Introduction
The constructor is the first lifecycle hook invoked when an LWC component is created.
It acts as the starting point of the component’s lifecycle and plays a crucial role in initializing the component.
Constructor is a place – where you initialize your component’s state and assign default values to its properties.
Here’s a breakdown of its role:
Role and Properties of Constructor:
1. First Step in Lifecycle Hook
- Constructor is the first step in the lifecycle hook. It is called when the component is created.
- This lifecycle hook flows from Parent to Child component
2. Initialization of State
- Constructor is primarily used to set up a LWC component’s initial state.
- It is a place where you can define default values for variables and properties.
constructor() {
super(); // Always call super() first
this.greeting = 'Hello, World!';
}
- In the above example, the greeting variable is initialized with a default value before the component is rendered.
3. No Access to DOM
- Since the constructor runs before the component is connected to the DOM, you cannot interact with any DOM elements or use template queries (this.template.querySelector).
- Important Note: Attempting to access DOM elements here will result in an error because the component has not yet been rendered.
4. Call Super()
The constructor must call super() as the first statement. This is necessary because LWC components extend the base class LightningElement.
Failing to call super() will throw an error.
constructor() {
// Required to call the parent class constructor
super();
console.log('Constructor called');
}
5. Not Ideal for Heavy Logic
The constructor is meant for lightweight initialization tasks. Avoid placing heavy logic, such as data fetching, here. These tasks are better suited for the connectedCallback hook.
6. Can Be Used for Dependency Injection
You can use the constructor to inject dependencies or perform one-time configurations that don’t rely on the DOM.
When to use constructor?
Constructor is used to:
- To initialize component-level variables or default states.
- To set up basic configurations.
- To perform lightweight, synchronous tasks.
What you should not do in constructor?
- Should not access DOM elements from constructor.
- Should not fetch data or perform asynchronous tasks from constructor.
Explore the post below to learn about the five different lifecycle hooks in LWC.
Simple Example - use of constructor()
import { LightningElement } from 'lwc';
export default class ConstructorDemo extends LightningElement {
greeting;
constructor() {
super();
console.log('Constructor executed');
this.greeting = 'Welcome to LWC!';
}
}
Key Takeaways
- The constructor is the first hook in the LWC lifecycle, ideal for initial setup.
- It initializes properties and prepares the component for the next stages of the lifecycle.
- Avoid interacting with the DOM or performing heavy tasks in the constructor.
This makes the constructor a foundational piece for setting up your LWC component correctly and efficiently.
Checkout below posts to explore other Lifecycle hooks in LWC in detail
