constructor(fb: FormBuilder)
  {
    this.form = fb.group({
      password: ['', Validators.required,this.asyncValidator],
      confirmPassword: ['', Validators.required,this.asyncValidator]
    })
  }

asyncValidator(control:FormControl):any{
    return new Promise(
      (resolve) =>{
        setTimeout(() => {
          if((control.password).length < 6 && (control.password).length > 1)
              console.log("true");
          console.log("false");
          resolve(null);
        },2000);
      }
    );
 }

<div class="alert alert-danger"
  *ngIf="asyncValidator(this.form.controls.password)">Password too short</div>