D3.js create new array with nest() values

Question

I am studying d3.js now and I want to set my dataset for timeseries in js to make a line chart.

I want to group every local and make an array for those occurrence and plot them by date

Right now this is my output

Object {0: Object, 1: Object, 2: Object, 3: Object, 4: Object…}
0: Object
date: "2020-03-08"
local: "MG"
occurrence: 1
1: Object
date: "2020-03-09"
local: "MG"
occurrence: 2
2: Object
date: "2020-03-11"
local: "BH"
occurrence: 1

But I want to set like this

Object{
entries: Array(n) [0: Object {local: "MG", occurrence: Array{1, 2, ..}, 1: Object {local: "BH", occurrence: Array{1 ...}]
date: Array(d) [0: 2020-03-08, 1: 2020-03-09, ...]
}

I am trying to use nest() and rollup() but I want an array with all occurrences but I dont know how to access it because its inside values and then inside occurrence

Answer 1

I manage to solve my own problem thanks to @Mehdi who gives me this link - d3 Grouping and Summarization

What I need to do is to grouping the data by local

data = d3.nest().key(d => d.local)

take all the entries of my occurrence

  .rollup(v => {
    return{
      occur: d3.set(v, d => {
        return d.occurrence;
      }).values()
    }
  })
  .entries(data)

map my data to return just the values I want

  .map(d => {
    return{
      local: d.key,
      occur: d.value.occur
    }
  });

What I not sure is about the map() is if it really elimite all other arrays which was initialized or its somewhere else