What is ngOnInit in Angular and Its Purpose

Introduction

When creating an Angular application, there are diverse factors that an individual should learn about the relationship between various components and how they start. Out of these lifecycle hooks, the ngOnInit in Angular is one of the crucial in its lifecycle. The method basically assists developers to gain confidence that their components will work optimally once set up. 

In this blog, we’re going to explore what exactly it is, how it functions, and why it’s important for your Angular applications. It will also include the steps that you should follow to create an Angular application, as well as some FAQs about the it and related features.

What is ngOnInit?

It is a lifecycle hook provided by Angular which gets called only once during a lifecycle of a component. This method is invoked right after the component construction, but before the view is rendered. It is mainly utilized for calling the component and setting properties and/or getting data from a server. The most commonly used method is it which is a great place to put any component setup that is needed before the component is displayed to the user.

How Does ngOnInit Work?

Angular follows certain steps when it creates a new component and these are as follows. First, it creates the component and when creating the component the constructor is called. Once the constructor is done, the Angular sets the component’s properties that are bound to the data. Once this initialization is done, this method is called by Angular. It is especially effective to use in the cases when the function is tied to the component’s presence, for instance, to get data or set the parameters.

For instance, if you have to call an API and fetch data when the component is being initialized then you can do it within this method. This makes it possible to have the required data when the component is rendered to the user thereby enhancing the user experience.

For the ultimate guide to Angular and its lifecycle hooks such as ngOnInit, you can take this Angular Course to boost your knowledge and expertise in Angular development.

Purpose of ngOnInit()

Initialization of Component

The main reason for using its function is to check that the initialization check for the parent component is done. On the other hand, Angular does not start the initialization process for the component’s input properties until it waits for them, and that is where it comes into play. This makes it possible to have the component prepare the right data at a time when it is expected to be shown. What makes it in angular useful is that it allows you to control when your component is ready to provide its service, so that it won’t display errors or incomplete information to the user.

Access to Input Properties

In this lifecycle method, all input properties that are bound to the component are available in it. These can be used to set other values or make some kind of calculation that depends on the given properties. This way the component is completely aware of the information that is to be rendered before being handled by it. This allows you to have a way of modifying your component according to the data being input to it, which results in producing applications that can change their behavior depending on the user interaction or changes in data.

Calling Services

If your component depends on some data from the server or any other service, then it is good to call these services in the ngOnInit() lifecycle hook. This lifecycle hook is well suited for making HTTP calls or to setup any services or data before the component actually renders something. This will ensure that all the information that may be needed when rendering the component is available before hand. This way, the component does not give incorrect and sometimes even outdated information to the user and makes the interaction more fluent.

Setup of Observables

If your component is based on the observables or other data sources, it is good to make subscriptions in this method. This is because you use observables in it, and thus data will be available when the component is rendering. This is especially the case where the data being used is real-time or when updates occur at any time. Thus, you make your component sensitive to the changes, and it will present the user with the most recent data as soon as possible.

DOM Manipulation

Despite the fact that Angular does not allow the direct manipulation of DOM, there are cases when it can be done. This is why it in angular is a suitable place for such actions to take place. In this particular stage of the component, the component checks whether the DOM is ready to be used. If you require to modify the elements based on the user’s input or if you want to manipulate the DOM for some specific effects, then it is better to do this in this method only. This way, you will not interfere with Angular’s rendering process, and your manipulations will be performed at the right time.

Steps for Installing & Configuring the Angular Application

To start using it in angular, you first need to install and configure an Angular application. 

Follow these simple steps:

Install Angular CLI: Begin by installing the Angular CLI. Open your terminal or command prompt and type:
bash

npm install -g @angular/cli

This will install Angular globally on your system.

Create a New Angular Application: Next, create a new Angular project using the following command:
ng new appname

Replace appname with the name of your project.

Navigate to Your Project: After the project is created, navigate to your project folder:

cd appname

Serve the Application: Start your Angular application by typing:
ng serve

This will start the application, and you can view it in your web browser.

Add it to a Component: To see it in action, create a component and add the following code:

import { Component, OnInit } from "@angular/core"; 

@Component({ 
    selector: "my-app", 
    template: 
        "<div><h1>{{data}}</h1><h1>{{subtitle}}</h1></div>", 
    styles: [`div { 
        color: green; 
    }`], 
}) 
export class AppComponent implements OnInit { 
    data: string; 
    subtitle: string; 

    constructor() { 
        this.data = "Welcome to Angular"; 
    } 

    ngOnInit() { 
        this.subtitle = "from ngOnInit"; 
    } 
}

This code sets the data and subtitle properties of your component during initialization.

Conclusion

It is used to perform tasks required after the input properties have been set and is key in the component’s lifecycle in Angular. This form is used to initiate services, retrieve data, and manage components for rendering and that makes it a crucial asset in the hands of developers towards the development of good and streamlined applications.

FAQs

What is the difference between constructor and ngOnInit?
The constructor is executed when the Angular creates the component, the ngOnInit() is called after the input properties of the component are bound.

Why do we use OnInit?
OnInit is used to perform initialization tasks after Angular has fully initialized the component.

What is the use of ng init in Angular?
ng init was used to initialize projects but is now deprecated, replaced by ng new and ng update commands.

What is the use of @Input in Angular?
@Input is used by a parent component to send data to the child component thus giving the parent control over the child.