└── README.md /README.md: -------------------------------------------------------------------------------- 1 | # 100 Core Angular Interview Questions in 2025 2 | 3 |
4 |

5 | 6 | web-and-mobile-development 7 | 8 |

9 | 10 | #### You can also find all 100 answers here 👉 [Devinterview.io - Angular](https://devinterview.io/questions/web-and-mobile-development/angular-interview-questions) 11 | 12 |
13 | 14 | ## 1. What is _Angular_ and what are its key features? 15 | 16 | **Angular** is a robust, structural, **TypeScript-based** open-source front-end web application platform. It is especially well-suited for creating **Single Page Applications** (SPAs) and maintains a rich ecosystem of libraries, extensions, and tools. 17 | 18 | ### Core Features 19 | 20 | - **Modularity**: Through **NG Modules**, different parts of an Angular application can be structured and managed as distinct and cohesive units. 21 | 22 | - **Component-based Architecture**: Angular is **built around components**, fostering a modular, reusable, and testable design. 23 | 24 | - **Directives**: These markers on a DOM element instruct Angular to attach a particular kind of behavior to that element or even transform the element and its children. 25 | 26 | - **Data Binding**: Angular offers several types of data binding, enabling live management of data across the model, view, and components. 27 | 28 | - **Dependency Injection (DI)**: Angular has its own DI framework, which makes it possible to get services and share data across components. 29 | 30 | - **Templates**: Enhanced HTML templates in Angular lead to seamless incorporation of specialized constructs, like directives and data binding. 31 | 32 | - **Model-driven Forms**: Angular approaches forms with modularity through custom NG modules, while also employing two-way data binding. 33 | 34 | - **Template-driven Forms**: Here, the emphasis is on minimizing the need for explicit model management on the component through directives that can observe and manage forms. 35 | 36 | - **Inter-component Communications**: Angular supports several methods for components to interact and share data, including Input, Output, ViewChild, and services based mechanisms. 37 | 38 | - **Asynchronous Operations**: Built on top of Promises, Observables offer a more flexible and powerful way to deal with sequences of events, HTTP responses, and more. 39 | 40 | - **Directives**: Angular comes with several built-in directives for management of the DOM, such as `*ngIf`, `*ngFor`, and `*ngSwitch`. 41 | - **Advanced Routing**: Angular's powerful Router employs configurable routes, location services, and guards to navigate between views seamlessly. 42 | 43 | - **Provisioning**: The DI system in Angular centralizes the management of instances of services, ensuring singletons where necessary and other strategies based on the provider settings. 44 |
45 | 46 | ## 2. Explain _data-binding_ in Angular. What are the different types? 47 | 48 | **Data Binding** in Angular represents the communication between a **component** and the **DOM**. It ensures that the model and view are synchronized. Angular offers different types of data binding to cater to varied application requirements. 49 | 50 | ### Key Types of Data Binding 51 | 52 | 1. **One-Way Data Binding** 53 | - Data flows in a single direction from the **component** to the **DOM** or **vice versa**. 54 | - **Example**: Interpolation, Property Binding, Event Binding. 55 | 56 | 2. **Two-Way Data Binding** 57 | - Enables bi-directional data flow, offering real-time synchronization between the **component** and the **DOM**. 58 | - **Syntax**: Utilize `[(ngModel)]` or `[( )]` for **attribute** binding. `[(ngModel)]` is specifically designed for forms, necessitating the `FormsModule` for integration. 59 | 60 | 3. **One-Way from Source** 61 | **One-way** binding ensures that changes in the source will dictate whether the destination in the DOM is updated or not. 62 | 63 | - **Example**: Style or Attribute Binding. 64 | 65 | 4. **One-Time Binding** 66 | 67 | One-time binding involves a single transfer of data from source to target without ongoing synchronization. This is useful when the data doesn't change and you don't want the overhead of continuous checks. 