In this chapter, you learned how to use most of the methods of the d3-array module, which extend JavaScript with several tools for data manipulation, making it easier to prepare data for use in selections, and in layout and shape generators.
You learned about two strategies for grouping data. The d3.nest() function from d3-collection is the most popular and the simplest one to use, but it may be replaced in the near future with functions derived from d3.group() and d3.rollup(), which are used in recent versions of the d3-array module. A full example using multi-level grouping was explored in this chapter using d3.nest(). You might want to try and repeat the example using d3.group() or d3.rollup() as an exercise.
We also introduced four other modules: d3-random, which is used to generate random distributions that we displayed as histograms, d3-interpolate, for interpolation...
