how to change formgroup value to the formData in angular code example

Example 1: angular formgroup on value change

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

Example 2: update formgroup value angular

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)
});