Custom Validator on reactive form for password and confirm password matching getting undefined parameters into Angular 4

import {AbstractControl, FormBuilder, FormGroup, Validators} from

set your password input into the group and no need to use “ngModel”.

<div class="form-group row" formGroupName="passwords">
  <div class="form-group">
     <label for="password" class="control-label">Contraseña:</label>
     <input type="password" class="form-control" formControlName="password" title="Please enter your password">
     <p class="help-block" *ngIf="signUpForm.get('password').hasError('required') && signUpForm.get('password').touched">Debe ingresar una contraseña</p>
  </div>
  <div class="form-group">
     <label for="confirmedPassword" class="control-label">Confirmar Contraseña:</label>
     <input type="password" class="form-control" formControlName="confirmedPassword"  title="Please re-enter your password">
     <p class="help-block" *ngIf="signUpForm.get('confirmedPassword').hasError('required') && signUpForm.get('confirmedPassword').touched">Password must be required</p>
     <p class="help-block" *ngIf="signUpForm.get('confirmedPassword').hasError('passwordMismatch') && signUpForm.get('confirmedPassword').touched">password does not match</p>
  </div>

     buildForm(): void {
            this.userForm = this.formBuilder.group({
                passwords: this.formBuilder.group({
                    password: ['', [Validators.required]],
                    confirm_password: ['', [Validators.required]],
                }, {validator: this.passwordConfirming}),

            });
        }

add this custom function for validate password and confirm password

  passwordConfirming(c: AbstractControl): { invalid: boolean } {
    if (c.get('password').value !== c.get('confirm_password').value) {
        return {invalid: true};
    }
}

Display error when password does not match

<div style="color:#ff7355" *ngIf="userForm.get(['passwords','password']).value != userForm.get(['passwords','confirm_password']).value && userForm.get(['passwords','confirm_password']).value != null">
  Password does not match</div>