Angular $watch gotcha

angular.md

Turns out that angular $watch statements will always fire once to "initialize" the 'watch' statement. The officially recommended workaround, should you not want this behavior, is to use an object identity check to compare the values, ie:

$scope.$watch 'allProviders', (selected, prior) ->
  return if selected is prior
  ...

In JS, the === is the same as Coffeescripts is operator.

Note, this has nothing to do with the third argument to $watch which tells the $watch command to compare on identity vs equality. The initialization behavior described above will occur regardless of this setting.

From the docs:

After a watcher is registered with the scope, the listener fn is called asynchronously (via $evalAsync) to initialize the watcher. In rare cases, this is undesirable because the listener is called when the result of watchExpression didn't change. To detect this scenario within the listener fn, you can compare the newVal and oldVal. If these two values are identical (===) then the listener was called due to initialization.

• Angularjs docs for $watch

Could you also comment on the use of the identity vs. equality comparison in combination with the guard suggested above?

The guard and the identity vs equality described above are completely independent. Regardless of the $watch mode used (identity or equality), the $watch statement will be executed once on creation to initialize it.

In both cases, you want to use an identity check (is in coffeescript, or === in JS) for the guard clause.