ArrayFixed is a fixed length sparse JavaScript array. The main advantage of a fixed length sparse array, is that we maintain a count along with the array length. Extending on the fixed length sparse array, we create a fixed length sparse array that is dense in one direction (left or right). The ArrayFixedDense ensures that the array is kept dense in one particular direction.
In order to lift the direction of ArrayFixedDense into the type system, we added direction boolean to the type. So you can set ArrayFixedDense<*, true> to ensure that we have a left dense array, or ArrayFixedDense<*, false> to ensure we have a right dense array.
However due to a flow problem, this doesn't quite work yet: facebook/flow#5848