ActionScript was a viable answer to introducing optional Types on JavaScript. NIH syndrome got this one.
For a more complete idea of ES4's type system, see "Evolutionary Programming and Gradual Typing in ES4": http://www.ecmascript.org/es4/spec/evolutionary-programming-...
Also, I see React with JSX repeating what MXML (AS3) / FXML (JavaFX) did 10 years back.