refactor($parse): remove unnecessary check

Closes: #13588

Author: thorn0

src/ng/parse.js

@@ -48,23 +48,22 @@ function ensureSafeMemberName(name, fullExpression) {
   return name;
 }
 
-function getStringValue(name, fullExpression) {
-  // From the JavaScript docs:
+function getStringValue(name) {
   // Property names must be strings. This means that non-string objects cannot be used
   // as keys in an object. Any non-string object, including a number, is typecasted
   // into a string via the toString method.
+  // -- MDN, https://developer.mozilla.org/en/docs/Web/JavaScript/Reference/Operators/Property_accessors#Property_names
   //
-  // So, to ensure that we are checking the same `name` that JavaScript would use,
-  // we cast it to a string, if possible.
-  // Doing `name + ''` can cause a repl error if the result to `toString` is not a string,
-  // this is, this will handle objects that misbehave.
-  name = name + '';
-  if (!isString(name)) {
-    throw $parseMinErr('iseccst',
-        'Cannot convert object to primitive value! '
-        + 'Expression: {0}', fullExpression);
-  }
-  return name;
+  // So, to ensure that we are checking the same `name` that JavaScript would use, we cast it
+  // to a string. It's not always possible. If `name` is an object and its `toString` method is
+  // 'broken' (doesn't return a string, isn't a function, etc.), an error will be thrown:
+  //
+  // TypeError: Cannot convert object to primitive value
+  //
+  // For performance reasons, we don't catch this error here and allow it to propagate up the call
+  // stack. Note that you'll get the same error in JavaScript if you try to access a property using
+  // such a 'broken' object as a key.
+  return name + '';
 }
 
 function ensureSafeObject(obj, fullExpression) {
@@ -1231,7 +1230,7 @@ ASTCompiler.prototype = {
   },
 
   getStringValue: function(item) {
-    this.assign(item, 'getStringValue(' + item + ',text)');
+    this.assign(item, 'getStringValue(' + item + ')');
   },
 
   ensureSafeAssignContext: function(item) {

test/ng/parseSpec.js

@@ -2201,6 +2201,19 @@ describe('parser', function() {
         expect(scope.$eval('true || false || run()')).toBe(true);
       });
 
+      it('should throw TypeError on using a \'broken\' object as a key to access a property', function() {
+        scope.object = {};
+        forEach([
+          { toString: 2 },
+          { toString: null },
+          { toString: function() { return {}; } }
+        ], function(brokenObject) {
+          scope.brokenObject = brokenObject;
+          expect(function() {
+            scope.$eval('object[brokenObject]');
+          }).toThrow();
+        });
+      });
 
       it('should support method calls on primitive types', function() {
         scope.empty = '';