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Angular1 Performance: ngClass and large objects

October 17, 2016

Some times the performance of Angular1 applications suddenly drops. Many times it is due to ngClass, or while using scope: ‘=’ in our directives. Here I present a simple optimization that ensures the best performance.

Tags: angularjs-ngclass-performance, contributions, angularjs

<i class="icon" ng-class="{friendly: user.friends, male: user.isMale}">...</i>
<i class="icon" ng-class="{friendly: !!user.friends, male: user.isMale}">...</i>

What is $watch?

Angular updates views by observing models and expressions through $watch:

$scope.$watch(whatToWatch, function onChange(newValue, oldValue) {
  // react to the change
}, objectEquality);

$watch parameters are the followings:

  • whatToWatch: $watch will track changes in this expression,
  • onChange: angular execute this function each time that the value changes,
  • objectEquality: it describes which change will trigger onChange.

ObjectEquality holds the performance key. It only has two valid values: true or false.

$watch(expr, onChange, false)

The algorithm is the following:

var oldValue;
function onDigest() {
  var newValue = expr;
  if (newValue !== oldValue) {    // compare new and old
    onChange(newValue, oldValue);
    oldValue = newValue;          // save new value

It is the fastest $watch. It is because the comparison (compare new and old) is the not-equal operator, and copy (save new value) is an assignation.

But it have a risk: if the value is an object or an array it calls onChange only when the object or array is replaced. It ignores changes in fields or element changes.

$watch(expr, onChange, true)

The algorithm is the following:

var oldValue;
function onDigest() {
  var newValue = expr;
  if (!angular.equals(newValue, oldValue)) {    // compare new and old
    onChange(newValue, oldValue);
    oldValue = angular.copy(newValue);          // save new value

It is the slowest $watch. It uses angular.equals for the comparison, and angular.copy to save the value. But it detects all changes, including changes in object fields or array elements.

Complex objects and $watch…true

Let’s imagine an object as the following:

var david = {
  name: 'David Rodenas',
  friends: [jairo, janton, moraleda, natalia, sarek, victor],

This object contains friends field. This field is an array that contains other users. Each user may have potentially other friends.

Six degrees of separation

Six degrees of separation theory tell us that we can reach all our users just following friends relationships.

#direction: right

[you] - [<actor>friend1]
[you] - [<actor>friend2]
[you] - [<actor>friend ...]
[you] - [<actor>friend4]
[friend2] - [<actor>friendof1]
[friend2] - [<actor>friendof2]
[friend2] - [<actor>fof ...]
[friend2] - [<actor>friendofn]
[friendofn] - [<actor>friendofof1]
[friendofn] - [<actor>friendofof2]
[friendofn] - [<actor>fofof...]
[friendofn] - [<actor>friendofofn]
[friendofof1] - [<actor>friendofofof1]
[friendofof1] - [<actor>friendofofof2]
[friendofof1] - [<actor>fofofof ...]
[friendofof1] - [<actor>friendofofofn]

Complex object copy

var oldDavid = angular.copy(david);

It creates oldDavid which is a copy of david but also a copy of each of its friends, friends of friends, and all users in our domain.

Thus, this copy is very expensive operation.

Complex object equals

var hasChanges = !angular.equals(david, oldDavid);

It compares each property of both, oldDavid and david. If a property is an array, it compares each element. If elements are an object it compares once again value by value. And so on. It eventually will check that each user in our domain has the same name and friends.

This, this equals operations is very expensive.

Other complex objects

There are lots of other objects that are complex.

Library example:

var davidUser = {
  name: 'David Rodenas',
  loans: [
    {book: theGoodParts, expires: '2016/11/12'},
    {book: aDeepnessInTheSky, expires: '2016/10/21'}
var cleanCodeBook = {
  name: 'Clean Code',
  author: 'Robert Cecil Martin',
  history: [
    {user: oriol, date: '2015/06/08'},
    {user: lluis, date: '2015/09/24'},
    {user: david, date: '2016/02/12'},
    {user: granados, date: '2016/04/01'}

Here are not friends, but a user has books, books has more users, etc. It will potentially copy all objects in our domain.

