Advanced LWC Interview Questions

1. How does the Shadow DOM affect Lightning Web Components?

The Shadow DOM in LWC encapsulates the component's HTML, CSS, and JavaScript, ensuring styles and functionality are scoped to the component. This prevents styles from leaking out or other styles from affecting the component. LWC uses native Shadow DOM where supported and synthetic Shadow DOM where it isn’t. This isolation ensures a clean, modular structure for UI components.

2. What are the limitations of the Shadow DOM in LWC?

With the Shadow DOM, global CSS or JavaScript cannot directly modify the styles or content within a component. Another limitation is that elements inside the Shadow DOM are inaccessible from the global scope, making cross-component manipulation more complex. Additionally, native Shadow DOM is not fully supported by all browsers.

3. How do you perform unit testing for Lightning Web Components using Jest?

Jest is the recommended testing framework for LWC. You set up tests by creating a test folder in your LWC component directory and writing JavaScript test files that use Jest’s testing functions like describe(), it(), and expect(). Salesforce provides @salesforce/sfdx-lwc-jest to simplify the setup. Apex calls can be mocked using jest.mock().

4. How do you handle Apex callouts in LWC without blocking the UI?

In LWC, Apex callouts can be handled asynchronously using JavaScript Promises with either the .then() method or the async/await syntax. To ensure the UI remains responsive, you can display a spinner or loading indicator while the Apex call is in progress and hide it once the data is returned.

5. How do you create a reusable service component in LWC?

Exporting functions or classes from a standalone JavaScript file can create reusable service components in LWC. These components don’t have an HTML template, but they can perform logic like data transformations, utility functions, or API calls that can be imported and used across multiple components.

6. Explain the concept of event bubbling and composition in LWC.

Event bubbling in LWC allows events to propagate from the child element to the parent, eventually reaching the document root. By default, LWC supports bubbling but can be controlled using event properties like bubbles. Event composition allows the event to cross the boundary of the Shadow DOM when the composed property is set to true.

7. How do you optimize the performance of a complex LWC application?

Performance in LWC can be optimized by:

• Using Lazy Loading to load components or data only when needed.
• Leveraging Lightning Data Service (LDS) to minimize Apex calls and use cached data.
• Reducing DOM manipulation by updating only necessary elements.
• Debouncing event handlers to prevent unnecessary executions during user input.

8. How does Lightning Message Service (LMS) work, and how do you use it for inter-component communication?

LMS enables communication between LWC components, Aura components, and even Visualforce pages. Components can publish and subscribe to messages on a message channel. This event-driven mechanism decouples components from each other, making the architecture more modular. LMS is essential when components are not in a parent-child relationship but must share data.

9. Explain the importance of the @api, @track, and @wire decorators in LWC.

• @api: Exposes public properties and methods for the component to communicate with other components (usually parent-to-child communication).
• @track: Makes an object or array property reactive, ensuring UI updates when its internal state changes.
• @wire: Connects the component to Salesforce data (via Apex or LDS), providing automatic reactivity when the data changes.

10. How do you handle cross-component data communication using Pub/Sub in LWC?

Pub/Sub (publish-subscribe) is a pattern used to facilitate communication between components that are not directly related. In LWC, this can be achieved using custom event handlers or Lightning Message Service (LMS). A message is published by one component and subscribed to by another without needing a direct reference to each other.

11. What are dynamic imports in LWC, and how do they enhance performance?

Dynamic imports in LWC allow you to load JavaScript modules asynchronously, loading them only when needed. This technique enhances performance by reducing a component's initial load time. Dynamic imports are performed using JavaScript's import() function and are especially useful for large or rarely-used components.

12. How do you debug Lightning Web Components effectively in production?

In production, effective debugging of LWC components involves:

• Using console logging strategically to capture key information.
• Leveraging Salesforce’s built-in logging mechanisms such as Debug Logs or the Apex Debugger.
• Using browser Developer Tools to inspect the DOM, network requests, and component behavior.
• Creating detailed error messages using try-catch blocks.

13. How do you handle file uploads in LWC?

File uploads in LWC are handled using the lightning-file-upload component. This base component allows uploading files to Salesforce by specifying the record ID where the files will be linked. The component supports multiple file uploads, and you can use Apex to customize the upload behavior further or handle validation.

14. How do you implement custom validation in lightning-input?

You can implement custom validation in lightning-input using the setCustomValidity() method. This method sets a custom error message if the input doesn’t meet your conditions. The reportValidity() method is then called to display the error message and prevent form submission.

15. How do you handle bulk record processing in LWC?

Bulk record processing in LWC is managed by:

• Using pagination to handle large datasets efficiently.
• Implementing infinite scrolling or dynamic loading techniques.
• Using SOQL OFFSET or LIMIT in server-side queries to fetch data in smaller chunks.
• Using Apex for batch processing of records and retrieving results asynchronously in LWC.

16. What are the differences between lightning-record-edit-form and lightning-record-form?

• lightning-record-form: This form provides a quick way to create, edit, or display records without writing custom code. It includes all the necessary fields for a record.
• lightning-record-edit-form: Offers more granular control, allowing you to create custom forms for editing records. You can define each field individually and manage events like onload and onsuccess.

17. What is getRecord in the lightning/uiRecordApi module, and how is it used?

The getRecord function is part of the lightning/uiRecordApi module. It allows you to retrieve records using LDS without writing Apex. You specify the record ID, object API name, and fields you want to retrieve. This function is reactive, automatically updating when the record changes in Salesforce.

18. What is lightning-dynamic-form, and when would you use it?

The lightning-dynamic-form component allows developers to generate forms for Salesforce records declaratively, adapting dynamically to field-level security and layout changes. It simplifies the creation of record pages without hard-coding individual fields, allowing you to respond more easily to changes in object definitions.

19. How do you ensure security in LWC components?

Security in LWC components is enforced using Locker Service, which isolates components from each other and ensures they only access authorized resources. Additional security best practices include:

• Respecting CRUD and FLS (Field Level Security) using the lightning/uiRecordApi module or server-side checks.
• Avoiding direct DOM manipulation to prevent cross-site scripting (XSS) attacks.
• Using Apex controllers with proper security annotations.

20. How do you integrate LWC with third-party APIs?

Integration with third-party APIs in LWC is achieved by making HTTP requests using the fetch API in JavaScript. You can call external REST services from the client side. However, cross-origin resource sharing (CORS) must be handled appropriately, and sometimes Apex is used as a proxy to relay the data between the client and the third-party service.