Shamir Secret Sharing: I cant get the right reconstructed value in javascript

I try to use shamir secret sharing. I implement the code that I find in wikipedia. But when I run it, for enormous numbers the result of reconstruction is different from the real secret value.

https://en.wikipedia.org/wiki/Shamir%27s_Secret_Sharing

The reconstruction function is:

function join(shares) {
    var accum, count, formula, startposition, nextposition, value, numerator, denominator;
    for(formula = accum = 0; formula < shares.length; formula++) {
    for(count = 0, numerator = denominator = 1; count < shares.length; count++) {
        if(formula == count) continue; // If not the same value
        startposition = shares[formula][0];
        nextposition = shares[count][0];
        numerator = (numerator * -nextposition) % prime;
        denominator = (denominator * (startposition - nextposition)) % prime;
    }
    value = shares[formula][1];
    accum = (prime + accum + (value * numerator * modInverse(denominator))) % prime;
}
return accum;
}

var sh = split(9846513, 5, 3)
var newshares = [sh[1], sh[3], sh[4]]; 

document.write(join(newshares));

</script>

When I try to run this code, instead of 9846513 the result is 761. Can someone help me to fix this logical error?

According to the formula, the number your are trying to reconstruct must be less than the prime number used in the algorithm.

You must use a prime number larger that 9846513... next largest prime is 9846517

var prime = 9846517;
...
var sh = split(9846513, 5, 3)
var newshares = [sh[1], sh[3], sh[4]]; 
document.write(join(newshares));

Fiddle here: https://jsfiddle.net/brettwgreen/gz84bw83/