AngularJS Watchers


Watchers needed for watch some value and detect that this value is changed.

After call $watch() or $watchCollection new watcher add to internal watcher collection in current scope.

So, what is watcher?

Watcher is a simple function, which is called on every digest cycle, and returns some value. Angular checks the returned value, if it is not the same as it was on the previous call - a callback that was passed in second parameter to function $watch() or $watchCollection will be executed.

(function() {
  angular.module("app", []).controller("ctrl", function($scope) {
    $scope.value = 10;
      function() { return $scope.value; },
      function() { console.log("value changed"); }

Watchers are performance killers. The more watchers you have, the longer they take to make a digest loop, the slower UI. If a watcher detects changes, it will kick off the digest loop (recalculation on all screen)

There are three ways to do manual watch for variable changes in Angular.

$watch() - just watches for value changes

$watchCollection() - watches for changes in collection (watches more than regular $watch)

$watch(..., true) - Avoid this as much as possible, it will perform "deep watch" and will kill the performance (watches more than watchCollection)

Note that if you are binding variables in the view, you are creating new watchers - use {{::variable}} not to create watcher, especially in loops

As a result you need to track how many watchers are you using. You can count the watchers with this script (credit to @Words Like Jared - How to count total number of watches on a page?

(function() {
  var root = angular.element(document.getElementsByTagName("body")),
      watchers = [];

  var f = function(element) {

    angular.forEach(["$scope", "$isolateScope"], function(scopeProperty) {
      if( && {
        angular.forEach([scopeProperty].$$watchers, function(watcher) {

    angular.forEach(element.children(), function(childElement) {



  // Remove duplicate watchers
  var watchersWithoutDuplicates = [];
  angular.forEach(watchers, function(item) {
    if(watchersWithoutDuplicates.indexOf(item) < 0) {



If you don't want to create your own script, there is an open source utility called ng-stats that uses a real-time chart embedded into the page to give you insight into the number of watches Angular is managing, as well as the frequency and duration of digest cycles over time. The utility exposes a global function named showAngularStats that you can call to configure how you want the chart to work.

  "position": "topleft",
  "digestTimeThreshold": 16,
  "autoload": true,
  "logDigest": true,
  "logWatches": true

The example code above displays the following chart on the page automatically (interactive demo).

screenshot of ng-stats chart