How to declare a variable in a template in Angular

Declaring variables directly within your Angular templates can significantly streamline your development process, making your code cleaner and more efficient. It allows you to perform calculations, manipulate strings, and control the display of elements directly within the HTML, reducing the need for complex component logic. Mastering this technique is essential for any Angular developer looking to write concise and maintainable code. This guide delves into the various methods for declaring variables in Angular templates, offering best practices and real-world examples to empower you to leverage this powerful feature effectively.

Using let in ngFor

One of the most common ways to declare a variable in an Angular template is within an ngFor loop. The let keyword allows you to create local variables that represent each item in the iterable array, as well as other useful metadata like the index and counting variables.

For example, to display a list of products:

<ul> <li ngFor="let product of products; let i = index"> {{ i + 1 }}. {{ product.name }} </li> </ul>

Here, product represents each individual product in the products array, while i holds the current index of the loop. This streamlined approach keeps the component logic focused on data retrieval, while the template handles presentation and minor manipulations.

Leveraging as for Aliases

The as keyword provides a way to create aliases for variables or the results of expressions within your templates. This is particularly helpful when dealing with complex observables or when you want to improve the readability of your template code.

Consider a scenario where you have an observable called userData$:

<div ngIf="userData$ | async as userData"> Welcome, {{ userData.name }}! </div>

Using as userData creates a local variable called userData that holds the emitted value of the observable. This simplifies the template syntax and avoids the need to repeatedly reference userData$.value.

Declaring Variables with ng-template and let

ng-template provides a powerful mechanism for creating reusable template fragments. Combined with let, you can declare variables within these templates and pass values to them from the parent component.

Here’s an example:

<ng-template myTemplate let-item="data"> <p>{{ item.name }}</p> </ng-template> <ng-container ngTemplateOutlet="myTemplate; context: { data: myData }"></ng-container>

This code defines a template named myTemplate that accepts a variable called item. The context object is used to pass data to this template when it’s instantiated using ngTemplateOutlet.

Best Practices for Template Variables

While template variables offer flexibility, it’s crucial to use them judiciously. Overuse can lead to complex and difficult-to-debug templates. Prioritize keeping your templates focused on presentation logic and leave complex computations to the component class.

Keep template variables concise and descriptive.
Avoid complex logic within templates. If the logic becomes intricate, move it to the component.

For more detailed guidance on Angular development, refer to the official Angular documentation.

Infographic Placeholder: Visual representation of how data flows from component to template variables.

FAQ

Q: What’s the difference between let and as?

A: let is primarily used within structural directives like ngFor to create local variables representing elements within an iterable. as creates an alias for an expression, often used with the async pipe to simplify working with observables.

Identify the location within your template where you need a variable.
Choose the appropriate method (let, as, or ng-template).
Declare the variable using a clear and descriptive name.
Use the variable within your template expressions.

By understanding and effectively utilizing these techniques, you can write cleaner, more efficient, and easier-to-maintain Angular applications. Embrace the power of template variables and elevate your Angular development skills. Check out this helpful resource on Angular template variables and this article on best practices for Angular development. Learn more about optimizing ngFor performance in this informative guide: ngFor Performance Optimization. These principles are crucial for achieving optimal performance and maintainability in your projects. This strategic approach will not only improve the user experience but also streamline your development workflow, ultimately leading to more robust and scalable Angular applications.

Question & Answer :
I have the following template :

<div> <span>{{aVariable}}</span> </div>

and would like to end up with :

<div "let a = aVariable"> <span>{{a}}</span> </div>

Is there a way to do it ?

Update

We can just create directive like *ngIf and call it *ngVar

ng-var.directive.ts

@Directive({ selector: '[ngVar]', }) export class VarDirective { @Input() set ngVar(context: unknown) { this.context.$implicit = this.context.ngVar = context; if (!this.hasView) { this.vcRef.createEmbeddedView(this.templateRef, this.context); this.hasView = true; } } private context: { $implicit: unknown; ngVar: unknown; } = { $implicit: null, ngVar: null, }; private hasView: boolean = false; constructor( private templateRef: TemplateRef<any>, private vcRef: ViewContainerRef ) {} }

with this *ngVar directive we can use the following

<div *ngVar="false as variable"> <span>{{variable | json}}</span> </div>

<div *ngVar="false; let variable"> <span>{{variable | json}}</span> </div>

<div *ngVar="45 as variable"> <span>{{variable | json}}</span> </div>

<div *ngVar="{ x: 4 } as variable"> <span>{{variable | json}}</span> </div>

Plunker Example Angular4 ngVar

Original answer

Angular v4

div + ngIf + let

{{variable.a}} {{variable.b}}
div + ngIf + as

view

<div *ngIf="{ a: 1, b: 2, c: 3 + x } as variable"> <span>{{variable.a}}</span> <span>{{variable.b}}</span> <span>{{variable.c}}</span> </div>

component.ts

export class AppComponent { x = 5; }

If you don’t want to create wrapper like div you can use ng-container

view

<ng-container *ngIf="{ a: 1, b: 2, c: 3 + x } as variable"> <span>{{variable.a}}</span> <span>{{variable.b}}</span> <span>{{variable.c}}</span> </ng-container>

As @Keith mentioned in comments

this will work in most cases but it is not a general solution since it relies on variable being truthy

See update for another approach.