68 | 69 | - **For Efficiency**: Use in scenarios with data that's static or changes infrequently. 70 | 71 | 72 | ### Best Practices for Data Binding 73 | 74 | - **Simplicity Breeds Clarity**: Limit two-way and one-time bindings to clear and justified contexts. 75 | 76 | - **Temporal Precision**: Use one-time bindings when data is static. 77 | 78 | - **Systematic Updates**: Employ strategies that maintain data integrity, such as `ChangeDetectionStrategy.OnPush`, and manually triggering `ChangeDetectorRef`. 79 | 80 | - **Performance Considerations**: Understand the potential performance implications of each data binding type and use them judiciously. 81 | 82 | ### Code Example: Types of Data Binding 83 | 84 | Here is the TypeScript code: 85 | 86 | ```typescript 87 | import { Component, ChangeDetectionStrategy, ChangeDetectorRef } from '@angular/core'; 88 | 89 | @Component({ 90 | selector: 'app-root', 91 | templateUrl: './app.component.html', 92 | changeDetection: ChangeDetectionStrategy.OnPush 93 | }) 94 | export class AppComponent { 95 | public message = "Initial message"; 96 | public btnContent = "Disable"; 97 | 98 | constructor(private cdr: ChangeDetectorRef) {} 99 | 100 | updateMessage() { 101 | this.message = new Date().toTimeString(); 102 | // Manually trigger Change Detection 103 | this.cdr.detectChanges(); 104 | } 105 | 106 | toggleBtn() { 107 | this.btnContent = this.btnContent === "Disable" ? "Enable" : "Disable"; 108 | } 109 | } 110 | ``` 111 |
112 | 113 | ## 3. Describe the _Angular application architecture_. 114 | 115 | The **Angular application architecture** adheres to the principles of modularity, components, and a unidirectional data flow. It includes **four foundational elements**: modules, components, services, and the routing module. 116 | 117 | ### Key Concepts 118 | 119 | - **Modules**: Serve as containers for a cohesive set of functionalities within an app. Angular uses dependency injection to manage the modules and their components. 120 | 121 | - **Components**: Represent the building blocks of the app. Each component is a small, self-contained unit, responsible for both UI and logic. 122 | 123 | - **Services**: Provide specialized functionality throughout the app. They are singletons and can be injected into any component or another service. 124 | 125 | - **Routing Module**: Manages navigation between application views. 126 | 127 | ### Data Flow Mechanism: One-way Binding 128 | 129 | - **@Input()**: Data flows into a component from its parent using this decorator. 130 | - **@Output()**: Components emit events to notify the parent through this decorator. 131 | 132 | ### App Structure 133 | 134 | - **Root Module**: Starting point of an Angular app. Coordinates and configures other modules, and defines the root component. 135 | 136 | - **Feature Modules**: Unique to Angular, they group functionality and components based on the specific feature they provide. Feature modules can be eagerly or lazily loaded. 137 | 138 | ### Code Example: Root Module 139 | 140 | Here is the Angular Code: 141 | 142 | ```typescript 143 | // app.module.ts 144 | import { NgModule } from '@angular/core'; 145 | import { BrowserModule } from '@angular/platform-browser'; 146 | import { AppComponent } from './app.component'; 147 | import { HomeComponent } from './home.component'; 148 | import { ContactComponent } from './contact.component'; 149 | import { AppRoutingModule } from './app-routing.module'; 150 | 151 | @NgModule({ 152 | declarations: [ 153 | AppComponent, 154 | HomeComponent, 155 | ContactComponent 156 | ], 157 | imports: [ 158 | BrowserModule, 159 | AppRoutingModule 160 | ], 161 | providers: [], 162 | bootstrap: [AppComponent] 163 | }) 164 | export class AppModule { } 165 | ``` 166 |
