Fix issue where el.splice could not clear full array

Author: TimvdLippe

lib/mixins/property-effects.html

@@ -1933,13 +1933,33 @@
         // Normalize fancy native splice handling of crazy start values
         if (start < 0) {
           start = array.length - Math.floor(-start);
-        } else {
+        } else if (start) {
           start = Math.floor(start);
         }
-        if (!start) {
-          start = 0;
+        // array.splice does different things based on the number of arguments
+        // you pass in. Therefore, array.splice(0) and array.splice(0, undefined)
+        // do different things. In the former, the whole array is cleared. In the
+        // latter, no items are removed.
+        // This means that we need to detect whether 1. one of the arguments
+        // is actually passed in and then 2. determine how many arguments
+        // we should pass on to the native array.splice
+        //
+        let ret;
+        // Omit any additional arguments if they were not passed in
+        if (start !== undefined && !deleteCount && !items.length) {
+          ret = array.splice(start);
+        // Either start was undefined and the others were defined, but in this
+        // case we can safely pass on all arguments
+        //
+        // Note: this includes the case where none of the arguments were passed in,
+        // e.g. this.splice('array'). However, if both start and deleteCount
+        // are undefined, array.splice will not modify the array (as expected)
+        } else {
+          ret = array.splice(start, deleteCount, ...items);
         }
-        let ret = array.splice(start, deleteCount, ...items);
+        // At the end, check whether any items were passed in (e.g. insertions)
+        // or if the return array contains items (e.g. deletions).
+        // Only notify if items were added or deleted.
         if (items.length || ret.length) {
           notifySplice(this, array, info.path, start, items.length, ret);
         }

test/unit/path-effects.html

@@ -498,6 +498,13 @@
     assert.equal(el.$.boundChild.arrayLength, el.data.length);
   });
 
+  test('splice correctly deletes length', function() {
+    el.data = [1,2,3];
+    assert.equal(el.data.length, 3);
+    el.splice('data', 0);
+    assert.equal(el.data.length, 0);
+  });
+
 });
 
 suite('path API', function() {