Is there a Javascript function similar to the Python Counter function?

Question

I am attempting to change a program of mine from Python to Javascript and I was wondering if there was a JS function like the Counter function from the collections module in Pyt

Answer 1

For those who want a pure JavaScript solution:

function countBy (data, keyGetter) {
  var keyResolver = {
    'function': function (d) { return keyGetter(d); },
    'string': function(d) { return d[keyGetter]; },
    'undefined': function (d) { return d; }
  };

  var result = {};

  data.forEach(function (d) {
    var keyGetterType = typeof keyGetter;
    var key = keyResolver[keyGetterType](d);

    if (result.hasOwnProperty(key)) {
      result[key] += 1;
    } else {
      result[key] = 1;
    }
  });

  return result;
}

Therefore:

list1 = ['a', 'b', 'c', 'b', 'a', 'b', 'c', 'a', 'a', 'a'];
console.log(countBy(list1));  // {'a':5, 'b':3, 'c':2}

list2 = ['abc', 'aa', 'b3', 'abcd', 'cd'];
console.log(countBy(list2, 'length'));  // {2: 3, 3: 1, 4: 1}

list3 = [1.2, 7.8, 1.9];
console.log(countBy(list3, Math.floor));  // {1: 2, 7: 1}