fix(ingIf/ngShow/ngHide): follow javscript truthy/falsy logic

Make ngShow and ngHide follow javascript `truthy`/`falsy` logic and
not the custom toBoolean logic

Fixes angular#5414 angular#4277 angular#3969

BREAKING CHANGE: The expressions

* `<div ng-hide="[]">X</div>`
* `<div ng-hide="'f'">X</div>`
* `<div ng-hide="'[]'">X</div>`

used to be evaluated to `false` and the elements were hidden.

The same effect is present for `ng-show` and the elements are now
  visible; and with `ng-if` and the elements are now removed

If you were previously doing `ng-show="exp"` where
  `$scope.exp = 'no' // (or 'n' or 'f')`, then instead write
  `ng-show="exp && exp !== 'no'` (or 'n' or 'f').

src/ng/directive/ngIf.js

@@ -85,9 +85,9 @@ var ngIfDirective = ['$animate', function($animate) {
     $$tlb: true,
     link: function ($scope, $element, $attr, ctrl, $transclude) {
         var block, childScope;
-        $scope.$watch($attr.ngIf, function ngIfWatchAction(value) {
+        $scope.$watch('!!(' + $attr.ngIf + ')', function ngIfWatchAction(value) {
 
-          if (toBoolean(value)) {
+          if (value) {
             if (!childScope) {
               childScope = $scope.$new();
               $transclude(childScope, function (clone) {

src/ng/directive/ngShowHide.js

@@ -150,8 +150,8 @@
  */
 var ngShowDirective = ['$animate', function($animate) {
   return function(scope, element, attr) {
-    scope.$watch(attr.ngShow, function ngShowWatchAction(value){
-      $animate[toBoolean(value) ? 'removeClass' : 'addClass'](element, 'ng-hide');
+    scope.$watch('!!(' + attr.ngShow + ')', function ngShowWatchAction(value){
+      $animate[value ? 'removeClass' : 'addClass'](element, 'ng-hide');
     });
   };
 }];
@@ -307,8 +307,8 @@ var ngShowDirective = ['$animate', function($animate) {
  */
 var ngHideDirective = ['$animate', function($animate) {
   return function(scope, element, attr) {
-    scope.$watch(attr.ngHide, function ngHideWatchAction(value){
-      $animate[toBoolean(value) ? 'addClass' : 'removeClass'](element, 'ng-hide');
+    scope.$watch('!!(' + attr.ngHide + ')', function ngHideWatchAction(value){
+      $animate[value ? 'addClass' : 'removeClass'](element, 'ng-hide');
     });
   };
 }];

test/BinderSpec.js

@@ -272,7 +272,7 @@ describe('Binder', function() {
     $rootScope.hidden = 'false';
     $rootScope.$apply();
 
-    assertVisible(element);
+    assertHidden(element);
 
     $rootScope.hidden = '';
     $rootScope.$apply();
@@ -291,7 +291,7 @@ describe('Binder', function() {
     $rootScope.show = 'false';
     $rootScope.$apply();
 
-    assertHidden(element);
+    assertVisible(element);
 
     $rootScope.show = '';
     $rootScope.$apply();

test/ng/directive/ngIfSpec.js

@@ -136,6 +136,18 @@ describe('ngIf', function () {
     expect(element.text()).toBe('before;after;');
   });
 
+  it('should evaluate using javascript `truthy`/`falsy` logic', function () {
+    var cases = ['[]', 'f', [], [''], 'false', {}, function() {}, function(f) {}, 0, false, null, undefined, '', NaN];
+    element.append($compile(
+        '<div ng-if="value">Lucas</div>'
+    )($scope));
+    angular.forEach(cases, function(value) {
+      $scope.value = value;
+      $scope.$apply();
+      expect(element.text()).toBe(value ? 'Lucas' : '');
+    });
+  });
+
   it('should restore the element to its compiled state', function() {
     $scope.value = true;
     makeIf('value');

test/ng/directive/ngShowHideSpec.js

@@ -38,6 +38,18 @@ describe('ngShow / ngHide', function() {
       $rootScope.$digest();
       expect(element).toBeShown();
     }));
+
+
+    it('should follow javascript `truthy`/`falsy` logic', inject(function($rootScope, $compile) {
+      var cases = ['[]', 'f', [], [''], 'false', {}, function() {}, function(f) {}, 0, false, null, undefined, '', NaN];
+      element = jqLite('<div ng-show="exp"></div>');
+      element = $compile(element)($rootScope);
+      angular.forEach(cases, function(value) {
+        $rootScope.exp = value;
+        $rootScope.$digest();
+        expect(element)[value ? 'toBeShown' : 'toBeHidden']();
+      });
+    }));
   });
 
   describe('ngHide', function() {
@@ -49,6 +61,18 @@ describe('ngShow / ngHide', function() {
       $rootScope.$digest();
       expect(element).toBeHidden();
     }));
+
+
+    it('should follow javascript `truthy`/`falsy` logic', inject(function($rootScope, $compile) {
+      var cases = ['[]', 'f', [], [''], 'false', {}, function() {}, function(f) {}, 0, false, null, undefined, '', NaN];
+      element = jqLite('<div ng-hide="exp"></div>');
+      element = $compile(element)($rootScope);
+      angular.forEach(cases, function(value) {
+        $rootScope.exp = value;
+        $rootScope.$digest();
+        expect(element)[value ? 'toBeHidden' : 'toBeShown']();
+      });
+    }));
   });
 });