How might window.btoa() be implemented in JS?

/**
 * @param {string} b
 * @returns {string}
 */
const btoa = (b) => {
    let s = '';

    const bytes = new Uint8Array(b.split('').map(x => x.charCodeAt(0)));
    const ALPHABET_TABLE = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/=';

    for (let i = 0; i < bytes.length; i += 3) {
        const group = new Uint8Array(
            [
                bytes.at(i) ?? 0x00,
                bytes.at(i+1) ?? 0x00,
                bytes.at(i+2) ?? 0x00,
            ],
        );

        const egroup = new Uint8Array(4);

        egroup[0] = (group[0] >> 2) & 0x3f;
        egroup[1] = (((group[0] << 6) | (group[1] >> 2)) >> 2) & 0x3f;
        egroup[2] = (((group[1] << 4) | (group[2] >> 4)) >> 2) & 0x3f;
        egroup[3] = group[2] & 0x3f;

        if (bytes.at(i+2) == undefined) egroup[3] = 64;
        if (bytes.at(i+1) == undefined) egroup[2] = egroup[3] = 64;

        s += ALPHABET_TABLE[egroup[0]];
        s += ALPHABET_TABLE[egroup[1]];
        s += ALPHABET_TABLE[egroup[2]];
        s += ALPHABET_TABLE[egroup[3]];
    }

    return s;
};

console.log(
    btoa('The quick brown fox jumps over the brown dog'),
);

https://gist.github.com/fuadop/63a7fc0e032176103e70e45c21b79375