How to get count of elements in an array, when elements are unknown

Question

So I'm trying to figure out how to generate an associative array that lists the elements of an array, and the number of times each element occurs, without knowing what the elements are beforehand.

As an example, let's say I have an array of animals: var animals = ['Rhino', 'Lion', 'Dog', 'Parrot', 'Parrot', 'Cat', 'Zebra', 'Rhino']

I'd like to generate an object that ultimately looks like:

{ 'Rhino': 2, 'Lion': 1, 'Dog': 1, 'Parrot': 2, 'Cat': 1, 'Zebra': 1 }

If I knew what the animals in the array were beforehand, I could of course do something like:

var animalsCount = {};

var numberOfRhinos = animals.filter(function(animal) {
   return animal == 'Rhino'
}).length;

animalsCount['Rhino'] = numberOfRhinos

to get an object like I'd like. The problem of course is that this becomes quite lengthy and repetitive depending on the number of animals. As well, if I don't know what each type of animal is I can't create the object this way. There must be a way to do this without knowing that information, but I'm stuck.

Answer 1

Easiest way is to create a map, initializing (to 1) values in the array as a a property on that map. You can increment a property's value every time you see a property that is not undefined.

    function countObjs(arr) {
      // So the object doesn't inherit from Object.prototype and avoids property
      // name collisions
      var obj = Object.create(null);
      arr.forEach(function(item) {
        if (obj[item]) {
          obj[item]++;
        } else {
          obj[item] = 1;
        }
      });
      return obj;
    }
    var animals = ['Rhino', 'Lion', 'Dog', 'Parrot', 'Parrot', 'Cat', 'Zebra', 'Rhino'];
    console.log(countObjs(animals));
    /*
    Cat: 1
    Dog: 1
    Lion: 1
    Parrot: 2
    Rhino: 2
    Zebra: 1
    */