Ok, the solution seems to be difficult but is reachable with few lines of code.

1. Get the nimsum of the heaps
2. Verifify that the heap^nimsum < heap this means the next player will lose if none of them make a mistake

By the way you can find a better understanding of this problem in wiki: https://en.wikipedia.org/wiki/Nim

function chocolateInBox(arr: number[]): number {
    var nimsum = 0;
    var winner = 0;
    for(let box of arr){ nimsum ^= box};
    for(let box of arr){ winner += ((box^nimsum) < box)?1:0;};
    return winner;
}