getTodos.ts

const SMARTSHEET_URL: string = 'https://api.smartsheet.com/2.0'
const SHEET_ID: number = 123456789;
const API_TOKEN: string = 'Bearer ' + 'API Token';

getTodos(): Observable<Array<Todo>> {
  let todos = this.http
      .get(this.SMARTSHEET_URL + '/sheets/' + this.SHEET_ID, {headers: this.getHeaders()})
      .map(extractTodos);
  return todos;
}

private getHeaders() : Headers {
  let headers = new Headers();
  headers.append('Authorization', this.API_TOKEN );
  headers.append('Content-type', 'application/json');
  return headers;
}