How to Keccak 256 hash in front end JavaScript before passing it to my smart contract

I would like to build a simple JavaScript (JS) front end that checks if a field submitted is the pre-image of a keccak256 hash I already know in my smart contract. I currently use the following:

Front end JS:

    <script src="https://cdn.jsdelivr.net/gh/emn178/js-sha3/build/sha3.min.js"></script>
    <script>keccak_ = keccak_256;</script>
    <script lang="javascript">
    .
    .
    $("a#call_preimage").click(function(){
                    var preimage = $("input#preimage").val();
                    var hash_ = keccack_(newNumber);
                    var hash__ = '0x'+ hash_;
                    var params = {from:web3.eth.accounts[0]}
                    contractInstance.checkPreimage(hash__, params, function(error, result){...

Solidity:

function checkPreimage(uint256 incoming) public {
 if (uint256(keccak256(abi.encodePacked(known_preimage))) == incoming){
 do_something(); }}
 // known_preimage is of type uint 256 as well

I can’t seem to pass the if condition. I’m certain all other parts of the code work.