feat($interpolate): provide contextual error messages

if an exception occurs during interpolation of a string
(e.g. name() in "Hello, {{name()}}!" throws an exception) we now print
an error message with the expression that was being evaluated when the
exception was thrown.

src/ng/interpolate.js

@@ -52,7 +52,7 @@ function $InterpolateProvider() {
   };
 
 
-  this.$get = ['$parse', function($parse) {
+  this.$get = ['$parse', '$exceptionHandler', function($parse, $exceptionHandler) {
     var startSymbolLength = startSymbol.length,
         endSymbolLength = endSymbol.length;
 
@@ -124,18 +124,24 @@ function $InterpolateProvider() {
       if (!mustHaveExpression  || hasInterpolation) {
         concat.length = length;
         fn = function(context) {
-          for(var i = 0, ii = length, part; i<ii; i++) {
-            if (typeof (part = parts[i]) == 'function') {
-              part = part(context);
-              if (part == null || part == undefined) {
-                part = '';
-              } else if (typeof part != 'string') {
-                part = toJson(part);
+          try {
+            for(var i = 0, ii = length, part; i<ii; i++) {
+              if (typeof (part = parts[i]) == 'function') {
+                part = part(context);
+                if (part == null || part == undefined) {
+                  part = '';
+                } else if (typeof part != 'string') {
+                  part = toJson(part);
+                }
               }
+              concat[i] = part;
             }
-            concat[i] = part;
+            return concat.join('');
+          }
+          catch(err) {
+            var newErr = new Error('Error while interpolating: ' + text + '\n' + err.toString());
+            $exceptionHandler(newErr);
           }
-          return concat.join('');
         };
         fn.exp = text;
         fn.parts = parts;

test/BinderSpec.js

@@ -175,7 +175,7 @@ describe('Binder', function() {
       $rootScope.error['throw'] = function() {throw 'MyError';};
       errorLogs.length = 0;
       $rootScope.$apply();
-      expect(errorLogs.shift()).toBe('MyError');
+      expect(errorLogs.shift().message).toBe('Error while interpolating: {{error.throw()}}\nMyError');
 
       $rootScope.error['throw'] = function() {return 'ok';};
       $rootScope.$apply();

test/ng/interpolateSpec.js

@@ -25,6 +25,29 @@ describe('$interpolate', function() {
     expect($interpolate('{{ false }}')()).toEqual('false');
   }));
 
+  it('should rethrow exceptions', inject(function($interpolate, $rootScope) {
+    $rootScope.err = function () {
+      throw new Error('oops');
+    };
+    expect(function () {
+      $interpolate('{{err()}}')($rootScope);
+    }).toThrow('Error while interpolating: {{err()}}\nError: oops');
+  }));
+
+  it('should stop interpolation when encountering an exception', inject(function($interpolate, $compile, $rootScope) {
+    $rootScope.err = function () {
+      throw new Error('oops');
+    };
+    var dom = jqLite('<div>{{1 + 1}}</div><div>{{err()}}</div><div>{{1 + 2}}</div>');
+    $compile(dom)($rootScope);
+    expect(function () {
+      $rootScope.$apply();
+    }).toThrow('Error while interpolating: {{err()}}\nError: oops');
+    expect(dom[0].innerHTML).toEqual('2');
+    expect(dom[1].innerHTML).toEqual('{{err()}}');
+    expect(dom[2].innerHTML).toEqual('{{1 + 2}}');
+  }));
+
 
   it('should return interpolation function', inject(function($interpolate, $rootScope) {
     $rootScope.name = 'Misko';