Merge pull request #169 from mout/issue-168

Fix negative start/end values for array/slice

doc/array.md

@@ -670,7 +670,8 @@ Returns a new array containing the items from `arr` from the start index to the
 end index.
 
 If `start` is omitted, it will start at `0`. If `end` is omitted, it will used
-the last index of the array.
+the last index of the array.  If `start` or `end` is negative, it is used as an
+offset from the end of the array.
 
 It will also convert array-like objects to arrays.
 
@@ -681,6 +682,8 @@ slice([1, 2, 3, 4], 1, 2); // [2, 3]
 slice([1, 2, 3], 1); // [2, 3]
 slice([1, 2, 3]); // [1, 2, 3]
 slice({ length: 2, 0: 'a', 1: 'b' }); // ['a', 'b']
+slice([1, 2, 3], 0, -1); // [1, 2]
+slice([1, 2, 3], -2); // [2, 3]
 ```
 
 

src/array/slice.js

@@ -1,16 +1,19 @@
 define(function () {
 
-    var arrSlice = Array.prototype.slice;
-
     /**
      * Create slice of source array or array-like object
      */
     function slice(arr, start, end){
         if (start == null) {
             start = 0;
+        } else if (start < 0) {
+            start = Math.max(arr.length + start, 0);
         }
+
         if (end == null) {
             end = arr.length;
+        } else if (end < 0) {
+            end = Math.max(arr.length + end, 0);
         }
 
         var result = [];

tests/spec/array/spec-slice.js

@@ -19,6 +19,19 @@ define(['mout/array/slice'], function(slice){
             expect( result ).not.toBe( arr );
         });
 
+        it('should accept negative start and end', function() {
+            var arr = [1, 2, 3, 4];
+            expect( slice(arr, 0, -2) ).toEqual( [1, 2] );
+            expect( slice(arr, -2, 4) ).toEqual( [3, 4] );
+            expect( slice(arr, -2, -1) ).toEqual( [3] );
+        });
+
+        it('should not allow negative start/end beyond array length', function() {
+            var arr = [1, 2, 3];
+            expect( slice(arr, -5, 2) ).toEqual( [1, 2] );
+            expect( slice(arr, 0, -5) ).toEqual( [] );
+        });
+
         it('should convert array-like object to array', function(){
             var obj = { 'length': 3, 0: 'a', 1: 'b', 2: 'c' },
                 result = slice(obj);