onmessage = function(event) {
    var scriptId = event.data.id;
    var cmd = event.data.cmd;
    var args = event.data.args;

    switch (cmd) {
        case "init":
            // code here to lock down the worker and disable everything potentially insecure, except for postMessage
            break;
        case "run":
            if (!workerInitialized()) return;
            // run user given code
            var code = args.code;
            // ...
            postMessage({cmd: "start", args: scriptId});        // signal that script started
            runScript(code);        // runs eval(code) with proper error reporting etc.
            postMessage({cmd: "stop", args: scriptId});         // signal that script stopped
            break;
        // ...
    }
}

postMessage({cmd: "start", args: scriptId});        // signal that script started
setTimeout((function(scriptId) { return function() { postMessage({cmd: "stop", args: scriptId}); };})(scriptId), 0);         // signal that script stopped, right after this function returned
scriptId = null;
runScript(code);        // runs eval(code) with proper error reporting etc.