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JavaScript

function mostFrequentItemCount(collection) {
  var output = {};
  for (var i = 0; i < collection.length; i++) {
    var item = collection[i];
    if (!(item in output))
      output[item] = 0;
    output[item]++;
  }
  console.log(output);
  var result = [0, 5e-324];
  for (var item in output) {
    if (output[item] > result[1]) {
      result[0] = parseFloat(item);
      result[1] = output[item];
    }
  }
  return result;
}
var input = [3, -1, -1, -1, 2, 3, -1, 3, -1, 2, 4, 9, 3];
var result = mostFrequentItemCount(input);
console.log(result);