Angular 2 form validations start date <= end date

Based on the answer of santiagomaldonado I have created a generic ValidatorFn that can be used in multiple Reactive Forms with a dynamic return value.

export class DateValidators {
    static dateLessThan(dateField1: string, dateField2: string, validatorField: { [key: string]: boolean }): ValidatorFn {
        return (c: AbstractControl): { [key: string]: boolean } | null => {
            const date1 = c.get(dateField1).value;
            const date2 = c.get(dateField2).value;
            if ((date1 !== null && date2 !== null) && date1 > date2) {
                return validatorField;
            }
            return null;
        };
    }
}

Import the validator and use it like this in your formgroup validators.

    this.form = this.fb.group({
        loadDate: null,
        deliveryDate: null,
    }, { validator: Validators.compose([
        DateValidators.dateLessThan('loadDate', 'deliveryDate', { 'loaddate': true }),
        DateValidators.dateLessThan('cargoLoadDate', 'cargoDeliveryDate', { 'cargoloaddate': true })
    ])});

Now you can use the validation in HTML.

<md-error *ngIf="form.hasError('loaddate')">Load date must be before delivery date</md-error>

You can also do it with Reactive Forms. The FormBuilder API lets you add custom validators.

Valid keys for the extra parameter map are validator and asyncValidator

Example:

import { Component }  from '@angular/core';
import { FormGroup, FormBuilder, Validators } from '@angular/forms';

@Component({
  selector: 'reactive-form',
  templateUrl: './reactive-form.html'
})
export class ReactiveFormComponent {
  
  form: FormGroup

  constructor(private fb: FormBuilder){
    this.createForm();
  }

  createForm() {
    this.form = this.fb.group({
      dateTo: ['', Validators.required ],
      dateFrom: ['', Validators.required ]
    }, {validator: this.dateLessThan('dateFrom', 'dateTo')});
  }
  
  dateLessThan(from: string, to: string) {
   return (group: FormGroup): {[key: string]: any} => {
    let f = group.controls[from];
    let t = group.controls[to];
    if (f.value > t.value) {
      return {
        dates: "Date from should be less than Date to"
      };
    }
    return {};
   }
 }
      
}

Note that I'm comparing the values of the inputs date and from with >, but by default this would be comparing strings. In the live example I'm using angular-date-value-accessor and importing the directive useValueAsDate.

<input formControlName="dateFrom" type="date" useValueAsDate />

With this directive group.controls[from].value and group.controls[to].value returns Date and then I can compare them with <.

Live example in plunkr

Credits to Dave's answer