Component Lifecycle Hooks

ngOnInit, ngOnDestroy

💡 Lifecycle hooks ek insaan ki zindagi ke important moments jaise hain — birth (ngOnInit — "component ban gaya, ab setup karo"), changes over time (ngOnChanges), death (ngOnDestroy — "component jaa raha hai, cleanup karo"). Angular tumhe har MOMENT par ek "hook" (function) deता hai.

Lifecycle hooks special methods hain jo Angular AUTOMATICALLY call karta hai component ki lifetime ke specific MOMENTS par. ngOnInit (component initialize hone ke baad — data fetching yahi karo, constructor mein nahi). ngOnChanges (@Input value change hone par). ngOnDestroy (component remove hone se PEHLE — cleanup, subscriptions unsubscribe karne ke liye zaroori).

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

export class UserProfileComponent implements OnInit, OnDestroy {
  constructor() {
    console.log('Constructor — object ban raha hai');
  }

  ngOnInit() {
    console.log('ngOnInit — component ready hai, data fetch karo yahan');
    // API calls, initial setup yahi karo
  }

  ngOnDestroy() {
    console.log('ngOnDestroy — cleanup karo, subscriptions unsubscribe karo');
    // memory leaks avoid karne ke liye zaroori
  }
}

🔄

Lifecycle hooks ek insaan ki zindagi ke important moments jaise hain — birth (ngOnInit — "component ban gaya, ab setup karo"), changes over time (ngOnChanges), death (ngOnDestroy — "component jaa raha hai, cleanup karo"). Angular tumhe har MOMENT par ek "hook" (function) deता hai.

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⚡ Quick Recap

ngOnInit = component ready hai — data fetching yahan karo
ngOnDestroy = cleanup, memory leaks avoid karne ke liye zaroori
constructor sirf basic setup ke liye, business logic ke liye nahi

On this page (2 subtopics)

Angular components ke paas 8 lifecycle hooks hain, ek SPECIFIC order mein chalते hain: constructor → ngOnChanges → ngOnInit → ngDoCheck → ngAfterContentInit → ngAfterContentChecked → ngAfterViewInit → ngAfterViewChecked → (repeat DoCheck/AfterContentChecked/AfterViewChecked on updates) → ngOnDestroy.

export class DemoComponent implements OnInit, AfterViewInit {
  constructor() { console.log('1. constructor'); }
  ngOnChanges() { console.log('2. ngOnChanges (agar @Input hai)'); }
  ngOnInit() { console.log('3. ngOnInit'); }
  ngAfterViewInit() { console.log('4. ngAfterViewInit — view fully ready'); }
  ngOnDestroy() { console.log('LAST. ngOnDestroy'); }
}

💡Tip: Practical use mein, 95% cases ngOnInit aur ngOnDestroy tak hi limited rehte hain — baaki hooks (ngDoCheck, ngAfterViewInit) advanced, specific scenarios (jaise child component references access karna) ke liye hain.

ngAfterViewInit tab chalता hai jab component ka VIEW (aur uske child components) poori tarah initialize ho chuke hon — @ViewChild se DOM elements/child components access karne ke liye yehi sahi jagah hai, ngOnInit mein nahi (tab tak view ready nahi hoti).

import { Component, ViewChild, ElementRef, AfterViewInit } from '@angular/core';

export class SearchComponent implements AfterViewInit {
  @ViewChild('searchInput') inputRef!: ElementRef;

  ngAfterViewInit() {
    this.inputRef.nativeElement.focus();   // ab element guaranteed exist karta hai
  }
}
// Template: <input #searchInput type="text">

⚠️Common Mistake: @ViewChild reference ngOnInit mein UNDEFINED hoga — view abhi initialize nahi hui hoti. ngAfterViewInit ka wait karna ZAROORI hai DOM elements safely access karne ke liye.

← Previous📦 NgModules Next →↔️ @Input & @Output

Code10x

Component Lifecycle Hooks

Complete Lifecycle Hook Order

ngAfterViewInit — Accessing Child Elements