Get name of Form Control

I'm working with reactive forms in angular, I need to compare the start date "start date" with the end date "end date", both controls are validated in the "dateLessThan" function, but the problem is that I do not know how to ask for control is evaluating

//Some stuff
public fechaInicio = new FormControl('', [
Validators.required
, this.dateLessThanTo

]);
public fechaFin = new FormControl('', [
Validators.required
, this.dateLessThan
]);



createForm() {
this.contratoForm = this.formBuilder.group({
fechas: this.formBuilder.group({
fechaInicio: this.fechaInicio,
fechaFin: this.fechaFin
}, { validator: this.dateLessThan('fechaInicio', 'fechaFin') }),

});
}


Here I Need to know the name of control for compare dates:

dateLessThanTo(fieldControl: FormControl) {
//
//if (fechaInicio.value > FechaFin.value){
// return true;
//}
//else{
// return false;
// }

}

//Some stuff

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