Base64文字テーブル

// The standard Base64 alphabet
const BASE64_ALPHABET =
  "ABCDEFGHIJKLMNOPQRSTUVWXYZ" +
  "abcdefghijklmnopqrstuvwxyz" +
  "0123456789+/";

// Look up Base64 character by 6-bit index
function base64Char(index: number): string {
  return BASE64_ALPHABET[index];
}

// Build a lookup table object
function buildBase64Table() {
  const table: Record<number, {
    char: string;
    ascii: number;
    hex: string;
    group: string;
  }> = {};
  for (let i = 0; i < 64; i++) {
    const ch = BASE64_ALPHABET[i];
    let group = "Uppercase";
    if (i >= 26 && i < 52) group = "Lowercase";
    else if (i >= 52 && i < 62) group = "Digits";
    else if (i >= 62) group = "Symbols";
    table[i] = {
      char: ch,
      ascii: ch.charCodeAt(0),
      hex: `0x${ch.charCodeAt(0).toString(16)}`,
      group,
    };
  }
  return table;
}

// Encode 3 bytes to 4 Base64 chars
function encode3Bytes(b0: number, b1: number, b2: number): string {
  const n = (b0 << 16) | (b1 << 8) | b2;
  return (
    BASE64_ALPHABET[(n >> 18) & 63] +
    BASE64_ALPHABET[(n >> 12) & 63] +
    BASE64_ALPHABET[(n >> 6) & 63] +
    BASE64_ALPHABET[n & 63]
  );
}
console.log(encode3Bytes(77, 97, 110)); // "TWFu"
console.log(base64Char(25)); // "Z"