base64.js

const LOOKUP =
  "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/=";

export function encodeBase64(buffer) {
  const view = new Uint8Array(buffer);
  let out = [];
  for (let i = 0; i < view.length; i += 3) {
    const [b1, b2 = 0x10000, b3 = 0x10000] = view.subarray(i, i + 3);
    out.push(
      b1 >> 2,
      ((b1 << 4) | (b2 >> 4)) & 63,
      b2 <= 0xff ? ((b2 << 2) | (b3 >> 6)) & 63 : 64,
      b3 <= 0xff ? b3 & 63 : 64
    );
  }
  return out.map((c) => LOOKUP[c]).join("");
}