Typescript

In case you see the following error for your Angular application in your JS console:

main.ts:6 ERROR NullInjectorError: R3InjectorError(AppModule)[MyService -> HttpClient -> HttpClient -> HttpClient]: 
  NullInjectorError: No provider for HttpClient!
    at NullInjector.get (core.mjs:7599:27)
    at R3Injector.get (core.mjs:8020:33)
    at R3Injector.get (core.mjs:8020:33)
    at R3Injector.get (core.mjs:8020:33)
    at injectInjectorOnly (core.mjs:634:33)
    at Module.ɵɵinject (core.mjs:638:60)
    at Object.MyService_Factory [as factory] (my.service.ts:8:38)
    at R3Injector.hydrate (core.mjs:8121:35)
    at R3Injector.get (core.mjs:8009:33)
    at ChainedInjector.get (core.mjs:12179:36)

you need to add HttpClientModule to your app.module.ts.

First, import HttpClientModule at the top of app.module.ts:

import { HttpClientModule } from '@angular/common/http';

After that, add

HttpClientModule,

to the imports: [...] section of your @NgModule, for example:

@NgModule({
  declarations: [
    AppComponent,
  ],
  imports: [
    HttpClientModule,
    BrowserModule,
  ],
  providers: [],
  bootstrap: [AppComponent]
})
export class AppModule { }

After that, your error should have disappeared

Using Angular’sHttpClient  in a service to fetch a JSON is pretty straightforward. In this example, we’ll fetch a JSON array.

Note that it’s recommended to create your own TypeScript interface for improved static typing, but nevertheless starting with any[] is often a good idea since it allows you to get started quickly.

public loadMyJSONArray(): Observable<any[]> {
  return this.http.get<any[]>(`/api/my-api`);
}

Full service example

import { Injectable } from '@angular/core';
import { HttpClient } from '@angular/common/http';
import { Observable } from 'rxjs';

@Injectable({
  providedIn: 'root'
})
export class MyService {

  constructor(private http: HttpClient) {
  }

  public loadMyJSONArray(): Observable<any[]> {
    return this.http.get<any[]>(`/api/my-api`);
  }

}

In order for Angular’s HttpClient to process plaintext responses and not result in an error, you need to set responseType: 'text' in the options(which is the second parameter to .get(). Otherwise, Angular will try to parse the plaintext response, even if the response MIME type is set to text/plain.

getPlaintext(command: string): Observable<string> {
  return this.http.get(`/api/text`, { responseType: 'text' });
}

Full service example

import { HttpClient } from '@angular/common/http';
import { Injectable } from '@angular/core';
import { Observable } from 'rxjs';

@Injectable({
  providedIn: 'root'
})
export class PlaintextService {

  constructor(private http: HttpClient) { }
  
  getPlaintext(command: string): Observable<string> {
    return this.http.get(`/api/text`, { responseType: 'text' });
  }
}

sendCommand(command: string): Observable<any> {
  return this.http.get<any>(`/api/command`, {
    params: {"command": command}
  });
}

Running

sendCommand("test")

will send a HTTP GET request to /api/command?command=test.

Full service example

import { HttpClient } from '@angular/common/http';
import { Injectable } from '@angular/core';
import { Observable } from 'rxjs';

@Injectable({
  providedIn: 'root'
})
export class CommandService {

  constructor(private http: HttpClient) { }

  sendSerialCommand(command: string): Observable<any> {
    return this.http.get<any>(`/api/command`, {
      params: {"command": command}
    });
  }

}

Problem:

You have an Angular service implementing OnInit:

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

@Injectable()
export class MyService implements OnInit {

  constructor() { }
  
  ngOnInit() {
    console.log("MyService initializing");
  }
}

but it never prints MyService initializing – i.e. the ngOnInit() function is never actually being called.

Solution:

Services should not implement OnInit, the function is deliberately never called. Instead, add the code from ngOnInit() in the constructor() and remove implements OnInit and ngOnInit():

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

@Injectable()
export class MyService {

  constructor() {
    console.log("MyService initializing");
  }
  
}

Problem:

You have angular HTTP client code like

this.http.get<MyType>(`${this.baseURL}/api/myAPI`).toPromise()

but toPromise() is deprecated in recent versions of angular / rxjs.

/** @deprecated Replaced with {@link firstValueFrom} and {@link lastValueFrom}. Will be removed in v8. Details: https://rxjs.dev/deprecations/to-promise */
toPromise(): Promise<T | undefined>;

Solution:

In most cases, for HttpClient, you want to use rxjs’s firstValueFrom() since the HttpClient Observables typically only return one value anyway.

First, import firstValueFrom from rxjs:

import { firstValueFrom } from 'rxjs';

then remove the .toPromise() call:

// Before
this.http.get<MyType>(`${this.baseURL}/api/myAPI`).toPromise()
// After
this.http.get<MyType>(`${this.baseURL}/api/myAPI`)

and surround the entire statement with firstValueFrom:

// Before
this.http.get<MyType>(`${this.baseURL}/api/myAPI`)
// After
firstValueFrom(this.http.get<MyType>(`${this.baseURL}/api/myAPI`).toPromise())

This will fix the issue.