fix(scope): $watch (and angular.equals) should support NaN values

- since NaN !== NaN in javascript digest can get into an infinite loop
  when model value is set to NaN
- angular.equals(NaN, NaN) should return true since that's what we
  expect when comparing primitives or objects containing NaN values

Previously NaN because of its special === properties was used as the
initial value for watches, but that results in issues when NaN is used
as model value.

In order to allow for model to be anything incuding undefined and NaN we
need to mark the initial value differently in a way that would avoid
these issues, allow us to run digest without major perf penalties and
allow for clients to determine if the listener is being called because
the watcher is being initialized or because the model changed. This
implementation covers all of these scenarios.

BREAKING CHANGE: previously to detect if the listener was called because
the watcher was being initialized, it was suggested that clients check
if old value is NaN. With this change, the check should be if the newVal
equals the oldVal.

Closes #657

1 files changed, 2 insertions, 1 deletions

diff --git a/src/Angular.js b/src/Angular.js
index 127be95d..e5f35bba 100644
--- a/src/Angular.js
+++ b/src/Angular.js
@@ -624,6 +624,7 @@ function copy(source, destination){
  *
  * * Both objects or values pass `===` comparison.
  * * Both objects or values are of the same type and all of their properties pass `===` comparison.
+ * * Both values are NaN. (In JavasScript, NaN == NaN => false. But we consider two NaN as equal)
  *
  * During a property comparision, properties of `function` type and properties with names
  * that begin with `$` are ignored.
@@ -634,11 +635,11 @@ function copy(source, destination){
  * @param {*} o1 Object or value to compare.
  * @param {*} o2 Object or value to compare.
  * @returns {boolean} True if arguments are equal.
- *
  */
 function equals(o1, o2) {
   if (o1 === o2) return true;
   if (o1 === null || o2 === null) return false;
+  if (o1 !== o1 && o2 !== o2) return true; // NaN === NaN
   var t1 = typeof o1, t2 = typeof o2, length, key, keySet;
   if (t1 == t2 && t1 == 'object') {
     if (isArray(o1)) {