Reducer: Count number of unique items in a list

reducer-count-unique-items.js

var list = ["foo", "foo", "bar", "bar", "bar"];

var results = list.reduce(reducer, {});

console.log(results); // {"foo": 2, "bar": 3}

/**
 * Callback function for the `reduce()` method.
 *
 * @param acc accumulated value previously returned in the last invocation of the callback
 * @param curr current element being processed in the array
 */
function reducer(acc, curr) {
  if (!acc[curr]) {
    acc[curr] = 1;
  } else {
    acc[curr] = acc[curr] + 1;
  }
  return acc;
}