167 | 168 | ## 4. What is a _component_ in Angular and how is it used? 169 | 170 | In Angular, a **component** represents a logical UI element that defines a part of the user interface. It consists of a TypeScript class that holds the component's data and logic, and an HTML file that defines the view, along with a CSS file for styling. Components can nest inside each other to form a component tree, and they often communicate with each other using **inputs**, **outputs**, **services**, and **observables**. 171 | 172 | ### Key Component Parts 173 | 174 | - **Class**: Represents the component's behavior and data using TypeScript. It may include properties, methods, lifecycles, and decorators. 175 | - **Template**: Specifies the UI structure using HTML, often integrated with Angular directives and data binding. 176 | - **Styles**: Uses CSS to define the component's visual appearance. Can be scope-limited to the component. 177 | 178 | ### Core Concepts 179 | 180 | - **Component Tree**: Refers to the hierarchical relationship among components where a top-level component can have child components, and these children can further have their own children, creating a tree structure. 181 | - **Data Binding**: Establishes a connection between the component's data (the model) and the template, enabling synchronization. 182 | 183 | ### Unique Features 184 | 185 | #### Component-Scoped Styles 186 | 187 | Angular lets you define styles specific to a component, ensuring they don't affect other parts of the application. This scoping is achieved using CSS Encapsulation techniques such as emulation of Shadow DOM or generated, unique attribute selectors. 188 | 189 | #### Modular Design for UI Elements 190 | 191 | Components offer a modular way to design and develop user interface elements. Each component is self-contained, focused on a single responsibility, and can encapsulate its HTML, styles, and related logic. 192 | 193 | #### Reusability: 194 | 195 | Via elements like `@Input()` and `@Output()`, a component's functionality and data can be exposed, making its task more adaptable, reusable, and modular within the application. 196 | 197 | #### Clear Separation of Concerns: 198 | 199 | The segregation of a component's class (handling of logic and data) from its template (dealing with the presentation) ensures a divide between the application's UI and its underlying functional structure. 200 | 201 | ### Code Example: Basic Angular Component 202 | 203 | Here is the Angular component: 204 | 205 | ```typescript 206 | import { Component } from '@angular/core'; 207 | 208 | @Component({ // The @Component Decorator 209 | selector: 'app-hello', // CSS Selector - This component can be used as in HTML 210 | template: '

Hello, {{name}}!

', // The component's template 211 | styles: ['h2 { color: green; }'] // The component's styles - using a simple inline array 212 | }) 213 | export class HelloComponent { // The component's class, named HelloComponent 214 | name = 'User'; // A public property, accessible in the template 215 | 216 | // A method that can be called from the template 217 | setName(newName: string): void { 218 | this.name = newName; 219 | } 220 | 221 | constructor() { 222 | // Constructor logic, executed when an instance of the component is created. 223 | } 224 | } 225 | ``` 226 |
227 | 228 | ## 5. What are _directives_ in Angular and can you name a few commonly used ones? 229 | 230 | **Directives** in Angular are powerful tools that allow you to extend HTML vocabulary. They attach special behaviors to elements or **transform** DOM structure and View elements in several unique ways. 231 | 232 | ### Types of Directives 233 | 234 | 1. **Component Directives**: These are the most common directives. They define components responsible for handling views and logic. 235 | 236 | 2. **Attribute Directives**: These modify the behavior and appearance of DOM elements. They are essentially markers on a DOM element that invoke some JavaScript logic. 237 | 238 | 3. **Structural Directives**: These are a special type of directives that modify the DOM layout by adding, removing, or manipulating elements. 239 | 240 | ### Commonly Used Directives 241 | 242 | 1. **ngIf**: This Angular structural directive conditionally adds or **removes** elements from the DOM tree. 243 | 244 | 2. **ngFor**: Useful for iterating through `arrays` and iterating over `object` properties. It dynamically renders elements based on the **collection** it's given. 245 | 246 | 3. **ngStyle**: This attribute directive allows for inline CSS styling based on template expressions. 247 | 248 | 4. **ngClass**: This attribute directive dynamically adds and **removes** classes from elements based on template expressions. 249 | 250 | 5. **ngModel**: This directive establishes **two-way data binding** between input elements and component data. It's commonly used in forms. 