on change in form control 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: angular 9 form value changes

this.reactiveForm.get("firstname").valueChanges.subscribe(selectedValue => {
  console.log('firstname value changed')
  console.log(selectedValue)                              //latest value of firstname
  console.log(this.reactiveForm.get("firstname").value)   //latest value of firstname
})