How to reduce a list of multiple items to a key value pair object in Javascript?

Question

I have an array of states:

['CO','CA','CO','AL', ... ,'NV']

and I'd like to reduce to:

{ 'CO': 9, 'CA':17, 'AL':1, etc}

The value is the number of times each state occurs in the array.

what's the most efficient way to do this?

Answer 1

function compress2dict( raw_arr )
{ 
  var ret={}; 
  for(var i=0;i<raw_arr.length;i++)
  {
     var item=raw_arr[i];
     ret[item]|=0;
     ret[item]++;
  }
  return ret;
}

a = ['CO','BO','CO','CC','CC','CO','CC']
b = compress2dict(a)
b
{'BO':1, 'CC':3, 'CO':3}

Answer 2

You may be interested in array_count_values from PHPJS. Since the PHP array_count_values function does exactly what you want, it stands to reason that the JavaScript port of that function fits.

Answer 3

I expect you just iterate over the array, assign the member values to object property names and the number of occurences as the value:

function toObj(arr) {
  var item, obj = {};

  for (var i=0, iLen=arr.length; i<iLen; i++) {
    item = arr[i];
    obj[item]? ++obj[item] : (obj[item] = 1);
  }

  return obj;
}

Or if you like while loops (sometimes they're faster, sometimes not):

function toObj(arr) {
  var item, obj = {}, i = arr.length;

  while (i) {
    item = arr[--i];
    obj[item]? ++obj[item] : (obj[item] = 1);
  }

  return obj;
}