Mutate original array passed to insert function and return it

Author: acarrau

test/array.builders.js

@@ -209,8 +209,9 @@ $(document).ready(function() {
   });
 
   test('insert', function(){
-    deepEqual(_.insert([], 0, 1), [1],'empty array will will insert item');
-    deepEqual(_.insert([1,3], 1, 2), [1,2,3],'array will insert item');
-    deepEqual(_.insert([1,3], 2, 2), [1,3,2],'exceeding index will insert item at the end');
+    deepEqual(_.insert([], 0, 1), [1],'inserts item in empty array');
+    deepEqual(_.insert([2], 0, 1), [1,2],'inserst item at the corret index');
+    deepEqual(_.insert([1,2], 2, 3), [1,2,3],'inserts item at the end of array if exceeding index');
+    deepEqual(_.insert([1,3], -1, 2), [1,2,3],'inserst item at the correct index if negative index');
   });
 });

underscore.array.builders.js

@@ -17,6 +17,7 @@
 
   // Create quick reference variables for speed access to core prototypes.
   var slice   = Array.prototype.slice;
+  var splice  = Array.prototype.splice;
 
   var existy = function(x) { return x != null; };
 
@@ -198,11 +199,11 @@
       },_.map(arguments[0],function(i){return [i];}));
     },
 
-    // Inserts an item in an array at the specific index
+    // Inserts an item in an array at the specific index mutating the original
+    // array and returning it.
     insert: function(array, index, item){
-      var copy = array.slice(0);
-      copy.splice(index, 0, item);
-      return copy;
+      splice.call(array, index, 0, item);
+      return array;
     }
 
   });