maputil

Some utilities for Map. So far, there are very few. Please open
issues with rationale to propose new additions.

getOrSet(map, key, createValue)

If key exists in map, return the value. Otherwise, set it to the
value returned by createValue() and return the newly added value.

The ES6 Map does not provide [a DRY way to get or set a
value. This function
does that.