support full [].slice semantics

aheckmann · Sep 29, 2012 · 71a864a · 71a864a
1 parent f127c48
commit 71a864a
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Showing 2 changed files with 47 additions and 2 deletions.
diff --git a/lib/sliced.js b/lib/sliced.js
@@ -10,9 +10,15 @@
 
 module.exports = function (args, slice, sliceEnd) {
   var ret = [];
-  var start = slice || 0;
+
+  var start = slice < 0
+    ? slice + args.length
+    : slice || 0;
+
   var end = 3 === arguments.length
-    ? sliceEnd
+    ? sliceEnd < 0
+      ? sliceEnd + args.length
+      : sliceEnd
     : args.length;
 
   for (var i = start; i < end; ++i) {

diff --git a/test/index.js b/test/index.js
@@ -33,4 +33,43 @@ describe('sliced', function(){
       assert.equal(o[1], r[0]);
     })
   })
+  describe('with negative start and no end', function(){
+    it('begins at an offset from the end and includes all following elements', function(){
+      var o = [3, "4", 5, null];
+      var r = sliced(o, -2);
+      assert.equal(2, r.length);
+      assert.equal(o[2], r[0]);
+      assert.equal(o[3], r[1]);
+    })
+  })
+  describe('with negative start and positive end', function(){
+    it('begins at an offset from the end and includes `end` elements', function(){
+      var o = [3, "4", {x:1}, null];
+
+      var r = sliced(o, -2, 1);
+      assert.equal(0, r.length);
+
+      var r = sliced(o, -2, 2);
+      assert.equal(0, r.length);
+
+      var r = sliced(o, -2, 3);
+      assert.equal(1, r.length);
+      assert.equal(o[2], r[0]);
+    })
+  })
+  describe('with negative start and negative end', function(){
+    it('begins at `start` offset from the end and includes all elements up to `end` offset from the end', function(){
+      var o = [3, "4", {x:1}, null];
+      var r = sliced(o, -3, -1);
+      assert.equal(2, r.length);
+      assert.equal(o[1], r[0]);
+      assert.equal(o[2], r[1]);
+
+      var r = sliced(o, -3, -3);
+      assert.equal(0, r.length);
+
+      var r = sliced(o, -3, -4);
+      assert.equal(0, r.length);
+    })
+  })
 })