formgroup controls set value code example

Example 1: formgroup addcontrol

let form: FormGroup = new FormGroup({});
form.addControl(field_name, new FormControl('', Validators.required));

Example 2: angular formgroup on value change

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