Here’s a set of common Lightning Web Components (LWC) interview questions along with their answers:
1. What is LWC and how is it different from Aura Components?
Answer: Lightning Web Components (LWC) is a lightweight framework built on modern web standards, designed to simplify building web components on the Salesforce Lightning Platform. It is different from Aura Components in that it is built with native web standards and can coexist with Aura, offering better performance and easier interoperability with modern JavaScript.
2. Can you explain the component lifecycle in LWC?
Answer: LWC components have a lifecycle managed by the framework, including phases such as creation, rendering, updating, and destruction. Key lifecycle hooks include:
constructor(): Invoked when a component instance is created.connectedCallback(): Called when the component is connected to the DOM.render(): Determines the component’s template.disconnectedCallback(): Invoked when the component is removed from the DOM.errorCallback(): Called when an error is thrown during the component’s lifecycle.
3. What is the Shadow DOM in LWC?
Answer: The Shadow DOM is a key feature of web components, including LWC, that provides encapsulation for JavaScript, styles, and templating. It ensures that a component’s styles do not leak out and that styles from outside do not affect the internals of the component.
4. How do you pass data between components in LWC?
Answer: Data can be passed between components using properties (for parent to child) and custom events (for child to parent). Properties are reactive if decorated with @api, making them reflect changes to parent components. Custom events allow children to send data to parent components through event dispatching.
5. What are decorators in LWC?
Answer: Decorators are special declarations prefixed with @, used to modify properties or functions. Key decorators in LWC include:
@api: Marks a field as public, which can be set by the parent component.@track: Used in earlier versions to make fields reactive. With modern LWC, all fields are reactive by default.@wire: Provides reactive data binding to Salesforce data and services.
6. Explain the use of the @wire decorator.
Answer: The @wire decorator is used to wire a property or method to a data source, typically Salesforce Apex methods or standard Salesforce APIs (like getRecord). The data is provided reactively to the component, and updates to the source are reflected in the component’s property or method.
7. How can you handle events in LWC?
Answer: Events in LWC can be handled using event listeners. You can add an event listener in the component’s template using the on[eventname] syntax, like onclick or onchange. Custom events can be handled similarly but need to be dispatched by child components using this.dispatchEvent(new CustomEvent('eventname')).
These questions cover fundamental concepts and are likely to come up in various forms during an interview focusing on LWC development.
Sure, let’s expand the list of interview questions for Lightning Web Components (LWC) with additional questions and code examples to demonstrate deeper knowledge and practical application:
8. How do you use slots in LWC?
Answer:
Slots are a feature of Web Components that allow you to define placeholder elements in a component’s template that can be filled with any markup passed by the parent component. They provide a way to compose complex components with customizable layouts.
Example:
<!-- childComponent.html -->
<template>
<div>
<slot name="header">Default header content</slot>
<p>Some more content...</p>
<slot name="footer">Default footer content</slot>
</div>
</template>
<!-- parentComponent.html -->
<template>
<c-child-component>
<h1 slot="header">Custom Header</h1>
<div slot="footer">Custom Footer Content</div>
</c-child-component>
</template>
9. What are reactive properties in LWC and how do you use them?
Answer:
Reactive properties are properties that cause the component to re-render when their values change. In LWC, any field declared with @track in earlier versions or simply as class fields in the current version are reactive by default.
Example:
import { LightningElement, track } from 'lwc';
export default class ReactiveExample extends LightningElement {
@track counter = 0;
incrementCounter() {
this.counter++;
}
}
<!-- reactiveExample.html -->
<template>
<button onclick={incrementCounter}>Increment</button>
<p>Counter: {counter}</p>
</template>
10. How can you call an Apex method from an LWC?
Answer:
Apex methods can be called from LWC using the @wire decorator for reactive data fetching, or by using import and calling the method imperatively.
Example for imperative call:
import { LightningElement } from 'lwc';
import getAccountList from '@salesforce/apex/AccountController.getAccountList';
export default class ApexCallExample extends LightningElement {
accounts;
error;
connectedCallback() {
getAccountList()
.then(result => {
this.accounts = result;
})
.catch(error => {
this.error = error;
});
}
}
11. Describe how conditional rendering works in LWC.
Answer:
Conditional rendering in LWC is achieved by using JavaScript expressions in the template to conditionally include or exclude portions of the DOM.
