code: function combinations( collection ) {
  	/**
  	 * A recursive function that iterates through all elements of an array to produce its combinations.
  	 *
  	 * @param {Array} xs The array to start accumulating with.
  	 * @param {Array} array The array with elements that should be combined.
  	 *
  	 * @returns {Array} result An array of all possible combinations of elements of the original array.
  	 */
  	function acc( xs, array ) {
  		const x = xs[ 0 ];
  
  		if ( typeof x === "undefined" ) {
  			return array;
  		}
  
  		for ( let i = 0, l = array.length; i < l; ++i ) {
  			array.push( array[ i ].concat( x ) );
  		}
  		return acc( xs.slice( 1 ), array );
  	}
  	return acc( collection, [ [ ] ] ).slice( 1 ).concat( [ [] ] );
  }
docs: Generates all possible combinations of the elements of an array (treated as unique).
  https://gist.github.com/jpillora/4435759
  
  @param {Array} collection The array with elements that should be combined.
  
  @returns {Array} result An array of all possible combinations of elements of the original array.