251 | 252 | 6. **ngSwitch**: This set of **structural** directives is like an enhanced version of `ngIf` by providing `else` and `default` matching functionalities. 253 | 254 | ### Code Example: ngFor 255 | 256 | Here is the Angular code: 257 | 258 | ```typescript 259 | @Component({ 260 | selector: 'app-item-list', 261 | template: ` 262 | 265 | ` 266 | }) 267 | export class ItemListComponent { 268 | items: any[] = [{ name: 'Item 1' }, { name: 'Item 2' }]; 269 | } 270 | ``` 271 | 272 | In the HTML template, the `ngFor` directive iterates over the `items` array and renders an `li` element for each item. 273 | 274 | ### Why Use Directives? 275 | 276 | Directives provide a **declarative approach** to organizing your code, making it more intuitive and easier to maintain, with a clear separation of UI and application logic. 277 |
278 | 279 | ## 6. How do you create a _service_ in Angular and why would you use one? 280 | 281 | **Services** are instrumental in Angular for finer architectural design and sharing common functionality across components. 282 | 283 | Angular automatically **injects** a service when a component or another service needs it. This mechanism fosters the "Don't Repeat Yourself" (DRY) principle, leading to more modular, maintainable, and testable code. 284 | 285 | ### Service Creation 286 | 287 | You can create a service in Angular using either of these methods: 288 | 289 | 1. **CLI**: Use the Angular CLI to generate a service. 290 | ```bash 291 | ng generate service my-service 292 | ``` 293 | 294 | 2. **Manual**: Create a `.ts` file for the service and define the class. 295 | 296 | ### Using the Service 297 | 298 | 1. **Service Registration**: 299 | 300 | - **Module**: Link the service to a specific module by adding it to the `providers` array in `@NgModule`. 301 | 302 | ```typescript 303 | @NgModule({ 304 | declarations: [ 305 | MyComponent 306 | ], 307 | providers: [MyService], 308 | imports: [CommonModule] 309 | }) 310 | ``` 311 | 312 | - **Dependency Injection Tree**: Use a tree level below the root or at a component level. 313 | 314 | ```typescript 315 | @Injectable({ 316 | providedIn: 'root' 317 | }) 318 | ``` 319 | 320 | 2. **Dependency Injection**: 321 | 322 | Annotate the constructor in the component or service to be injected. 323 | 324 | ```typescript 325 | constructor(private myService: MyService) {} 326 | ``` 327 | 328 | 3. **Lifecycle Management**: Handle service lifecycle based on the specific requirements, such as persistent state management. 329 | 330 | ### Code Example: Service 331 | 332 | Here is the TypeScript code: 333 | 334 | ```typescript 335 | // service.ts 336 | @Injectable({ 337 | providedIn: 'root' 338 | }) 339 | export class MyService { 340 | private data: any; 341 | 342 | setData(data: any): void { 343 | this.data = data; 344 | } 345 | 346 | getData(): any { 347 | return this.data; 348 | } 349 | } 350 | 351 | // component.ts 352 | export class MyComponent { 353 | constructor(private myService: MyService) {} 354 | 355 | saveDataLocally(data: any): void { 356 | this.myService.setData(data); 357 | } 358 | 359 | fetchStoredData(): any { 360 | return this.myService.getData(); 361 | } 362 | } 363 | ``` 364 | 365 | In this case, the `MyService` will persist its `data` property throughout its lifetime, and any component or service can access or modify it using the defined methods. 366 |
367 | 368 | ## 7. Can you explain what _dependency injection_ is in Angular? 369 | 370 | **Dependency Injection (DI)** is a core concept in Angular, where components (or services) depend on other components. Angular handles the creation and management of these dependencies. 371 | 372 | ### Simplified Explanation 373 | 374 | DI takes three steps: 375 | 376 | 1. **Registration**: Identify the components to be injected. 377 | 2. **Resolution**: Find the appropriate dependencies. 378 | 3. **Injection**: Insert the resolved dependencies. 379 | 380 | ### Key Angular Features Linked to DI 381 | 382 | - **Modules**: Angular applications are made up of modules, each with its dependency injector. 383 | - **Providers**: Within modules, providers offer a mechanism for registering dependencies. 384 | 385 | ### Code Example: DI in Angular 386 | 387 | Here is the Angular code: 388 | 389 | ```typescript 390 | // Service definition 391 | @Injectable() 392 | export class DataService { 393 | getData() { 394 | return "Some data"; 395 | } 396 | } 397 | 398 | // Register in a module 399 | @NgModule({ 400 | providers: [DataService], 401 | // ... 402 | }) 403 | export class MyModule {} 404 | 405 | // Constructor injection in a component 406 | @Component({ 407 | // ... 408 | }) 409 | export class MyComponent { 410 | constructor(private dataService: DataService) {} 411 | 412 | ngOnInit() { 413 | console.log(this.dataService.getData()); 414 | } 415 | } 416 | ``` 417 |
