fix: slice filter on negative begin, #117

src/builtin/filters/array.ts

@@ -9,8 +9,8 @@ export default {
   'sort': <T>(v: T[], arg: (lhs: T, rhs: T) => number) => v.sort(arg),
   'size': (v: string | any[]) => v.length,
   'concat': <T1, T2>(v: T1[], arg: T2[] | T2): Array<T1 | T2> => Array.prototype.concat.call(v, arg),
-  'slice': <T>(v: T[], begin: number, length: number): T[] => {
-    if (length === undefined) length = 1
+  'slice': <T>(v: T[], begin: number, length: number = 1): T[] => {
+    begin = begin < 0 ? v.length + begin : begin
     return v.slice(begin, begin + length)
   },
   'uniq': function<T> (arr: T[]): T[] {

test/integration/builtin/filters/array.ts

@@ -43,6 +43,7 @@ describe('filters/array', function () {
     it('should slice third char by 2', () => test('{{ "Liquid" | slice: 2 }}', 'q'))
     it('should slice substr by 2,5', () => test('{{ "Liquid" | slice: 2, 5 }}', 'quid'))
     it('should slice substr by -3,2', () => test('{{ "Liquid" | slice: -3, 2 }}', 'ui'))
+    it('should slice substr by -2,2', () => test('{{ "abc" | slice: -2, 2 }}', 'bc'))
     it('should support array', () => test('{{ "1,2,3,4" | split: "," | slice: 1,2 | join }}', '2 3'))
   })
   it('should support sort', function () {