Example:
<!-- conditionalRendering.html -->
<template>
<template if:true={isVisible}>
<p>Visible Content</p>
</template>
<template if:false={isVisible}>
<p>Hidden Content</p>
</template>
</template>
import { LightningElement } from 'lwc';
export default class ConditionalRendering extends LightningElement {
isVisible = true;
toggleVisibility() {
this.isVisible = !this.isVisible;
}
}
12. Explain the use of static resources in LWC.
Answer:
Static resources allow you to upload images, style sheets, JavaScript, and other files that you can reference in your LWC. They are useful for assets that are reused across multiple components or applications.
Example:
<!-- staticResourceComponent.html -->
<template>
<img src={logoUrl} alt="Company Logo">
</template>
import { LightningElement } from 'lwc';
import LOGO from '@salesforce/resourceUrl/companyLogo';
export default class StaticResourceComponent extends LightningElement {
logoUrl = LOGO;
}
Certainly! Let’s add more in-depth Lightning Web Components (LWC) interview questions that cover additional advanced topics, suitable for evaluating a candidate’s breadth and depth of understanding:
13. How do you manage state between multiple LWC components?
Answer:
State management between components in LWC can be achieved through a combination of event handling, reactive properties, and shared JavaScript modules (for more global state). Components can communicate using events for direct parent-child or sibling communication, and shared modules can act as simple stores or more complex state management utilities.
Example for shared module:
// stateStore.js
let count = 0;
const callbacks = [];
const registerCallback = (callback) => {
callbacks.push(callback);
};
const incrementCount = () => {
count++;
callbacks.forEach(callback => callback(count));
};
export { registerCallback, incrementCount };
// componentUsingState.js
import { LightningElement } from 'lwc';
import { registerCallback, incrementCount } from 'c/stateStore';
export default class ComponentUsingState extends LightningElement {
count = 0;
connectedCallback() {
registerCallback((newCount) => {
this.count = newCount;
});
}
increment() {
incrementCount();
}
}
14. Explain the different types of decorators in LWC and their uses.
Answer:
In LWC, decorators are special functions that add functionality to properties or methods. The primary decorators are:
@api: Exposes public properties and methods, making them accessible to other components.@track: Used in earlier versions to track private property changes and re-render components when those properties change. In current versions, all properties are reactive by default.@wire: Connects a component to a data source, such as an Apex method or Salesforce data service, and provides reactive data.
Example using @wire:
import { LightningElement, wire } from 'lwc';
import { getRecord } from 'lightning/uiRecordApi';
export default class WireDecoratorExample extends LightningElement {
@wire(getRecord, { recordId: '001xx000003NGGqAAO', fields: ['Account.Name'] })
account;
}
15. How can you handle errors in LWC?
Answer:
Error handling in LWC can be done in several ways, including try-catch blocks within JavaScript methods, error handling in lifecycle hooks, and using the errorCallback lifecycle hook to handle errors during rendering or in the lifecycle.
Example using errorCallback:
import { LightningElement } from 'lwc';
export default class ErrorHandlingComponent extends LightningElement {
errorCallback(error, stack) {
console.log(error, stack);
}
}
16. What are best practices for unit testing LWC components?
Answer:
Best practices for unit testing LWC components include:
- Testing one unit of code (a single component) at a time.
- Mocking dependent modules and Apex calls to ensure the tests are isolated.
- Using Jest test framework provided by Salesforce to simulate user interaction, check the DOM output, and validate component public methods and properties.
Example using Jest:
import { createElement } from 'lwc';
import MyComponent from 'c/myComponent';
import { registerApexTestWireAdapter } from '@salesforce/sfdx-lwc-jest';
describe('c-my-component', () => {
afterEach(() => {
while (document.body.firstChild) {
document.body.removeChild(document.body.firstChild);
}
});
it('renders greeting message', () => {
const element = createElement('c-my-component', {
is: MyComponent
});
document.body.appendChild(element);
const div = element.shadowRoot.querySelector('div');
expect(div.textContent).toBe('Hello, World!');
});
});
These questions and examples provide a comprehensive overview of handling common and advanced scenarios in LWC development, which should be very useful for deep-diving into a candidate’s expertise during interviews.
SFDC Arjuna 