Array#wrap Useful JavaScript Game Extension #29

/**
 * Pretend the array is a circle and grab a new array containing length elements.
 * If length is not given return the element at start, again assuming the array
 * is a circle.
 *
 * @param {Number} start The index to start wrapping at, or the index of the
 * sole element to return if no length is given.
 * @param {Number} [length] Optional length determines how long result
 * array should be.
 * @returns The element at start mod array.length, or an array of length elements,
 * starting from start and wrapping.
 * @type Object or Array
 */
Array.prototype.wrap = function(start, length) {
  if(length != null) {
    var end = start + length;
    var result = [];

    for(var i = start; i < end; i++) {
      result.push(this[i.mod(this.length)]);
    }

    return result;
  } else {
    return this[start.mod(this.length)];
  }
};

Simple use:

[1, 2, 3].wrap(-1) => 3

[1, 2, 3].wrap(6) => 1

Or get fancy and tile your kitchen:

["w", "o", "o", "o"].wrap(0, 16) => ["w", "o", "o", "o", "w", "o", "o", "o", "w", "o", "o", "o", "w", "o", "o", "o"]