Passing a function to startParallelShell() unnecessarily introduces a variable into the started shell's scope (using random numbers to avoid a collision).

if (typeof(jsCode) == "function") {
    var id = Math.floor(Math.random() * 100000);
    jsCode = "var f" + id + " = " + jsCode.toString() + ";f" + id + "();";
}

Instead we can have jsCode immediately invoke the function expression, i.e.
startParallelShell(function() { ... }) becomes --eval (function() { ... })().