TrackBy in ngFor in Angular

Introduction

TrackBy helps Angular remember items in a list to update only what changes. This makes your app faster and smoother.

When displaying a list of items that can change often, like messages or tasks.

When you want to avoid flickering or losing input focus in list items during updates.

When working with large lists to improve performance by reducing DOM updates.

When items have unique IDs you can use to track them efficiently.

Syntax

Angular

<div *ngFor="let item of items; trackBy: trackByFunction">{{item.name}}</div>

trackByFunction(index: number, item: any): any {
  return item.id;
}

The trackBy function receives the index and the item from the list.

It should return a unique identifier for each item, usually an ID.

Examples

Tracks users by their unique id property.

Angular

<div *ngFor="let user of users; trackBy: trackByUserId">{{user.name}}</div>

trackByUserId(index: number, user: any): number {
  return user.id;
}

Tracks products by their position in the list (index). Useful if no unique ID exists.

Angular

<div *ngFor="let product of products; trackBy: trackByIndex">{{product.name}}</div>

trackByIndex(index: number, product: any): number {
  return index;
}

Sample Program

This component shows a list of products. Clicking the button shuffles the list order. Using trackById helps Angular keep track of each product by its id, so it only updates the order without recreating all list items.

Angular

import { Component } from '@angular/core';

@Component({
  selector: 'app-trackby-demo',
  template: `
    <h2>Product List</h2>
    <button (click)="shuffle()">Shuffle Products</button>
    <ul>
      <li *ngFor="let product of products; trackBy: trackById">
        {{product.name}} (ID: {{product.id}})
      </li>
    </ul>
  `
})
export class TrackByDemoComponent {
  products = [
    { id: 1, name: 'Apple' },
    { id: 2, name: 'Banana' },
    { id: 3, name: 'Cherry' }
  ];

  trackById(index: number, product: any): number {
    return product.id;
  }

  shuffle() {
    this.products = this.products
      .map(value => ({ value, sort: Math.random() }))
      .sort((a, b) => a.sort - b.sort)
      .map(({ value }) => value);
  }
}

Important Notes

Without trackBy, Angular recreates all list items on changes, which can cause flicker and lose input focus.

Always use a unique and stable identifier for best results.

Summary

TrackBy improves list rendering performance by identifying items uniquely.

Use it when lists update often or are large.

It helps Angular update only changed items, making apps faster and smoother.

Practice

1. What is the main purpose of using trackBy in an ngFor directive in Angular?

easy

A. To improve performance by uniquely identifying list items during updates

B. To style list items differently based on their index

C. To sort the list items before rendering

D. To filter out duplicate items from the list

Solution

1. Step 1: Understand what ngFor does

   ngFor repeats a template for each item in a list, rendering them in the view.

2. Step 2: Identify the role of trackBy

   trackBy helps Angular know which items changed by uniquely identifying them, so it only updates those items instead of re-rendering the whole list.

3. Final Answer:

   To improve performance by uniquely identifying list items during updates -> Option A

4. Quick Check:

   TrackBy improves performance = C [OK]

Hint: TrackBy helps Angular track items uniquely for faster updates [OK]

Common Mistakes:

• Thinking trackBy sorts or filters items
• Believing trackBy changes item styles
• Assuming trackBy removes duplicates

2. Which of the following is the correct syntax to use trackBy in an ngFor loop?

easy

A. *ngFor="let item of items; trackByFn"

B. *ngFor="let item of items; trackBy: trackByFn"

C. *ngFor="let item of items; trackByFn()"

D. *ngFor="let item of items; trackBy=trackByFn()"

Solution

1. Step 1: Recall the correct ngFor syntax with trackBy

   The correct syntax uses a semicolon and the keyword trackBy followed by the function name without parentheses.

2. Step 2: Compare options

   *ngFor="let item of items; trackBy: trackByFn" matches the correct syntax: *ngFor="let item of items; trackBy: trackByFn". Others either miss the keyword or use parentheses incorrectly.

3. Final Answer:

   *ngFor="let item of items; trackBy: trackByFn" -> Option B

4. Quick Check:

   Correct syntax includes 'trackBy:' keyword [OK]

Hint: Use 'trackBy:' keyword with function name, no parentheses [OK]

Common Mistakes:

• Omitting 'trackBy:' keyword
• Adding parentheses after function name
• Using '=' instead of ':'

3. Given the component code below, what will be logged when the items array is updated by adding a new item?

items = [{id: 1, name: 'A'}, {id: 2, name: 'B'}];

trackByFn(index: number, item: any) {
  return item.id;
}

Template:

<div *ngFor="let item of items; trackBy: trackByFn">{{item.name}}</div>

medium

A. Rendering throws an error due to trackBy function

B. All items are re-rendered and logged

C. No items are rendered

D. Only the new item is rendered and logged

Solution

1. Step 1: Understand how trackBy works with unique IDs

   The trackByFn returns the unique id of each item, so Angular can track items by their IDs.

2. Step 2: Effect of adding a new item

   When a new item is added, Angular uses trackByFn to detect only the new item and renders just that one, not the entire list.

3. Final Answer:

   Only the new item is rendered and logged -> Option D

4. Quick Check:

   trackBy with unique ID renders only changed items [OK]

Hint: trackBy returns unique id to render only changed items [OK]

Common Mistakes:

• Assuming all items re-render on update
• Thinking trackBy causes errors if function returns id
• Believing no items render after update

4. Identify the error in the following trackBy function used in an ngFor loop:

trackByFn(index: number, item: any) {
  return index + 1;
}

medium

A. It returns a non-unique value causing incorrect tracking

B. It returns the index which is acceptable and error-free

C. It throws a syntax error due to missing return type

D. It causes Angular to ignore the trackBy function

Solution

1. Step 1: Analyze the returned value from trackByFn

   The function returns index + 1, which changes if the list order changes or items are added/removed.

2. Step 2: Understand why this causes problems

   Returning a value based on index is not stable for tracking because indexes shift, causing Angular to think items changed incorrectly.

3. Final Answer:

   It returns a non-unique value causing incorrect tracking -> Option A

4. Quick Check:

   trackBy must return stable unique IDs, not shifting indexes [OK]

Hint: trackBy must return stable unique IDs, not changing indexes [OK]

Common Mistakes:

• Thinking index is always safe to return
• Ignoring that indexes shift on list changes
• Assuming missing return type causes syntax error

5. You have a list of users with possible duplicate names but unique IDs. You want to optimize rendering with trackBy. Which trackBy function below correctly handles this scenario to avoid unnecessary re-renders when the list updates?

hard

A. trackByFn(index: number, user: any) { return user.name; }

B. trackByFn(index: number, user: any) { return index; }

C. trackByFn(index: number, user: any) { return user.id; }

D. trackByFn(index: number, user: any) { return user; }

Solution

1. Step 1: Identify the unique property for tracking

   Users may have duplicate names, so using user.name is not reliable. The unique user.id is the best choice.

2. Step 2: Evaluate each option

   A uses user.name (not unique), B uses user.id (stable and unique), C uses index (unstable), D returns the whole user object (not recommended).

3. Final Answer:

   trackByFn(index: number, user: any) { return user.id; } -> Option C

4. Quick Check:

   Use unique IDs for trackBy to avoid re-renders [OK]

Hint: Always trackBy unique stable IDs, not names or indexes [OK]

Common Mistakes:

• Using non-unique properties like names
• Using index which changes on list updates
• Returning the whole object causing errors