DevDose: Understanding Angular’s ngOnInit in the Component Lifecycle

Introduction

Angular’s component lifecycle is a cornerstone of building robust applications, and understanding hooks like ngOnInit is critical for predictable behavior. This post dives into ngOnInit’s role, inspired by a recent project where lifecycle misuse led to subtle bugs.

Real-World Example

In a dashboard application, I encountered a component fetching data in its constructor, causing inconsistent UI updates. The constructor, meant for dependency injection, isn’t ideal for initialization logic. Moving the logic to ngOnInit resolved the issue, ensuring the component was fully instantiated before executing setup code.

import { Component, OnInit } from '@angular/core';
import { DataService } from './data.service';

@Component({
  selector: 'app-dashboard',
  templateUrl: './dashboard.component.html'
})
export class DashboardComponent implements OnInit {
  constructor(private dataService: DataService) {}

  ngOnInit(): void {
    this.displayData         toSignal(this.dataService.fetchData());
  }
}

Insight and Reasoning

ngOnInit is a lifecycle hook called by Angular after the constructor and initial property bindings are set, but before the component’s view is fully rendered. It’s the right place for initialization tasks like fetching data or setting up subscriptions because it ensures the component’s dependencies and inputs are ready.

Using the constructor for such logic risks premature execution, especially in complex components with asynchronous dependencies. In the dashboard case, ngOnInit ensured data fetching happened at a predictable point, avoiding race conditions and improving testability.

Conclusion

Mastering ngOnInit means knowing when and why to use it: post-construction, pre-rendering, for safe initialization. This small shift in approach can prevent subtle bugs and make your Angular components more reliable.