Python-like JS slice

slice.js

'use strict';

/**
 * @ngdoc function
 * @name ng.filter:slice
 * @function
 *
 * @description
 * Creates a new array limited to the specified range, and optionnaly extracts only n-th items
 * with the `step` argument, similar to Python slice operators. 
 * The array can also be reversed easily depending on the sign of the `step` parameter.
 * The slice function uses the builtin Javscript `array.slice` method.
 *
 * Note: This function is used to augment the `Array` type in Angular expressions. See
 * {@link ng.$filter} for more information about Angular arrays.
 *
 * @param {Array} input Source array to be sliced.
 * @param {number} start Slice start index. If the `start` number 
 *     is positive, the slice will start at index `start`.
 *     If the `start` number is negative, the slice will start from the n-th item
 *     from the end of the source array.
 * @param {number} end Slice end index. If the `end` number 
 *     is positive, the slice will end at index `end` exluded.
 *     If the `end` number is negative, the slice will end at the n-th item
 *     from the end of the source array.
 * @param {number} step Slice items step. If the `step` number 
 *     is specified, the function will only return each n-th item based on its index.
 *     If the `step` number is negative, the slice will be reversed and we return
 *     each n-th item in reversed order.
 * @returns {Array} A new sub-array containing `Math.floor(end-start/step)`/
 *
 * @example
   <doc:example>
     <doc:source>
       <script>
         function Ctrl($scope) {
           $scope.numbers = [1,2,3,4,5,6,7,8,9];
           $scope.start = 3;
           $scope.end = 7;
           $scope.step = 1;
         }
       </script>
       <div ng-controller="Ctrl">
         Start at: <input type="integer" ng-model="start"><br>
         End at: <input type="integer" ng-model="end"><br>
         Step: <input type="integer" ng-model="step"><br>
         <p>Output numbers: {{ numbers | slice:start:end:step }}</p>
       </div>
     </doc:source>
     <doc:scenario>
       it('should limit the number array to first three items', function() {
         expect(element('.doc-example-live input[ng-model=numLimit]').val()).toBe('3');
         expect(element('.doc-example-live input[ng-model=letterLimit]').val()).toBe('3');
         expect(binding('numbers | limitTo:numLimit')).toEqual('[1,2,3]');
         expect(binding('letters | limitTo:letterLimit')).toEqual('abc');
       });

       it('should update the output when -3 is entered', function() {
         input('numLimit').enter(-3);
         input('letterLimit').enter(-3);
         expect(binding('numbers | limitTo:numLimit')).toEqual('[7,8,9]');
         expect(binding('letters | limitTo:letterLimit')).toEqual('ghi');
       });

       it('should not exceed the maximum size of input array', function() {
         input('numLimit').enter(100);
         input('letterLimit').enter(100);
         expect(binding('numbers | limitTo:numLimit')).toEqual('[1,2,3,4,5,6,7,8,9]');
         expect(binding('letters | limitTo:letterLimit')).toEqual('abcdefghi');
       });
     </doc:scenario>
   </doc:example>
 */
function slice(array, from, to, step) {
    if (from===null) from=0;
    if (to===null) to=array.length;
    if (!step) return array.slice(from, to);
    var result = Array.prototype.slice.call(array, from, to);
    if (step < 0) result.reverse();
    step = Math.abs(step);
    if (step > 1) {
        var final = [];
        for (var i = result.length - 1; i >= 0; i--) {
            (i % step === 0) && final.push(result[i]);
        };
        final.reverse();
        result = final;
   }
   return result;
}