When is a resolver executed in Angular?

A resolver is executed before the route is activated, allowing data to be fetched in advance.

What Are Resolvers in Angular Routing | FullStackPrep.Dev

Q: What is a resolver in Angular routing?

A resolver is a service in Angular that retrieves data before a route is activated, ensuring the component has the required data upon load.

💡 Concept Name

Route Resolver – A service in Angular that retrieves data before a route is activated, ensuring required data is available to the component immediately.

📘 Quick Intro

Resolvers in Angular routing are used to pre-fetch data before a component loads. They enhance the user experience by avoiding empty states or extra loading indicators after navigation.

🧠 Analogy / Short Story

Imagine ordering food in a restaurant and waiting until your meal is ready before being seated. Similarly, Angular waits for resolver data to load before entering a route.

🔧 Technical Explanation

🛠 A resolver is a service implementing the Resolve<T> interface.
🚦 It is configured in the resolve property of route definitions.
📡 Angular calls the resolver before navigating to the route.
📁 Once resolved, data is injected via ActivatedRoute.data.
🧩 Useful for fetching configuration, user profile, or preloading resources.

🎯 Purpose & Use Case

✅ Pre-fetch user details before profile page loads.
✅ Load configuration settings or app metadata before navigation.
✅ Improve perceived performance by avoiding UI flashes or loading spinners.

💻 Real Code Example

// 1. Create resolver service
import { Injectable } from '@angular/core';
import { Resolve } from '@angular/router';
import { Observable } from 'rxjs';
import { UserService } from './user.service';

                    @Injectable({ providedIn: 'root' })
export class UserResolver implements Resolve<any> {
  constructor(private userService: UserService) {}

  resolve(): Observable<any> {
    return this.userService.getUser();
  }
}

// 2. Define resolver in route
const routes: Routes = [
  {
    path: 'profile',
    component: ProfileComponent,
    resolve: { user: UserResolver }
  }
];

// 3. Access resolved data in component
constructor(private route: ActivatedRoute) {
  this.route.data.subscribe(data => {
    console.log(data['user']);
  });
}

❓ Interview Q&A

Q1: What is a resolver in Angular routing?
A: A resolver is a service that retrieves data before a route is activated.

Q2: Which interface must a resolver implement?
A: Resolve<T>

Q3: Where is a resolver defined in the route config?
A: Inside the resolve property of the route object.

Q4: How do components access resolved data?
A: Using ActivatedRoute.data subscription.

Q5: When is a resolver executed?
A: Before the component is loaded during navigation.

📝 MCQs

Q1. What does a resolver in Angular do?

Guards against route changes
Fetches data before route activation
Creates routes dynamically
Binds data after route loads

Q2. Which interface is used for resolvers?

OnInit
Resolve<T>
ActivatedRoute
CanActivate

Q3. Where is a resolver defined?

In constructor
Inside template
In the resolve property of route
In ngOnInit

Q4. When does Angular call a resolver?

After route loads
During component init
Before route activation
On page refresh only

Q5. How to access resolved data?

Using @Input
Using resolve()
Using ActivatedRoute.data
Using HttpClient

Q6. Which method must be implemented in a resolver?

transform()
activate()
resolve()
guard()

Q7. What does the resolve() method return?

Boolean
Promise only
Observable or data
Route definition

Q8. What happens if resolver fails?

Component still loads
Nothing
Navigation can be canceled
Route is ignored

Q9. Can you use multiple resolvers in one route?

No
Yes
Only one per module
Only with guards

Q10. Where are resolvers typically provided?

In component
In HTML
In root or module
In route params

💡 Bonus Insight

Resolvers can help avoid flickering UIs and improve performance by ensuring data is ready before component instantiation. They work well in tandem with guards for secure and efficient routing.

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What Are Resolvers in Angular Routing