Basic string/buffer entropy calculator

entropy.mjs

function entropy(strOrBuf = '') {
    const buf = typeof strOrBuf === 'string' ? Buffer.from(strOrBuf, 'utf8') : strOrBuf;
    const counts = {};
    for (const byte of buf) {
        counts[byte] ??= 0;
        counts[byte] += 1;
    }
    let totalEntropy = 0;
    for (const byte in counts) if (counts[byte]) {
        const prevalence = counts[byte] / buf.byteLength;
        totalEntropy += (prevalence * Math.log(prevalence) / Math.log(2)) * -1;
    }
    return totalEntropy;
}