Do I need to be concerned with race conditions with asynchronous Javascript?

Suppose I load some Flash movie that I know at some point in the future will call window.flashReady and will set window.flashReadyTriggered = true.

Now I have a block of code that I want to have executed when the Flash is ready. I want it to execute it immediately if window.flashReady has already been called and I want to put it as the callback in window.flashReady if it has not yet been called. The naive approach is this:

if(window.flashReadyTriggered) {
  block();
} else {
  window.flashReady = block;
}

So the concern I have based on this is that the expression in the if condition is evaluated to false, but then before block() can be executed, window.flashReady is triggered by the external Flash. Consequently, block is never called.

Is there a better design pattern to accomplish the higher level goal I'm going for (e.g., manually calling the flashReady callback)? If not, am I safe, or are there other things I should do?