Structural Directives in Angular: ngIf and ngFor

Anton Ioffe - December 4th 2023 - 9 minutes read

As an artisan of the web, you understand the profound impact that structuring and rendering can have on the user experience and application efficiency. In the Angular realm, two quintessential tools – ngIf and ngFor – serve as the very sinews of dynamic interface construction. You're about to embark on a journey through the intricacies of these powerful structural directives, dissecting their contributions to performance, flexibility, and the crystalline clarity of your code. Prepare to master the subtle arts of conditional content display and list manipulation, avoid common pitfalls, and unlock the synergistic potential that will elevate your Angular applications to new pinnacles of interactivity and design.

Delving Into *ngIf: Controlling Content Render

*ngIf is an integral directive in Angular, serving as a powerful tool for content rendering that allows us to conditionally include or exclude DOM elements. Utilizing *ngIf for simple binary condition toggling enables the display of content according to whether a condition is met. This approach ensures that, unlike CSS visibility toggles, unnecessary elements aren't merely hidden but are excluded from the DOM, aligning closely with the principles of effective web development practices.

To handle scenarios where a simple toggle between two states is insufficient, *ngIf is versatile enough to accommodate an else block. This is realized through ng-template, offering a structured way to define alternate views when the primary condition fails. The use of a template reference variable in the else block maintains the template's readability and clearly communicates the rendering path taken for either outcome.

<div *ngIf="isLoggedIn; else loggedOut">
  Welcome back, user!
<ng-template #loggedOut>
  Please log in to continue.

In large applications, it's beneficial to extract common *ngIf patterns into reusable components or template references. This practice not only declutters the main template but also promotes the DRY (Don't Repeat Yourself) principle. By using named templates, you can maintain clear separation of concerns, creating a more scalable and testable codebase.

A typical error with *ngIf is compounding the directive with overly complex expressions. Such practices reduce code clarity and may cause unforeseen issues when the state changes unexpectedly. To maintain clean and readable templates, it's advisable to move sophisticated logic into the component's class methods. This separates concerns and keeps templates declarative and straightforward. The juxtaposition below illustrates the benefits of extracting logic from a cumbersome inline expression into a method within the component class.

// Avoid:
<div *ngIf="(user?.status === 'active' || user?.role === 'admin') && !loading"></div>

// Prefer:
<div *ngIf="shouldDisplayUser()"></div>

In the component class:

shouldDisplayUser() {
  const isActiveOrAdmin = this.user?.status === 'active' || this.user?.role === 'admin';
  return isActiveOrAdmin && !this.loading;

To harness the full power of *ngIf, it demands a thorough comprehension, especially in intricate application states. Consider how nested *ngIf directives are structured for transparency and how asynchronous data can affect conditionals. Strive to evaluate and improve upon how *ngIf is utilized within your applications, without letting complexity impede the interpretability of your templates.

Going Beyond Visibility: Performance Implications of ngIf

The *ngIf directive in Angular has significant performance implications when compared to CSS-based visibility controls such as display: none. Using styles to hide elements means these elements remain within the DOM, consuming resources since Angular continues to check them for potential changes. This could negatively impact performance, particularly for complex components or applications with a high level of user interaction.

In contrast, *ngIf can have favorable performance outcomes by adding or removing elements entirely from the DOM, which results in Angular stopping its change detection for these elements, hence freeing up system resources. However, this benefit comes with the cost of having to rebuild the element whenever the condition turns true again. Re-instantiation of the element can be more resource-intensive than changing a CSS property, especially when dealing with complex components that may contain extensive initialization logic.

The decision to use *ngIf should be weighed against the use case. For components that switch visibility frequently and have a high initialization cost, CSS hiding might be more efficient. Conversely, if a component is infrequently activated or lightweight, using *ngIf to remove it from the DOM when not needed could lead to a smaller memory footprint and better app performance.

