code: function find(arr, obj, cmp) {
    if (typeof cmp !== 'undefined') {
      for (var i = 0; i < arr.length; i++) {
        if (cmp(arr[i], obj)) {
          return i;
        }
      }
      return -1;
    }
  }
docs: Callback function for testing equality of items.
  @private
  @callback comparisonCallback
  @param {*} - The first item.
  @param {*} - The second item.
  @return {boolean} - Whether or not the items are equal.
   
  Returns the location of obj in arr with equality determined by cmp.
  @private
  @param {Array.<*>} arr - The array to be searched.
  @param {*} obj - The item to find a match for.
  @param {comparisonCallback} cmp - The callback that defines
  whether `obj` matches.
  @return {integer} - The index of the first element to match `obj`,
  or -1 if no such element was located.