418 | 419 | ## 8. What is a _module_ in Angular and what is its purpose? 420 | 421 | In Angular, a **module** is a way to group components, services, directives, and pipes. It helps in both organizing and dividing your application into smaller, more manageable and efficient **pieces**. 422 | 423 | ### Key Module Elements 424 | 425 | - **Components**: The visual and behavioral building blocks of your application. 426 | - **Directives**: Tools for modifying the DOM or containing certain behaviors. 427 | - **Services**: Reusable units of code, often central to your application's functionality. 428 | - **Pipes**: Data transformation agents, primarily used for UI purposes. 429 | 430 | ### Types of Modules 431 | 432 | - **Root Module**: The core module that serves as the entry point for your application. It's often called `AppModule`. 433 | - **Feature Module**: An optional module that's usually smaller in scope and can be **lazily loaded**. It usually targets a specific feature or a set of related features, allowing for better code organization and loading only when needed. 434 | 435 | ### Advantages of Using Modules 436 | 437 | 1. **Organization and Reusability**: Components, services, directives, and more are logically grouped, making their intent clear and their code easily accessible. They can also be shared across modules as needed. 438 | 2. **Performance and Efficiency**: Modules can be **eager-loaded** (automatically loaded with the application) or **lazily-loaded** (loaded on-demand), optimizing initial bundle size and reducing start-up time. 439 | 3. **Collaborative Development**: By defining clear boundaries between components, directives, and services, modules facilitate team collaboration and help in preventing naming conflicts or unintentional dependencies. 440 | 441 | ### Code Example: Module Structure 442 | 443 | Here is the Angular code: 444 | 445 | ```typescript 446 | // File: app.module.ts 447 | import { BrowserModule } from '@angular/platform-browser'; 448 | import { NgModule } from '@angular/core'; 449 | import { AppComponent } from './app.component'; 450 | 451 | @NgModule({ 452 | declarations: [AppComponent], // Components declared in this module 453 | imports: [BrowserModule], // Other modules this module requires 454 | providers: [], // Services provided by this module 455 | bootstrap: [AppComponent] // The root component of this module 456 | }) 457 | export class AppModule {} // The root module 458 | 459 | // File: user.module.ts (Feature Module Example) 460 | import { NgModule } from '@angular/core'; 461 | import { UserComponent } from './user.component'; 462 | 463 | @NgModule({ 464 | declarations: [UserComponent], // Components declared in this module 465 | imports: [], // Other modules this module requires 466 | providers: [], // Services provided by this module 467 | }) 468 | export class UserModule {} 469 | ``` 470 |
471 | 472 | ## 9. How do you handle _events_ in Angular? 473 | 474 | **Handling events** in Angular involves capturing and responding to user or system actions. Angular provides **declarative** and **imperative** methods to accomplish this. 475 | 476 | ### Declarative Approach 477 | 478 | Declarative methods involve writing event handlers directly in the Angular template using **event binding**. 479 | 480 | #### Syntax 481 | 482 | For event binding, Angular uses `(event)` syntax to listen for DOM events and execute an associated method in the component. 