// Angular component example illustrating *ngIf vs. CSS-based visibility control
  selector: 'app-toggle-demo',
  template: `
    <div *ngIf="condition">Content</div>
    <div [style.display]="condition ? 'block' : 'none'">Content</div>
export class ToggleDemoComponent {
  condition: boolean = false;

  // Method to toggle condition
  toggleCondition(): void {
    this.condition = !this.condition;

The code above shows an Angular component where the *ngIf directive is used to conditionally add or remove an element from the DOM. In the same component, we use the style.display property for comparison. Developers must contemplate how often visibility changes, initialization costs, response times, data binding complexity, and resource consumption interact—which will guide the decision between using *ngIf and CSS visibility toggling. The objective is to choose the method that most effectively balances application responsiveness with resource efficiency.

Iterative Templating with *ngFor: Lists and Loops

Angular's *ngFor structural directive is a cornerstone for rendering dynamic lists within your application's UI. It's a powerful ally in your templating toolkit, harnessing JavaScript's iteration protocols to reproduce template elements for each entry in an array or Observable.

Utilizing the trackBy feature is an essential practice when working with *ngFor to boost performance. With trackBy, Angular identifies each unique item by a key rather than by object reference, which is a game-changer when handling lists with frequent updates. A common best practice is to use an item's unique identifier, such as an ID, to prevent unnecessary DOM manipulations:

  <!-- Assume each item has a unique identifier, such as 'id' -->
  <li *ngFor="let item of items; trackBy: trackById">{{ }}</li>

function trackById(index, item){
  // Using the item's ID to keep track of the list's items

When approaching multi-layered data, *ngFor can unleash its full potential by nesting loops. Remember that with great power comes great responsibility; nested loops can exponentially increase the complexity of your DOM. To ensure scalability and performance, keenly review your data structures and streamline when necessary:

<div *ngFor="let row of matrix">
  <!-- Each 'row' is an array of cell objects -->
  <div *ngFor="let cell of row">
    {{ cell.value }}

Managing dynamic lists effectively demands that iteration be bound to computed properties. This strategy allows for decisive control over list modifications, safeguarding against unnecessary updates and obviating complexity in the template. Offload filtering and computations to the component class; this fosters better testability and clearer templates:

// Filtered list based on some condition, recomputed when items or conditions change
get filteredItems() {
  return this.items.filter(item => this.filterCondition(item));

*ngFor enables rendering of intricate data structures responsively. Leveraging techniques like trackBy for efficient updates, mindful practices in managing nested loops, and strategic binding to computed properties fortifies your application against performance degradations and fosters better maintainability. In environments where optimization is paramount, the detailed understanding of *ngFor is indispensable.

Common Missteps with ngFor and How to Avoid Them

One common misstep with *ngFor is failing to use a trackBy function. Without trackBy, Angular may perform unnecessary DOM operations, as it re-renders entire lists when the data changes, even if some elements remain the same. This can significantly degrade performance. A trackBy function should return a unique identifier for each item. When implemented, Angular will only re-render the items that have actually changed.

<ng-container *ngFor="let item of items; trackBy: trackById">

  // Component metadata...
export class MyComponent {
  items = [{id: 1, name: 'Item 1'}, {id: 2, name: 'Item 2'}];

  trackById(index, item) {

Another pitfall is the misuse of an index within *ngFor. When rendering elements that include some form of numbered list or pagination, developers might wrongly assume that the index represents the absolute position of an item within a paginated dataset. It's important to remember that the index represents the position within the current page, not the dataset. If pagination is involved, calculate the position based on the index and the current page offset.

<ul *ngFor="let item of items; let i = index">
  <li>#{{currentPage * itemsPerPage + i + 1}}: {{}}</li>

Incorrect manipulation of the iterated items can also introduce bugs. For example, directly modifying the array (using methods like push, splice, or assigning new arrays) inside a loop can cause unexpected behavior and errors. It's better to handle such operations in component methods and then reassign the list or use immutable operations to trigger change detection properly.

addItem(newItem) {
  this.items = [...this.items, newItem]; // Reassigning with a new array

removeItem(itemToRemove) {
  this.items = this.items.filter(item => item !== itemToRemove); // Immutable delete

Given the iterable nature of *ngFor, dealing with complex nested structures may increase the difficulty. When rendering trees or nested lists, use recursive templates to keep the design modular and maintainable.

