You've made it through the chapter on data!
We really got to the core of what D3 is about - that is, data wrangling. We've looked at the various types of JavaScript flow control, resolved more promises than we care to remember and converted some shapefiles to TopoJSON -- at this point, your ES2015 skills should be approaching (if they aren't already over) 9,000.
Lastly, we made a cool map that showed a particular set of data, where we mashed up multiple datasets to create something cool. You'll find most of the more interesting data visualizations you build involve combining multiple datasets -- with your new-found skills with promises, you're well on your way to mashing up all the data.
In the next chapter, we'll look at making our stuff pop visually by adding animations. Now things are starting to get pretty exciting!