Learn D3.js

Rollup

Finally, you can transform each leaf object and replace it with derived or filtered data using the rollup() method with a reduction function based on the values array. For example, this code will replace each leaf object with a string in a value key (see Collections/7-nesting-rollup.html):

const groupByDirector =        d3.nest()          .key(d => d.director)          .sortKeys((a,b) => d3.ascending(a, b))          .sortValues((a,b) => d3.ascending(a.year, b.year))          .rollup(values => values.map(d => `${d.title} (${d.year})`))          .entries(movies);

The result of running the preceding code is as follows:

[  {     key: "Guillermo del Toro",     value: ["El Laberinto del Fauno (2006)",             "The Shape of Water (2017)"]  },{     key: "Patty Jenkins",     value: ["Wonder Woman (2017)"]  },{ key: "Stanley ...