ngOnInit not being called when Injectable class is Instantiated

Why isn't ngOnInit() called when an Injectable class is resolved?

Code

import {Injectable, OnInit} from 'angular2/core'; import { RestApiService, RestRequest } from './rest-api.service'; @Injectable() export class MovieDbService implements OnInit { constructor(private _movieDbRest: RestApiService){ window.console.log('FROM constructor()'); } ngOnInit() { window.console.log('FROM ngOnInit()'); } } 

Console Output

FROM constructor() 

7 Answers

Lifecycle hooks, like OnInit() work with Directives and Components. They do not work with other types, like a service in your case. From docs:

A Component has a lifecycle managed by Angular itself. Angular creates it, renders it, creates and renders its children, checks it when its data-bound properties change and destroy it before removing it from the DOM.

Directive and component instances have a lifecycle as Angular creates, updates, and destroys them.

I don't know about all the lifecycle hooks, but as for destruction, ngOnDestroy actually get called on Injectable when it's provider is destroyed (for example an Injectable supplied by a component).

From the docs :

Lifecycle hook that is called when a directive, pipe or service is destroyed.

Just in case anyone is interested in destruction check this question:

Adding to answer by @Sasxa,

In Injectables you can use class normally that is putting initial code in constructor instead of using ngOnInit(), it works fine.

Note: this answer applies only to Angular components and directives, NOT services.

I had this same issue when ngOnInit (and other lifecycle hooks) were not firing for my components, and most searches led me here.

The issue is that I was using the arrow function syntax (=>) like this:

class MyComponent implements OnInit { // Bad: do not use arrow function public ngOnInit = () => { console.log("ngOnInit"); } } 

Apparently that does not work in Angular 6. Using non-arrow function syntax fixes the issue:

class MyComponent implements OnInit { public ngOnInit() { console.log("ngOnInit"); } } 

I had to call a function once my dataService was initialized, instead, I called it inside the constructor, that worked for me.

we can trigger it by placing it in Constructor. But what is the reason ngOnInit() is not triggering.

import {Injectable, OnInit} from 'angular2/core'; import { RestApiService, RestRequest } from './rest-api.service'; import {Service} from "path/to/service/"; @Injectable() export class MovieDbService implements OnInit { userId:number=null; constructor(private _movieDbRest: RestApiService, private instanceMyService : Service ){ // do evreything like OnInit just on services this._movieDbRest.callAnyMethod(); this.userId = this.instanceMyService.getUserId() }