Difference between Constructor and ngOnInit

Angular provides lifecycle hook ngOnInit by default.

Why should ngOnInit be used, if we already have a constructor?

The Constructor is a default method of the class that is executed when the class is instantiated and ensures proper initialization of fields in the class and its subclasses. 

ngOnInit is a lifecycle hook called by Angular to indicate that Angular is done creating the component.

We have to import OnInit in order to use like this (implementing OnInit is not mandatory but considered good practice):

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

then to use the method of OnInit we have to implement in the class like this.

export class App implements OnInit{

    constructor(){

     //called first time before the ngOnInit()

    }

    ngOnInit(){

      //called after the constructor and called  after the first ngOnChanges()

       }

      }

Mostly we should use ngOnInit for all the initialization/declaration and avoid stuff to work in the constructor. The constructor should only be used to initialize class members but shouldn't do actual "work".

So you should use constructor() to setup Dependency Injection and not much else. ngOnInit() is better place to "start" - it's where/when components' bindings are resolved.