<ng-template #recursiveList let-list>
    <li *ngFor="let item of list">
      <ng-container *ngIf="item.children" [ngTemplateOutlet]="recursiveList" [ngTemplateOutletContext]="{ $implicit: item.children }"></ng-container>

Lastly, while iterating over objects, a common mistake is not realizing that JavaScript's loop iterates over all enumerable properties, which might lead to unexpected items in the loop. Instead, obtain the keys or values using Object.keys() or Object.values() and then iterate over the resulting array.

  <li *ngFor="let key of objectKeys(myObject)">
    Key: {{key}}, Value: {{myObject[key]}}

// In your TypeScript:
objectKeys(obj) {
  return Object.keys(obj);

By recognizing these common missteps and applying the solutions provided, developers can write more efficient and less error-prone Angular applications.

Directive Synthesis: Harnessing ngIf and ngFor Synergy

In the landscape of dynamic web interfaces, blending *ngIf and *ngFor directives emerges as a potent tool in Angular development. When iterating over a collection of data with *ngFor, there are often scenarios where conditional rendering is required for each item. Typically, *ngIf is employed to determine whether a particular item should be included in the DOM. Used singly, both directives are straightforward, but the real prowess is evident when they work in concert, as *ngIf can be used within *ngFor to create rich, context-driven user interfaces.

Consider a real-world example of a task list where tasks may be in different states, such as 'active', 'completed', or 'archived'. While *ngFor iterates over this array of tasks, *ngIf can selectively render tasks based on their status. This synthesis can reduce the complexity of the component's template by avoiding additional filtering logic in the TypeScript, hence keeping the template syntax clean and declarative.

<div *ngFor="let task of tasks">
  <ng-container *ngIf="task.isActive">
    <!-- Active tasks are displayed here -->
    <app-active-task [task]="task"></app-active-task>
  <ng-container *ngIf="!task.isActive">
    <!-- Completed or archived tasks are displayed here -->
    <app-inactive-task [task]="task"></app-inactive-task>

Structuring your Angular templates to performantly handle both the presence of elements and their iterative display requires thoughtful engineering. To enhance performance and avoid creating and destroying DOM elements unnecessarily, it is beneficial to use the trackBy function with *ngFor in tandem with carefully structured *ngIf conditions. This combination lets Angular track each item by a unique identifier, and in conjunction with *ngIf, only updates the DOM when the underlying data actually changes. Moreover, this coordination can lead to improved modularity as developers can encapsulate the *ngIf logic into separate components, ensuring reusability and maintainability.

<div *ngFor="let user of users; trackBy: trackByFunction">
  <app-user-profile *ngIf="user.isActive" [userInfo]="user"></app-user-profile>

Strategies to avoid common pitfalls include: avoiding nested *ngIf statements within *ngFor loops which can create readability and performance issues, and instead, encapsulating such logic within dedicated components. Also, steering clear of excessive function calls or complex expressions in *ngIf statements within *ngFor blocks, which can trigger change detection cycles and degrade performance.

As a thought-provoking takeaway, ponder how this directive synergy can be applied not only to visibility toggles but also to more intricate UI behavior. For example, how would you handle a dynamically fetched list where items must be displayed conditionally based on user roles or permissions? Could leveraging the ng-template with embedded *ngIf and *ngFor logic offer a more refined solution, or would it be advisable to handle such complex conditions within the component class itself? Reflect on how to optimize these patterns for even more nuanced scenarios, driven by your app's specific requirements.


In this article, the author explores the use of structural directives in Angular, specifically focusing on ngIf and ngFor. The article delves into the intricacies of these directives, discussing their contributions to performance, flexibility, and code clarity. Key takeaways include the importance of using ngIf for conditional content display, the performance implications of ngIf compared to CSS visibility toggling, the benefits of using trackBy with ngFor for efficient rendering of lists, common missteps to avoid when using ngFor, and the synergy between ngIf and ngFor in creating dynamic user interfaces. The article challenges the reader to think about how the directive synergy can be applied to more complex scenarios and how to optimize patterns for their specific application requirements.

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