483 | 484 | **(e.g.) Click Event:** 485 | 486 | - Template: 487 | ```html 488 | 489 | ``` 490 | 491 | - Component: 492 | ```typescript 493 | onClick(event: MouseEvent): void { 494 | console.log('Button was clicked', event); 495 | } 496 | ``` 497 | 498 | **(e.g.) Input Event:** 499 | 500 | - Template: 501 | ```html 502 | 503 | ``` 504 | 505 | - Component: 506 | ```typescript 507 | onInput(event: Event): void { 508 | const inputText = (event.target as HTMLInputElement).value; 509 | console.log('Input value changed:', inputText); 510 | } 511 | ``` 512 | 513 | **Event Objects** are optionally passed to event handling functions. These objects contain specific properties based on the event type, such as `MouseEvent` for click events and `KeyboardEvent` for keyboard-related ones. 514 | 515 | ### Imperative Approach 516 | 517 | While Angular promotes a **declarative style**, it also supports an **imperative** one, where event listeners are manually added and removed through the `@ViewChild` decorator of TypeScript. 518 | 519 | #### Syntax 520 | 521 | - **HTML Template:** 522 | ```html 523 |
Target DIV
524 | ``` 525 | 526 | - **Component Class:** 527 | ```typescript 528 | @ViewChild('targetDiv') targetDiv: ElementRef; 529 | 530 | ngAfterViewInit(): void { 531 | this.targetDiv.nativeElement.addEventListener('click', this.onClick); 532 | } 533 | 534 | ngOnDestroy(): void { 535 | this.targetDiv.nativeElement.removeEventListener('click', this.onClick); 536 | } 537 | 538 | onClick(event: MouseEvent): void { 539 | console.log('Div clicked:', event); 540 | } 541 | ``` 542 | 543 | In this method, the `ngAfterViewInit` method sets up the event listener, and the `ngOnDestroy` method removes it to prevent memory leaks or unexpected behavior. 544 |
545 | 546 | ## 10. What is _two-way binding_ and how do you implement it in Angular? 547 | 548 | **Two-way binding** in Angular synchronizes data between the data model and the view in **both directions**. Any changes in the data model automatically reflect in the view and vice versa. 549 | 550 | ### Implementation in Angular 551 | 552 | Angular primarily uses two-way binding through the `[(ngModel)]` directive, leveraging the `FormsModule` or `ReactiveFormsModule`. 553 | 554 | ### Using `[(ngModel)]` and `FormsModule` 555 | 556 | 1. **Module Setup**: Import the `FormsModule` in your Angular module. 557 | ```typescript 558 | import { FormsModule } from '@angular/forms'; 559 | 560 | @NgModule({ 561 | imports: [FormsModule], 562 | // ... 563 | }) 564 | export class AppModule { } 565 | ``` 566 | 567 | 2. **Input Binding**: Use `[(ngModel)]` in the view to enable two-way binding with an input element. 568 | 569 | ```html 570 | 571 | ``` 572 | 573 | 3. **Data Model**: Define the associated property in the component. 574 | ```typescript 575 | @Component({...}) 576 | export class TwoWayBindingComponent { 577 | public name: string; 578 | } 579 | ``` 580 | 581 | This setup ensures that any changes to the input element are reflected in the `name` property and vice versa. 582 | 583 | ### Using `[(ngModel)]` with `ReactiveFormsModule` 584 | 585 | If you choose to integrate `[(ngModel)]` with `ReactiveFormsModule`, follow these steps: 586 | 587 | 1. **Module Setup**: Import the `ReactiveFormsModule` in your Angular module. 588 | - Code Example: `app.module.ts` 589 | ```typescript 590 | import { ReactiveFormsModule } from '@angular/forms'; 591 | 592 | @NgModule({ 593 | imports: [ReactiveFormsModule], 594 | // ... 595 | }) 596 | export class AppModule { } 597 | ``` 598 | 599 | 2. **FormGroup Creation**: Create an Angular `FormGroup` and associate it with the template and component. 600 | 601 | 3. **Model Binding**: Use the `formControlName` directive in the view to bind an input to a specific form control. 602 | 603 | ```html 604 |
605 | 606 |
607 | ``` 608 | 609 | ```typescript 610 | import { FormBuilder, FormGroup } from '@angular/forms'; 611 | 612 | @Component({...}) 613 | export class TwoWayBindingReactiveComponent { 614 | public myForm: FormGroup; 615 | 616 | constructor(private fb: FormBuilder) { 617 | this.myForm = this.fb.group({ 618 | name: [''] 619 | }); 620 | } 621 | } 622 | ``` 623 | 624 | This approach ensures synchronized data between the form input and the `FormControl` associated with it. 625 | 626 | ### Best Practices 627 | 628 | - **Consistent Tracking**: Whether through the `FormsModule` or `ReactiveFormsModule`, ensure consistent data tracking to avoid unexpected behavior. 629 | - **Input Element Type**: Not all elements support two-way binding. Use two-way bindings like `[(ngModel)]` with compatible input elements such as `` and `