The code snippet below looks for the startTokenId of the contract (which is usually 0) and the current largest tokenId (maxId). You will get all the tokenIds of the given contract by getting all the bigint from startTokenId to maxId

Credit: thirdweb SDK v5

// Example for an ERC-721 contract
const options = {
  contract: theNftContract,
};
const [startTokenId_, maxSupply] = await Promise.allSettled([
  startTokenId(options),
  nextTokenIdToMint(options),
  totalSupply(options),
]).then(([_startTokenId, _next, _total]) => {
  // default to 0 if startTokenId is not available
  const startTokenId__ =
    _startTokenId.status === "fulfilled" ? _startTokenId.value : 0n;
  let maxSupply_: bigint;
  // prioritize nextTokenIdToMint
  if (_next.status === "fulfilled") {
    // because we always default the startTokenId to 0 we can safely just always subtract here
    maxSupply_ = _next.value - startTokenId__;
  }
  // otherwise use totalSupply
  else if (_total.status === "fulfilled") {
    maxSupply_ = _total.value;
  } else {
    throw new Error(
      "Contract requires either `nextTokenIdToMint` or `totalSupply` function available to determine the next token ID to mint",
    );
  }
  return [startTokenId__, maxSupply_] as const;
});

const maxId = maxSupply + startTokenId_;
const allTokenIds:bigint[] = [];
for (let i = startTokenId_; i < maxId; i++) {
  allTokenIds.push(i);
}

console.log(allTokenIds); // [startTokenId_, 1n, 2n, ... maxId]