What are Angular's component lifecycle hooks?

Question

Accepted Answer

**Lifecycle hooks** are methods Angular calls at specific moments in a component's life — creation, input changes, rendering, and destruction. You implement them by adding interface methods to your component class.

## The main hooks in order

```ts
import { Component, OnInit, OnDestroy, OnChanges, SimpleChanges } from "@angular/core";

@Component({ ... })
export class MyComponent implements OnInit, OnDestroy, OnChanges {
  ngOnChanges(changes: SimpleChanges) {
    // called when an @Input value changes (before ngOnInit the first time)
  }
  ngOnInit() {
    // called ONCE after the first ngOnChanges — do initialization, fetch data
  }
  ngOnDestroy() {
    // called just before the component is removed — CLEAN UP here
  }
}
```

## The full sequence

```text
constructor          → DI happens; inputs NOT yet available
ngOnChanges          → when @Input values are set/changed
ngOnInit             → once, after first ngOnChanges ★ (init, fetch data)
ngDoCheck            → every change detection run (custom change detection)
ngAfterContentInit   → projected content (<ng-content>) initialized
ngAfterViewInit      → the component's view + child views ready ★ (access @ViewChild)
ngAfterViewChecked   → after each view check
ngOnDestroy          → before removal ★ (unsubscribe, clear timers)
```

## The three you use most

```ts
ngOnInit() {
  this.userService.getUsers().subscribe(u => this.users = u); // init/fetch — inputs are ready
}
ngAfterViewInit() {
  this.chart.nativeElement.focus(); // the DOM/@ViewChild exists now
}
ngOnDestroy() {
  this.subscription.unsubscribe();   // prevent memory leaks!
  clearInterval(this.timer);
}
```

- **`ngOnInit`** — the standard place for initialization and data fetching (unlike the constructor, `@Input`s are available here).
- **`ngAfterViewInit`** — when child views and `@ViewChild` references exist.
- **`ngOnDestroy`** — cleanup: unsubscribe from Observables, clear timers, remove listeners.

## Why ngOnInit over the constructor

The constructor should only do DI; inputs aren't bound yet there. `ngOnInit` runs after Angular has set up the component and its inputs — the correct place for startup logic.

## Why it matters

Lifecycle hooks let you run logic at the right time — initialize in `ngOnInit`, access the rendered view in `ngAfterViewInit`, and crucially **clean up in `ngOnDestroy`** (unsubscribing from Observables is the #1 way to avoid memory leaks in Angular).

Knowing the order and the role of each hook is essential for correct data fetching, DOM access, and resource management.