I tend to think of it as a pre-emption issue more than a concurrency or parralellism issue. Endless loops cause hangs. Being able to pre-empt things would solve a lot of the problem.

    10 print "mike is cool"
    20 goto 10

simply doesn't have a good analogue in javascript. Even in Web Workers a loop like that causes the worker to be unresponsive to events. The Shared Memory addition alows a rudimentary form of this by allowing a flag to make an aware program exit. It wouldn't be too hard to have a BASIC to worker compiler that compiled the above code to

    while (true) {
       print("mike is cool");
       if (shared_global_flag === true) break;
    }

There are a few javascript livecoding websites out there that update the code as you type. There is a problem with this. Broken (as half written code tends to be) javascript doesn't just fail to work, it breaks other things

for example

    var x=1;
    while (x<10) {
      output(x);
      x+=1;
    }

Typing this into a livecoding environment breaks because the while loop goes on forever until you type the code to increment x. It breaks everything because the while loop blocks you from being able to type the code that would get you out of the situation.

I don't see the wisdom of disaallowing something due to the specter of race conditions when its absence seems to cause a much easier to cause debilitating problem.