Angular Academy · Lesson

ngOnInit and Initialization

Run setup logic when a component starts.

ngOnInit and Initialization is a free Angular Academy lesson on CoddyKit — lesson 1 of 4. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the Angular Academy learning path, one of 4 lessons in the course, and your progress syncs across the web and the CoddyKit app.

What Are Lifecycle Hooks?

Angular components have a lifecycle managed by the framework: creation, change detection, rendering, and destruction. Lifecycle hooks let you run code at specific moments.

Introducing OnInit

The OnInit hook runs once, after Angular has set the component inputs and rendered the initial view. It is the standard place for initialization logic.

Implementing OnInit

Implement the OnInit interface and add the ngOnInit method. Angular calls it automatically.

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

@Component({ selector: "app-demo", template: "" })
export class DemoComponent implements OnInit {
  ngOnInit(): void {
    console.log("Component initialized");
  }
}

Constructor vs ngOnInit

The constructor runs first, before inputs are set, and should only do light dependency injection. Use ngOnInit for real setup once inputs are available.

constructor(private http: HttpClient) {
  // inputs NOT ready yet
}

ngOnInit() {
  // inputs ARE ready here
}

Inputs Are Ready in ngOnInit

By the time ngOnInit runs, all @Input properties have their initial values bound from the parent.

@Input() userId!: string;

ngOnInit() {
  console.log(this.userId); // available
}

Fetching Data in ngOnInit

A common pattern is to load data from a service in ngOnInit, since you have access to inputs and dependencies.

ngOnInit() {
  this.userService.getUser(this.userId)
    .subscribe(u => this.user = u);
}

ngOnInit Runs Once

Unlike change detection hooks, ngOnInit runs exactly once during the component lifetime, right after the first ngOnChanges.

Order Relative to ngOnChanges

If the component has inputs, ngOnChanges fires first with the initial values, then ngOnInit runs. Without inputs, ngOnInit still runs.

// sequence with inputs:
// 1. constructor
// 2. ngOnChanges (first)
// 3. ngOnInit

Initializing Forms

Building reactive forms is a classic ngOnInit task because you may need injected services to set up the form.

form!: FormGroup;

ngOnInit() {
  this.form = this.fb.group({
    name: [""],
    email: [""]
  });
}

Keep Constructors Light

Avoid heavy work in the constructor. It runs before Angular wires inputs, and side effects there make components harder to test and predict.

Async Initialization

You can call async logic from ngOnInit, but the method itself returns void; subscribe or use async patterns inside it.

ngOnInit() {
  this.loadAsync();
}

private async loadAsync() {
  this.data = await firstValueFrom(this.api.get());
}

Quick Check

Test your understanding of OnInit.

Recap

The OnInit hook with ngOnInit runs once after inputs are bound and is ideal for data fetching and form setup. Keep the constructor light and reserve initialization for ngOnInit.

Frequently asked questions

Is the “ngOnInit and Initialization” lesson free?

Yes — the full text of “ngOnInit and Initialization” is free to read here on the web, and the Angular Academy course includes 4 lessons in total. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the Angular Academy course, upgrade to CoddyKit PRO.

What will I learn in “ngOnInit and Initialization”?

Run setup logic when a component starts. You practise Angular Academy with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.

Do I need any experience to start Angular Academy?

No prior experience is required. Angular Academy on CoddyKit is structured for beginners through advanced learners; this is — lesson 1 of 4, so you can start here or from the beginning and move at your own pace.

How long does the “ngOnInit and Initialization” lesson take?

Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.

Can I write and run code in this Angular Academy lesson?

Yes. Every Angular Academy lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.

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