aureooms/js-codec-base64

export default function* char4tobyte3(index, a, b, c, d) {
    yield (index[a] << 2) | (index[b] >> 4);
    yield (0xff & (index[b] << 4)) | (index[c] >> 2);
    yield (0xff & (index[c] << 6)) | index[d];
}