1To set all FormGroup values use, setValue:
2
3this.myFormGroup.setValue({
4  formControlName1: myValue1, 
5  formControlName2: myValue2
6});
7To set only some values, use patchValue:
8
9this.myFormGroup.patchValue({
10  formControlName1: myValue1, 
11  // formControlName2: myValue2 (can be omitted)
12});

1 
2this.reactiveForm.get("firstname").valueChanges.subscribe(selectedValue => {
3  console.log('firstname value changed')
4  console.log(selectedValue)                              //latest value of firstname
5  console.log(this.reactiveForm.get("firstname").value)   //latest value of firstname
6})
7 
8

1this.reactiveForm.get("firstname").valueChanges.subscribe(x => {
2   console.log('firstname value changed')
3   console.log(x)
4})