Ids instead of references

Replacing object references by ids can make objects less complex.

Friends example

var david = {
  id: '#david',
  name: 'David Rodenas',
  friends: ['#jairo', '#janton', '#moraleda', '#natalia', '#sarek', '#victor'],

Library example

var davidUser = {
  id: '#david',
  name: 'David Rodenas',
  loans: [
    {book: '#theGoodParts', expires: '2016/11/12'},
    {book: '#aDeepnessInTheSky', expires: '2016/10/21'}
var cleanCodeBook = {
  id: '#cleanCode',
  name: 'Clean Code',
  author: 'Robert Cecil Martin',
  history: [
    {user: '#oriol', date: '2015/06/08'},
    {user: '#lluis', date: '2015/09/24'},
    {user: '#david', date: '2016/02/12'},
    {user: '#granados', date: '2016/04/01'}

Copy and equals

There are no direct references to other objects when using ids.

Given friends example, angular.copy create another object, with fields id, name with the same value, and a field friends with a copy of the array with the same values.

angular.equals check that both objects have the same fields, id and name has the same values, and field friends has the same values in the same order in both objects.

Accelerate ngClass

ngClass uses $watch-true. It need to track changes inside an object.


Given the example:

<i class="icon" ng-class="{friendly: user.friends}">...</i>

ngClass watches the expression:

  {friendly: user.friends}

It copies and compares all user friends, friend of friends, … which is really expensive.

What we want

What we really meant to write was:

<i class="icon" ng-class="{friendly: user.hasFriends()}">...</i>

In such case ngClass would see one of the two following values:

var caseAddClass = {friendly: true};
var caseRemoveClass = {friendly: false};

There is no trace of the user object. It will not copy neither compare user values, friends, … No need to replace references by ids.

Double negation notation

It is not always feasible to add new methods or properties like user.hasFriends.

In such cases we can use double negation !! to a truthy/falsy value into a true/false.

<i class="icon" ng-class="{friendly: !!user.friends}">...</i>

ngClass see one of the following values:

var caseAddClass = {friendly: true};
var caseRemoveClass = {friendly: false};

Thus, no complex object copy or comparison.


Implementation oportunity

Use ids instead of references is not always possible. There are cases where an object will contain its own entities. For example: a todo list may contain its todo objects.

Double negation notation is always possible to use.

Implementation cost

Using ids instead of references ensures that $watch will never do a large copy or comparison. But it may need a deep refactor of the application. It makes it one of the most difficult possible solutions.

Double negation notation is simple, just look for ngClass directives and add a double negation operator before each object.

Programmer reliability

Using ids instead of references will always work. Programmer has to do nothing but use it. It ensures that in any case accidentally the programmer will make $watch to copy and compare all domain objects.

Using double negation notation is dangerous. A programmer may forget use it inside a ngClass, work well the first day, and later, some day discover that the app starts to behave sluggish.


  Digest-loop time
No double negation 1.40ms
With double negation 0.02ms

Time without double negation varies: how big is the basic object? how many references it has? how many references have your domain? In some cases I have seen it take more than 30 seconds.

Double negation notation execution time is constant. It does not matter how big the object is, how complex is our domain, it always will take the same time.


It is not always possible use ids instead of references. But even using ids instead of references it copies the object and compares it. If the object is complex the performance will suffer. It does not give any guarantee of good performance.

Double negation notation is simple, always have a predictable behaviour and good performance. The only problem is the programmer, he may omit some double negation and performance will suffer. But it has a simple solution: time to time, or if performance degrades, look for all ngClass and check double negations, if any is missing, just add it.

Replicate benchmarks

Performance times are obtained using benchpress.

You can replicate the experiments with the benchpress benchmark inside AngularJS git repo ng-class-bp .

More information

You can consult the official documentation here to learn more details.

  • $compile:$compile
  • $watch:$rootScope.Scope#$watch

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