Exploring the programming interview question even occurrence in JavaScript.

evenOccurrence.js

// Prompt: Find the first item that occurs an even number of times in an array. Remember to handle multiple even-occurrence items and return the first one. Return null if there are no even-occurrence items.

function evenOccurrence (array) {
  // Store counts
  var storage = {};

  // Store each value within the storage object to keep count
  array.forEach(function(value, index) {
    storage[value] = storage[value] + 1 || 1;
  });

  // loop through array to find first occurence of an even count
  for (var i = 0; i < array.length; i++) {
    var current = array[i];

    if (storage[current] % 2 === 0) {
      return current;
    }
  }

  // If no even occurrence found, return null
  return null;
}