To set all FormGroup values use, setValue:
this.myFormGroup.setValue({
formControlName1: myValue1,
formControlName2: myValue2
});
To set only some values, use patchValue:
this.myFormGroup.patchValue({
formControlName1: myValue1,
// formControlName2: myValue2 (can be omitted)
});
this.form.controls['your form control name'].value
this.reactiveForm.get("firstname").valueChanges.subscribe(x => {
console.log('firstname value changed')
console.log(x)
})