Check out example codes for "angular start date end date validation". It will help you in understanding the concepts better.

Code Example 1

export class CustomeDateValidators {
    static fromToDate(fromDateField: string, toDateField: string, errorName: string = 'fromToDate'): ValidatorFn {
        return (formGroup: AbstractControl): { [key: string]: boolean } | null => {
            const fromDate = formGroup.get(fromDateField).value;
            const toDate = formGroup.get(toDateField).value;
           // Ausing the fromDate and toDate are numbers. In not convert them first after null check
            if ((fromDate !== null && toDate !== null) && fromDate > toDate) {
                return {[errorName]: true};
            }
            return null;
        };
    }
}

/*--- implementations ---*/
this.form = this.fb.group({
  fromDate: null,
  toDate: null,
}, { validator: [
  //Default error with this validator:  {fromToDate: true}
  CustomeDateValidators.fromToDate('fromDate', 'toDate')
  
  // For custome error name like: {customeErrorName: true}, pass third optional parameter with custome name
  // CustomeDateValidators.fromToDate('fromDate', 'toDate', 'customeErrorName')
]});

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