Simplifying Usage of trackBy in Angular Directives for Effective Item Tracking


In Angular, when working with lists and data rendering, it's crucial to keep track of the rendered items efficiently. One common challenge is ensuring the stability of the rendered items while updating the list dynamically. Fortunately, Angular provides us with syntax sugar that simplifies this task by using the trackBy directive. In this article, we will explore how to leverage the power of trackBy to maintain a good track of rendered items effortlessly.

Understanding the trackBy Directive

The trackBy directive is an essential tool in Angular that optimizes rendering by associating a unique identifier with each item in a list. By using the trackBy directive, Angular can detect changes efficiently and update only the necessary items in the DOM, resulting in improved performance.

Syntax and Usage

To make use of trackBy, follow these steps:

Step 1. Define a Unique Identifier: In your component, ensure that each item in the list has a unique identifier. This can be achieved by adding an id property to your data objects.

export interface Item {
  id: number;
  // Other properties

// Example data
items: Item[] = [
  { id: 1, name: 'Item 1' },
  { id: 2, name: 'Item 2' },
  // Additional items

Step 2. Specify the trackBy Function: Next, create a method in your component that acts as the trackBy function. This function will be responsible for returning the unique identifier for each item.

trackByFn(index: number, item: Item): number {

Step 3.  Apply trackBy in Template: In your HTML template, apply the trackBy directive by binding it to the ngFor loop.

<ng-container *ngFor="let item of items; trackBy: trackByFn">
  <!-- Render item here -->

The benefits of using the trackBy Directive offer

  1. Improved Performance: By associating a unique identifier with each item, Angular can efficiently track changes and update only the affected elements, resulting in optimized rendering performance.

  2. Reduced DOM Manipulation: With trackBy, Angular avoids unnecessary DOM manipulations by reusing existing elements when the data changes, leading to a smoother user experience.

  3. Simplified Syntax: The syntax sugar provided by the trackBy directive simplifies the usage and implementation of item tracking in Angular, making the code more readable and maintainable.

By utilizing the syntax sugar of Angular directives, specifically the trackBy directive, we can easily maintain good track of rendered items in our Angular applications. By following the steps outlined in this article, you can optimize performance, reduce unnecessary DOM manipulations, and simplify the codebase. Leveraging the power of trackBy, you can build responsive and efficient Angular applications with ease.