Sanity Check: $index vs. DOM In AngularJS Directives

By Ben Nadel on March 4, 2014

With AngularJS, there is such a strong separation between the DOM (Document Object Model) and your Controller / Model / Service components, that I sometimes find myself unsure as to when I can depend on the existence of dynamically-generated DOM nodes within AngularJS directives. The other day, I had a moment of anxiety about the $index value added by the ngRepeat directive; and, I found myself having to run a sanity check on how the DOM lines up with the $index values.

Run this demo in my JavaScript Demos project on GitHub.

In an ngRepeat loop, AngularJS automatically adds an $index value (among others) to the item-based $scope that gets created in the loop. In this sanity check, I wanted to see if watching for changes in the $index value would accurately reflect changes in the current DOM structure. In this case, I'm logging the number of generated LI elements whenever the $index changes.

<!doctype html>
<html ng-app="Demo">
<head>
	<meta charset="utf-8" />

	<title>
		Sanity Check: $index vs. DOM In AngularJS Directives
	</title>

	<style type="text/css">

		a[ ng-click ] {
			cursor: pointer ;
			text-decoration: underline ;
		}

	</style>
</head>
<body ng-controller="AppController">

	<h1>
		Sanity Check: $index vs. DOM In AngularJS Directives
	</h1>

	<form ng-submit="addFriend()">

		<input type="text" ng-model="form.name" />
		<input type="submit" value="Add Friend" />

	</form>

	<ul>
		<li
			ng-repeat="friend in friends"
			class="friend">

			<!--
				As each ngRepeat item is rendered, we're going to
				listen for changes on the auto-generated $index value.
			-->
			<span bn-index-watch>{{ friend.name }}</span>

			( <a ng-click="removeFriend( friend )">Remove</a> )

		</li>
	</ul>


	<!-- Load scripts. -->
	<script type="text/javascript" src="../../vendor/jquery/jquery-2.0.3.min.js"></script>
	<script type="text/javascript" src="../../vendor/angularjs/angular-1.2.min.js"></script>
	<script type="text/javascript">

		// Create an application module for our demo.
		var app = angular.module( "Demo", [] );


		// -------------------------------------------------- //
		// -------------------------------------------------- //


		// I control the root of the application.
		app.controller(
			"AppController",
			function( $scope ) {

				// I am the collection of friends being rendered.
				$scope.friends = [
					{
						name: "Sarah"
					}
				];

				// I am the form-input collection for ngModel.
				$scope.form = {
					name: ""
				};


				// ---
				// PUBLIC METHODS.
				// ---


				// I add a new friend, using the form input.
				$scope.addFriend = function() {

					$scope.friends.push({
						name: $scope.form.name
					});

					$scope.form.name = "";

				};


				// I remove the given friend from the collection.
				$scope.removeFriend = function( friend ) {

					$scope.friends.splice(
						$scope.friends.indexOf( friend ),
						1
					);

				};

			}
		);


		// -------------------------------------------------- //
		// -------------------------------------------------- //


		// I watch for changes in the $index and compare it to the DOM.
		app.directive(
			"bnIndexWatch",
			function() {

				// I wire the $scope to the DOM.
				function link( $scope, element, attributes ) {

					$scope.$watch(
						"$index",
						function( newValue, oldValue ) {

							// Collection the rendered DOM elments and
							// the index in the context of the current
							// directive / DOM element.
							// --
							// NOTE: We are making the index 1-based
							// for an easier-to-read comparison.
							var friendCount = $( "li.friend" ).length;
							var indexCount = ( newValue + 1 );

							console.log( indexCount, ",", friendCount );

						}
					);

				}

				// Return directive configuration.
				return({
					link: link,
					restrict: "A"
				});

			}
		);

	</script>

</body>
</html>

After adding three friends to the collection, I get the following console log output:

1 , 1
2 , 2
3 , 3
4 , 4

As I added items, you can see that the changes in the $index value match the changes in the DOM.

After removing three friends from the top of the list (in order to actually force $index changes on existing items), I get the following console log output:

1 , 3
2 , 3
3 , 3
1 , 2
2 , 2
1 , 1

As I deleted items, you can see that the changes in the $index value match the changes in the DOM.

Sanity check accomplished! The $index value is an accurate way to gauge the state of the Document Object Model (DOM) in the context of an AngularJS ngRepeat directive.

Want to use code from this post? Check out the license.

Short link: https://bennadel.com/go/2590