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Anubhav Kumar · 2025-12-16T21:47:54.210Z

In Angular , OnInit is a lifecycle hook that lets a component (or directive) run code once, right after Angular has initialized its input properties . What exactly is OnInit ? OnInit is an interface provided by Angular. It defines a single method: ngOnInit() Angular automatically calls ngOnInit() after the component’s constructor runs and after all @Input() values are set . Why do we use OnInit ? You use OnInit when you want to: Fetch data from an API Initialize component variables Set up subscriptions Perform logic that depends on @Input() values Run setup code once when the component loads Basic example Execution order Constructor ngOnInit() Constructor vs ngOnInit() Constructor ngOnInit() Used to inject dependencies Used to initialize logic Runs when class is created Runs after inputs are set Should be lightweight Can contain business logic Best practice: Use the constructor only for dependency injection Use ngOnInit() for initialization logic Example with @Input() When NOT to use OnInit For logic that should run every time a value changes → use ngOnChanges For cleanup → use ngOnDestroy For DOM access → use ngAfterViewInit Summary OnInit is an Angular lifecycle hook Implemented via ngOnInit() Runs once after component initialization Ideal for API calls and setup logic Better than using the constructor for initialization

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What is OnInit in Angular?

Anubhav Kumar 11 16-Dec-2025

javascript angular js

Updated on 16-Dec-2025

Anubhav Kumar

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The Anubhav portal was launched in March 2015 at the behest of the Hon'ble Prime Minister for retiring government officials to leave a record of their experiences while in Govt service .

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What is OnInit in Angular?

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