48 answers on StackOverflow to the most popular Angular questions

I gathered the most common questions and answers from Stackoverflow. These questions were chosen by the highest score received. Whether you are an expert or a beginner, you can learn from others’ experiences.

Enjoy!

Angular — Promise vs Observable

551+ points 🔹 150,497+ viewed
Rohit asked,

Can someone please explain the difference between Promise and Observable in Angular?

An example on each would be helpful in understanding both the cases. In what scenario can we use each case?

Günter Zöchbauer answered, (686+ points)

Promise

A Promise handles a single event when an async operation completes or fails.

Note: There are Promise libraries out there that support cancellation, but ES6 Promise doesn't so far.

Observable

An Observable is like a Stream (in many languages) and allows to pass zero or more events where the callback is called for each event.

Often Observable is preferred over Promise because it provides the features of Promise and more. With Observable it doesn't matter if you want to handle 0, 1, or multiple events. You can utilize the same API in each case.

Observable also has the advantage over Promise to be cancelable. If the result of an HTTP request to a server or some other expensive async operation isn't needed anymore, the Subscription of an Observable allows to cancel the subscription, while a Promise will eventually call the success or failed callback even when you don't need the notification or the result it provides anymore.

Observable provides operators like map, forEach, reduce, ... similar to an array

There are also powerful operators like retry(), or replay(), ... that are often quite handy.

Source
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Difference between Constructor and ngOnInit

444+ points 🔹 190,166+ viewed
Haseena P A asked,

Angular provides lifecycle hook ngOnInit by default.

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

Pardeep Jain answered, (512+ points)

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. Angular or better Dependency Injector (DI) analyzes the constructor parameters and when it creates a new instance by calling new MyClass() it tries to find providers that match the types of the constructor parameters, resolves them and passes them to the constructor like

new MyClass(someArg);

ngOnInit is a lifecycle hook called by Angular2 to indicate that Angular is done creating the component.

We have to import OnInit in order to use like this (actually 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() 
  }
}

Implement
 this interface to execute custom initialization logic after your 
directive's data-bound properties have been initialized. ngOnInit is 
called right after the directive's data-bound properties have been 
checked for the first time, and before any of its children have been 
checked. It is invoked only once when the directive is instantiated.

Mostly we 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.

For more information refer here:

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Can’t bind to ‘ngModel’ since it isn’t a known property of ‘input’

442+ points 🔹 246,901+ viewed
abreneliere asked,

I’ve got the following error when launching my Angular app, even if the component is not displayed.

I have to comment out the so that my app works.

zone.js:461 Unhandled Promise rejection: Template parse errors:
Can't bind to 'ngModel' since it isn't a known property of 'input'. ("
    <div>
        <label>Created:</label>
        <input  type="text" [ERROR ->][(ngModel)]="test" placeholder="foo" />
    </div>
</div>"): InterventionDetails@4:28 ; Zone: <root> ; Task: Promise.then ; Value:

I’m looking at the Hero plucker but I don’t see any difference.

Here is the component file:

import { Component, EventEmitter, Input, OnInit, Output } from '@angular/core';
import { Intervention } from '../../model/intervention';

@Component({
    selector: 'intervention-details',
    templateUrl: 'app/intervention/details/intervention.details.html',
    styleUrls: ['app/intervention/details/intervention.details.css']
})

export class InterventionDetails
{
    @Input() intervention: Intervention;

    public test : string = "toto";
}

abreneliere answered, (674+ points)

Yes that’s it, in the app.module.ts, I just added :

import { FormsModule } from '@angular/forms';

[...]

@NgModule({
  imports: [
    [...]
    FormsModule
  ],
  [...]
})

Source
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Angular HTML binding

385+ points 🔹 227,115+ viewed
Aviad P. asked,

I am writing an Angular application, and I have an HTML response I want to display. How do I do that? If I simply use the binding syntax {{myVal}} it encodes all HTML characters (of course).

I need somehow to bind the inner html of a div to the variable value.

prolink007 answered, (691+ points)

The correct syntax is now the following:

<div [innerHTML]="theHtmlString"></div>

Working in 5.2.6

Documentation Reference

Source
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Angular/RxJs When should I unsubscribe from `Subscription`

320+ points 🔹 69,606+ viewed
Sergey Tihon asked,

When should I store the Subscription instances and invoke unsubscribe() during the NgOnDestroy lifecycle and when can I simply ignore them?

Saving all subscriptions introduces a lot of mess into component code.

HTTP Client Guide ignore subscriptions like this:

getHeroes() {
  this.heroService.getHeroes()
                   .subscribe(
                     heroes => this.heroes = heroes,
                     error =>  this.errorMessage = <any>error);
}

In the same time Route & Navigation Guide says that:

Eventually,
 we'll navigate somewhere else. The router will remove this component 
from the DOM and destroy it. We need to clean up after ourselves before 
that happens. Specifically, we must unsubscribe before Angular destroys 
the component. Failure to do so could create a memory leak.

We unsubscribe from our Observable in the ngOnDestroy method.

private sub: any;

ngOnInit() {
  this.sub = this.route.params.subscribe(params => {
     let id = +params['id']; // (+) converts string 'id' to a number
     this.service.getHero(id).then(hero => this.hero = hero);
   });
}

ngOnDestroy() {
  this.sub.unsubscribe();
}

seangwright answered, (508+ points)

— — Edit 3 — The ‘Official’ Solution (2017/04/09)

I spoke with Ward Bell about this question at NGConf (I even showed him this answer which he said was correct) but he told me the docs team for Angular had a solution to this question that is unpublished (though they are working on getting it approved). He also told me I could update my SO answer with the forthcoming official recommendation.

The solution we should all use going forward is to add a private ngUnsubscribe: Subject = new Subject(); field to all components that have .subscribe() calls to Observables within their class code.

We then call this.ngUnsubscribe.next(); this.ngUnsubscribe.complete(); in our ngOnDestroy() methods.

The secret sauce (as noted already by @metamaker) is to call .takeUntil(this.ngUnsubscribe) before each of our .subscribe() calls which will guarantee all subscriptions will be cleaned up when the component is destroyed.

Example:

import { Component, OnDestroy, OnInit } from '@angular/core';
import 'rxjs/add/operator/takeUntil';
// import { takeUntil } from 'rxjs/operators'; // for rxjs ^5.5.0 lettable operators
import { Subject } from 'rxjs/Subject';

import { MyThingService } from '../my-thing.service';

@Component({
    selector: 'my-thing',
    templateUrl: './my-thing.component.html'
})
export class MyThingComponent implements OnDestroy, OnInit {
    private ngUnsubscribe: Subject = new Subject();

    constructor(
        private myThingService: MyThingService,
    ) { }

    ngOnInit() {
        this.myThingService.getThings()
            .takeUntil(this.ngUnsubscribe)
            .subscribe(thing

Angular Blogs

Friday, January 11, 2019

48 answers on StackOverflow to the most popular Angular questions

Table of Contents

Angular — Promise vs Observable

Difference between Constructor and ngOnInit

Can’t bind to ‘ngModel’ since it isn’t a known property of ‘input’

Angular HTML binding

Angular/RxJs When should I unsubscribe from `Subscription`

— — Edit 3 — The ‘Official’ Solution (2017/04/09)

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