Reduce Multi Dimensional Array

Question

I'm currently trying to map and reduce functions to flatten out a multidimensional array.
This is a mock example data set:

data: [
  {
    label: "Sort-01"
    data: [
      {
        label: "OCT-2017"
        weight: 2304
      },
      {
        label: "NOV-2017"
        weight: 1783
      }
    ]
  },
  {
    label: "Sort-02"
    data: [
      {
        label: "OCT-2017"
        weight: 4785
      },
      {
        label: "NOV-2017"
        weight: 102
      }
    ]
  },
 ......
]

I know in order to map-reduce by sort number I can use:

data.map(sort => sort.data.reduce((a,b) => a.weight + b.weight));

However, I want to reduce by the month instead of sort number.
I would appreciate any help.

Thanks,
Jay

Ori Drori · Accepted Answer · 2017-10-18T17:42:50.343

Use Array#map to get the an array of the data properties array, then flatten with Array#concat and spread. Use Array#reduce to collect the values into a Map, then use Map#values, and the spread syntax to convert back to an array:

const array = [{"label":"Sort-01","data":[{"label":"OCT-2017","weight":2304},{"label":"NOV-2017","weight":1783}]},{"label":"Sort-02","data":[{"label":"OCT-2017","weight":4785},{"label":"NOV-2017","weight":102}]}];

const result = [... // spread the values iterator to an array
  [].concat(...array.map(({ data }) => data)) // map the array into an array of data arrays
  .reduce((m, { label, weight }) => {
    // take item if label exists in map, if not create new
    const item = m.get(label) || { label, weight: 0 };
    
    item.weight += weight; // add the weight
  
    return m.set(label, item); // set the item in the map
  }, new Map).values()] // get the values iterator

console.log(result);

Here is a spreadless version:

const array = [{"label":"Sort-01","data":[{"label":"OCT-2017","weight":2304},{"label":"NOV-2017","weight":1783}]},{"label":"Sort-02","data":[{"label":"OCT-2017","weight":4785},{"label":"NOV-2017","weight":102}]}];

const helper = Object.create(null);
const result = [].concat.apply([], array.map(({ data }) => data)) // map the array into an array of data arrays, and flatten by apply concat
  .reduce((r, { label, weight }) => {
    // if label is not in helper, add to helper, and push to r
    if(!helper[label]) {
      helper[label] = { label, weight: 0 };
      r.push(helper[label]);
    }
    
    helper[label].weight += weight; // add the weight to the object
  
    return r;
  }, []) // get the values iterator

console.log(result);

Is there a way to do this without the spread? I don't think the project I am currently working on has spread operators yet. I am getting an "[].concat.apply(...).reduce(...).values(...).slice is not a function" error. — Jay, Oct 18 '17 at 15:40

score 1 · Answer 2 · answered Oct 18 '17 at 15:36

You can use reduce or concat (see Ori's answer) to get a flattened data array. From there reassign your year label as key and weight as value. then pass to another reducer in order to total your weights by month.

const data = [{label: "Sort-01",data: [{label: "OCT-2017",weight: 2304,},{label: "NOV-2017",weight: 1783,},],},{label: "Sort-02",data: [{label: "OCT-2017",weight: 4785,},{label: "NOV-2017",weight: 102,},],},];
const flatten = (acc, cur) => { 
  cur.data.forEach(val => acc.push(val)); 
  return acc;
};
const monthMap = ({ label, weight }) => ({ [label]: weight });
const reducer = (acc, cur) => {
  const key = Object.keys(cur)[0]
  if (!acc.hasOwnProperty(key)) { acc[key] = 0 }
  acc[key] += cur[key];
  return acc;
};

let x = data.reduce(flatten, []).map(monthMap).reduce(reducer, {});
console.log(x);

Reduce Multi Dimensional Array

2 Answers2