The "cost" of executing the JavaScript proof of work is fairly irrelevant, the whole concept just doesn't make sense with a pessimistic inspection. Anubis requires the users to do an irrelevant amount of sha256 hashes in slow javascript, where a scraper can do it much faster in native code; simply game over. It's the same reason we don't use hashcash for email, the amount of proof of work a user will tolerate is much lower than the amount a professional can apply. If this tool provides any benefit, it's due to it being obscure and non standard.
When reviewing it I noticed that the author carried the common misunderstanding that "difficulty" in proof of work is simply the number of leading zero bytes in a hash, which limits the granularity to powers of two. I realize that some of this is the cost of working in JavaScript, but the hottest code path seems to be written extremely inefficiently.
for (; ;) {
const hashBuffer = await calculateSHA256(data + nonce);
const hashArray = new Uint8Array(hashBuffer);
let isValid = true;
for (let i = 0; i < requiredZeroBytes; i++) {
if (hashArray[i] !== 0) {
isValid = false;
break;
}
}