feat(protractor): wrap negative indices for ElementArrayFinder.get(i)

Closes #1213

lib/protractor.js

@@ -285,6 +285,7 @@ var buildElementHelper = function(ptor) {
 
   /**
    * Get an element within the ElementArrayFinder by index. The index starts at 0.
+   * Negative indices are wrapped (i.e. -i means ith element from last)
    * This does not actually retrieve the underlying element.
    *
    * @alias element.all(locator).get(index)
@@ -307,16 +308,17 @@ var buildElementHelper = function(ptor) {
     var self = this;
     var getWebElements = function() {
       return self.getWebElements().then(function(parentWebElements) {
-        if (index === -1) {
-          // -1 is special and means last
-          index = parentWebElements.length - 1;
+        var i = index; 
+        if (i < 0) {
+          // wrap negative indices
+          i = parentWebElements.length + i;
         }
-        if (index >= parentWebElements.length) {
+        if (i < 0 || i >= parentWebElements.length) {
           throw new Error('Index out of bound. Trying to access element at ' +
               'index:' + index + ', but there are only ' +
               parentWebElements.length + ' elements');
         }
-        return [parentWebElements[index]];
+        return [parentWebElements[i]];
       });
     };
     return new ElementArrayFinder(getWebElements, this.locator_).toElementFinder_();

spec/basic/elements_spec.js

@@ -366,18 +366,28 @@ describe('ElementArrayFinder', function() {
     expect(colorList.get(2).getAttribute('value')).toEqual('red');
   });
 
+  it('should get an element from an array using negative indices', function() {
+    var colorList = element.all(by.model('color'));
+
+    browser.get('index.html#/form');
+
+    expect(colorList.get(-3).getAttribute('value')).toEqual('blue');
+    expect(colorList.get(-2).getAttribute('value')).toEqual('green');
+    expect(colorList.get(-1).getAttribute('value')).toEqual('red');
+  });
+
   it('should get the first element from an array', function() {
     var colorList = element.all(by.model('color'));
     browser.get('index.html#/form');
 
-    expect(colorList.first(0).getAttribute('value')).toEqual('blue');
+    expect(colorList.first().getAttribute('value')).toEqual('blue');
   });
 
   it('should get the last element from an array', function() {
     var colorList = element.all(by.model('color'));
     browser.get('index.html#/form');
 
-    expect(colorList.last(0).getAttribute('value')).toEqual('red');
+    expect(colorList.last().getAttribute('value')).toEqual('red');
   });
 
   it('should perform an action on each element in an array', function() {