypt(ciphertext, shift));  // "HELLO"