Group By Javascript Object Keys + Array

I am trying to use underscore to group my array of objects in a smaller list: var list = [ { Region: 'A', Vals: [ 7, 'H' ] }, { Region: 'B', Vals: [ 40, 'H' ] }, { Region

Solution 1:

list.reduce(function (memo, v) {
  if (memo[v.Region]) {
    memo[v.Region] = memo[v.Region].concat(v.Vals)
  } else {
    memo[v.Region] = v.Vals.slice()
  }
  return memo
}, {})

The output will look like this:

{
  A: [ 7, "H", 40, "VH" ],
  B: [40, "H", 24, "VH" ],
  C: [ 4, "VH" ]
}

Solution 2:

Use native JavaScript Array#reduce method with a reference object to hold the index.

var list = [
  { Region: 'A', Vals: [ 7, 'H' ] },
  { Region: 'B', Vals: [ 40, 'H' ] },
  { Region: 'B', Vals: [ 24, 'VH' ] },
  { Region: 'C', Vals: [ 4, 'VH' ] },
  { Region: 'A',Vals: [ 40, 'VH' ] }
];


// object for refering index
var ref = {};

// iterate over the array
var res = list.reduce(function(arr, v) {
      // check property defined if not define and push
      // value to array
      if (!(v.Region in ref)) {
        ref[v.Region] = arr.length;
        arr.push({Region: v.Region, Vals: v.Vals});
      // if index already defined then push values
      } else{
        [].push.apply(arr[ref[v.Region]].Vals, v.Vals);
      }
    // return the array reference
    return arr;
  // set initial value as empty array
}, []);

console.log(res);

---

UPDATE : If you want to generate an object where Region value as property name and Vals value as it's value then do something like this.

var list = [
  { Region: 'A', Vals: [ 7, 'H' ] },
  { Region: 'B', Vals: [ 40, 'H' ] },
  { Region: 'B', Vals: [ 24, 'VH' ] },
  { Region: 'C', Vals: [ 4, 'VH' ] },
  { Region: 'A',Vals: [ 40, 'VH' ] }
];


// iterate over the array
var res = list.reduce(function(obj, v) {
  // define the property as an array if not 
  // already defined
  obj[v.Region] = obj[v.Region] || [];
  
  // push all values to array
  [].push.apply(obj[v.Region], v.Vals);
  
  // return the object reference
  return obj;
  
  // set initial value as an empty object
}, {});

console.log(res);

Solution 3:

This will produce the exact output that you are looking for.

//first build an object with Region properties, and add the Vals to each of those properties

var tempList = {}; 
for (var i=0; i < list.length; i++) {
  if (!(list[i].Region in tempList)) {
    tempList[list[i].Region] = [];
  }
  Array.prototype.push.apply(tempList[list[i].Region],list[i].Vals);
}

//then format this as an array of objects    
var groupedList = [];
for (var region in tempList) {
    groupedList.push({Region:region, Vals: tempList[region]});
}

list = groupedList;

After the above code, the following will be true:

list = [
  { Region: 'A', Vals: [ 7, 'H', 40, 'VH' ] },
  { Region: 'B', Vals: [ 40, 'H',  24, 'VH' ] },
  { Region: 'B', Vals: [ 24, 'VH' ] },
  { Region: 'C', Vals: [ 4, 'VH' ] }
];