JAVASCRIPT

reduce tally

const pets = ["dog", "chicken", "cat", "dog", "chicken", "chicken", "rabbit"];

const petCounts = pets.reduce(function(obj, pet) {
  if (!obj[pet]) {
    // if the pet doesn't yet exist as a property of the accumulator object,
    //   add it as a property and set its count to 1
    obj[pet] = 1;
  } else {
    // pet exists, so increment its count
    obj[pet]++;
  }
  
  return obj; // return the modified object to be used as accumulator in the next iteration
}, {}); // initialize the accumulator as an empty object

console.log(petCounts);
/*
{
  dog: 2, 
  chicken: 3, 
  cat: 1, 
  rabbit: 1 
}
*/