Understanding the Differences Between Constructor and OnInit in Angular

In Angular, both the constructor and ngOnInit lifecycle hooks are used in components but serve different purposes. Both are essential for different aspects of component initialization in Angular applications. In this article, we will delve deeper into the key differences between these two important concepts.

Key Differences Between Constructor and OnInit

Angular applications utilize a well-defined lifecycle for components. The constructor and ngOnInit are two of the lifecycle hooks that serve distinct roles within this process. Understanding these differences is crucial for effective Angular component development.

The Constructor

Purpose

The constructor is a TypeScript feature designed to initialize class members. Its primary responsibility is to set up the initial state of the component before it is further processed by Angular.

When It Is Called

The constructor is called before Angular initializes the component. This means that before ngOnInit and other lifecycle hooks are invoked, the constructor executes. No inputs or initial properties have been set at this point.

Usage

You typically use the constructor for dependency injection, particularly for services, and initializing basic properties of the component. It is an ideal place to set up any variables that you need for the entire lifecycle of the component.

Example

export class MyComponent {
    constructor(private myService: MyService) {
        // Initialization logic here
    }
}

The ngOnInit Lifecycle Hook

Purpose

ngOnInit is a lifecycle hook provided by Angular. It is used for performing actions that require the component to be fully initialized, such as fetching data or setting up subscriptions.

When It Is Called

ngOnInit is called after the constructor and after Angular has set the component’s input properties. This means that when ngOnInit runs, all data-bound properties and dependencies have already been initialized, making it the perfect place for complex initialization logic.

Usage

You typically use ngOnInit for initialization logic that depends on the component’s inputs. This could involve fetching data from a service or setting up subscriptions for reactive form changes, among other things.

Example

import { Component, OnInit } from '@angular/core';
@Component({
    selector: 'app-my-component',
    templateUrl: '',
})
export class MyComponent implements OnInit {
    constructor(private myService: MyService) {}
    ngOnInit(): void {
        // Logic that requires input properties to be set
        ().subscribe(data  {
            // Handle the data
        });
    }
}

Summary

While both constructor and ngOnInit are crucial for different aspects of component initialization, they serve distinct purposes. The constructor sets up basic initialization and dependency injection, called before the component is fully initialized. On the other hand, ngOnInit is used for more complex initialization tasks that require input properties to be set, called after the constructor and after all data-bound properties initialization.

Mastering the usage of these lifecycle hooks is essential for writing clean, efficient, and easy-to-maintain Angular applications.