I've just started learning Javascript. But I will share my solution anyway. I hope it helps someone.

function bonAppetit (bill, k, b) {

var total_bill = parseInt( bill.reduce(function(prev, curr) {
    return prev + curr;
}, 0) );

var anna_bill = (total_bill - bill[k]) / 2;
var anna_change = bill[k] / 2;

if(anna_bill === b) {
   return "Bon Appetit";
} else {
    return anna_change;
}

}