Can't assign value to variable using subscribe() method in Angular 2

With this code

export class TopbarComponent {
  companyCount;

  constructor (private dataService: DataService){
    dataService.getCompaniesCount().subscribe(res => this.companyCount = res.count); //doesn't work
    dataService.getCompaniesCount().subscribe(res => console.log(res.count)); //works    
  }
}

res => this.companyCount = res.count doesn't get executed immediately. When getCompaniesCount() makes a request to a server, it takes a long time until the response arrives and the observable calls the function passed to subscribe(...) (res => this.companyCount = res.count).

The execution of the constructor, ngOnInit, ngAfterViewInit() and lots of other stuff will have happened before the response arrives.

You can see

subscribe(res => this.companyCount = res.count)

like registering an event handler that gets called when an event happens.

All code that depends on the data being available needs to be properly chained.

The simplest way is to move to code into subscribe(...)

  constructor (private dataService: DataService){
    dataService.getCompaniesCount().subscribe(res => {
      this.companyCount = res.count); 
      // more code that depends on `res.count` being set goes here
    });
    dataService.getCompaniesCount().subscribe(res => console.log(res.count)); //works    
  }