AngularJS watching an expression -

say have expression

$scope.showbutton = users.is_admin() && !document.is_provided; 

and in same controller have button updates value of document.is_provided:

<button ng-click="document.is_provided = true;">provide document</button> 

the problem $scope.showbutton should changed it's not changing.

updated: plnkr showing simplified issue: http://plnkr.co/edit/qk2jnhamqsrnxsajdyek?p=preview

i'd add on squiroid's answer , explain why works. i'll provide simple way understand, common problem me when started.

angular has called digest cycle. series of functions called successively make sure if variable two-way bound variable b (using ng-model example), stay in sync (that is, have same value). functions angular provides (or services, start $, $timeout etc), automatically start digest cycle

so let's have variable , you've bound $scope variable b, equal variable c. expect c bound too, right? because think when change c, b should change, , so, should change too. possible when start digest cycle. cycle variables need updated, , update them. however, changing value of c, never trigger digest cycle, , change never propagate a.

what when put variable in $watch (as $ suggests), forcefully start digest cycle. when change c, start cycle, tracks down needs update too, , that's why works.

in example, $scope.showbutton bound document.is_provided. you're using non-angular function change value of document.is_provided. means not trigger digest cycle, , means change never propagated $scope.showbutton. therefore, showbutton remains false.

and of course, example understand: http://jsbin.com/vojoso/edit?js,console,output

watch console. i've added logs understand. comment out $watch statement , see happens. try understanding , doing yourself, you'll it.

hope helped :)