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Optimizing Angular Performance with "trackBy" in *ngFor


Angular is a powerful front-end framework that allows developers to build dynamic web applications with ease. However, as applications grow in complexity and data sets become larger, performance optimization becomes a crucial concern. One of the common scenarios where performance can be improved is when rendering lists of items using the *ngFor directive. In this article, we'll explore how to optimize Angular performance with the trackBy function in *ngFor, providing you with a clear and practical guide to improve your application's performance.

Understanding the Problem

When using the *ngFor directive in Angular, you often iterate through an array of objects to display a list. Angular tracks changes in the list and updates the DOM accordingly. This process is efficient for small lists, but can become a performance bottleneck with larger datasets.

Consider the following scenario: you have a list of 1000 items, and you want to display and update them in your application. Without proper optimization, Angular will re-render the entire list every time there is a change, even if only one item is modified. This leads to unnecessary DOM manipulation and can result in a significant performance hit.

The Solution - Introducing "trackBy"

The trackBy function is an optimization technique in Angular that allows you to specify a unique identifier for each item in the list. Angular will then use this identifier to track changes and update only the necessary parts of the DOM when the list changes. This can greatly improve the performance of your application when working with large datasets.

Let's dive into the details and see how to implement trackBy in your Angular application.

H3: Implementation - How to Use "trackBy"

Step 1: Define a Unique Identifier

To use trackBy, you need to define a unique identifier for each item in your list. This identifier can be a property of your data objects, such as an id or any other attribute that ensures the uniqueness of the items. For example, if you have an array of items like this:

items = [
  { id: 1, name: 'Item 1' },
  { id: 2, name: 'Item 2' },
  // ... more items
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You can use the id property as the unique identifier.

Step 2: Implement the trackBy Function

Next, you need to implement the trackBy function in your component. This function should return the unique identifier for a given item. Here's an example of how to implement trackBy for the items array:

trackByFn(index, item) {
  return; // Use the 'id' property as the unique identifier
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Step 3: Use trackBy in the *ngFor Directive

Now that you have defined the trackBy function, you can use it in the *ngFor directive in your template. Here's how you can apply it to your list of items:

  <li *ngFor="let item of items; trackBy: trackByFn">{{ }}</li>
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By including the trackBy function in the *ngFor directive, you are telling Angular to use the unique identifier provided by trackByFn to track changes in the list.

Benefits of Using "trackBy"

Using the trackBy function provides several benefits for your Angular application:

1. Improved Performance

The most significant advantage is the performance improvement. Angular can now track and update only the items that have changed, reducing unnecessary DOM manipulation and improving rendering speed, especially when dealing with large lists.

2. Reduced Resource Consumption

Since Angular updates only the necessary parts of the DOM, your application will consume fewer resources. This can lead to a more responsive and efficient user experience.

3. Smoother Animations and Transitions

If your list items include animations or transitions, using trackBy can make them smoother and more responsive because only the changed items are updated.

Frequently Asked Questions

What happens if the unique identifier changes?

If the unique identifier of an item changes (e.g., the id of an item is modified), Angular may not be able to track it correctly. To address this, it's essential to ensure that the unique identifier remains constant for each item.

Is "trackBy" suitable for all scenarios?

"trackBy" is most beneficial when dealing with large lists or when items are frequently added, removed, or updated. In cases with small, static lists, the performance gain may not be as noticeable.


Optimizing the performance of your Angular application is crucial, especially when working with large datasets. The trackBy function is a valuable tool that can significantly improve the efficiency of your application by reducing unnecessary DOM updates and rendering only the items that have changed. By following the steps outlined in this article, you can take advantage of "trackBy" and enhance the user experience in your Angular application.

In summary, by utilizing the trackBy function, you can achieve a more responsive and efficient application, resulting in a better user experience and improved performance.

Optimize your Angular applications today by implementing "trackBy" and enjoy the benefits of improved performance and smoother user interactions.

This article has provided you with a comprehensive guide on optimizing Angular performance with "trackBy" in *ngFor. By implementing this technique, you can make your Angular applications more efficient, especially when dealing with large datasets. If you have any more questions or need further assistance, feel free to reach out. Happy coding!

Top comments (1)

misscoder profile image
Helina Coder

good tips i would give it a try