static unique(arr) {
    let contains = {};
    return arr.filter(function (i) {
      // using !== instead of hasOwnProperty (http://andrew.hedges.name/experiments/in/)
      return contains[i] !== undefined ? false : contains[i] = true;
    });
  }
Returns an array of unique elements contained in an array.

@static
@param {Array} arr An array.
@returns {Array} An array of unique elements contained within the array supplied as an argument.