Angular2 validator which relies on multiple form fields

Question

To kind of reiterate on the methods other have posted, this is the way I've been creating FormGroup validators that don't involve multiple groups.

For this example, simply provide the key names of the password and confirmPassword fields.

// Example use of FormBuilder, FormGroups, and FormControls
this.registrationForm = fb.group({
  dob: ['', Validators.required],
  email: ['', Validators.compose([Validators.required,  emailValidator])],
  password: ['', Validators.required],
  confirmPassword: ['', Validators.required],
  firstName: ['', Validators.required],
  lastName: ['', Validators.required]
}, {validator: matchingPasswords('password', 'confirmPassword')})

In order for Validators to take parameters, they need to return a function with either a FormGroup or FormControl as a parameter. In this case, I'm validating a FormGroup.

function matchingPasswords(passwordKey: string, confirmPasswordKey: string) {
  return (group: FormGroup): {[key: string]: any} => {
    let password = group.controls[passwordKey];
    let confirmPassword = group.controls[confirmPasswordKey];

    if (password.value !== confirmPassword.value) {
      return {
        mismatchedPasswords: true
      };
    }
  }
}

Technically, I could have validated any two values if I knew their keys, but I prefer to name my Validators the same as the error they will return. The function could be modified to take a third parameter that represents the key name of the error returned.

Updated Dec 6, 2016 (v2.2.4)

Full Example: https://embed.plnkr.co/ukwCXm/

Dave's answer was very, very helpful. However, a slight modification might help some people.

In case you need to add errors to the Control fields, you can keep the actual construction of the form and validators:

// Example use of FormBuilder, ControlGroups, and Controls
this.registrationForm= fb.group({
  dob: ['', Validators.required],
  email: ['', Validators.compose([Validators.required,  emailValidator])],
  password: ['', Validators.required],
  confirmPassword: ['', Validators.required],
  firstName: ['', Validators.required],
  lastName: ['', Validators.required]
}, {validator: matchingPasswords('password', 'confirmPassword')})

Instead of setting an error on the ControlGroup, do so on the actual field as follows:

function matchingPasswords(passwordKey: string, passwordConfirmationKey: string) {
  return (group: ControlGroup) => {
    let passwordInput = group.controls[passwordKey];
    let passwordConfirmationInput = group.controls[passwordConfirmationKey];
    if (passwordInput.value !== passwordConfirmationInput.value) {
      return passwordConfirmationInput.setErrors({notEquivalent: true})
    }
  }
}

When implementing validators for multiple form fields, you will have to make sure, that validators are re-evaluated when each of the form control is updated. Most of the examples doesn't provide a solution for such scenario, but this is very important for data consistency and correct behavior.

Please see my implementation of a custom validator for Angular 2, which takes this into account: https://gist.github.com/slavafomin/17ded0e723a7d3216fb3d8bf845c2f30.

I'm using otherControl.valueChanges.subscribe() to listen for changes in other control and thisControl.updateValueAndValidity() to trigger another round of validation when other control is changed.

---

I'm copying a code below for future reference:

match-other-validator.ts

import {FormControl} from '@angular/forms';


export function matchOtherValidator (otherControlName: string) {

  let thisControl: FormControl;
  let otherControl: FormControl;

  return function matchOtherValidate (control: FormControl) {

    if (!control.parent) {
      return null;
    }

    // Initializing the validator.
    if (!thisControl) {
      thisControl = control;
      otherControl = control.parent.get(otherControlName) as FormControl;
      if (!otherControl) {
        throw new Error('matchOtherValidator(): other control is not found in parent group');
      }
      otherControl.valueChanges.subscribe(() => {
        thisControl.updateValueAndValidity();
      });
    }

    if (!otherControl) {
      return null;
    }

    if (otherControl.value !== thisControl.value) {
      return {
        matchOther: true
      };
    }

    return null;

  }

}

Usage

Here's how you can use it with reactive forms:

private constructForm () {
  this.form = this.formBuilder.group({
    email: ['', [
      Validators.required,
      Validators.email
    ]],
    password: ['', Validators.required],
    repeatPassword: ['', [
      Validators.required,
      matchOtherValidator('password')
    ]]
  });
}

---

More up-to-date validators could be found here: moebius-mlm/ng-validators.

I'm using Angular 2 RC.5 but couldn't find the ControlGroup, based on the helpful answer from Dave. I found that FormGroup works instead. So I did some minor updates on Dave's code, and thought I'd share with others.

In your component file, add an import for FormGroup:

import {FormGroup} from "@angular/forms";

Define your inputs in case you need to access the form control directly:

oldPassword = new FormControl("", Validators.required);
newPassword = new FormControl("", Validators.required);
newPasswordAgain = new FormControl("", Validators.required);

In your constructor, instantiate your form:

this.form = fb.group({
  "oldPassword": this.oldPassword,
  "newPassword": this.newPassword,
  "newPasswordAgain": this.newPasswordAgain
}, {validator: this.matchingPasswords('newPassword', 'newPasswordAgain')});

Add the matchingPasswords function in your class:

matchingPasswords(passwordKey: string, passwordConfirmationKey: string) {
  return (group: FormGroup) => {
    let passwordInput = group.controls[passwordKey];
    let passwordConfirmationInput = group.controls[passwordConfirmationKey];

    if (passwordInput.value !== passwordConfirmationInput.value) {
      return passwordConfirmationInput.setErrors({notEquivalent: true})
    }
  }
}

Hope this helps those who are using RC.5. Note that I haven't tested on RC.6 yet.

To expand on matthewdaniel's answer since it's not exactly correct. Here is some example code which shows how to properly assign a validator to a ControlGroup.

import {Component} from angular2/core
import {FormBuilder, Control, ControlGroup, Validators} from 'angular2/common'

@Component({
  selector: 'my-app',
  template: `
    <form [ngFormModel]="form">
      <label for="name">Name:</label>
      <input id="name" type="text" ngControl="name">
      <br>
      <label for="email">Email:</label>
      <input id="email" type="email" ngControl="email">
      <br>
      <div ngControlGroup="matchingPassword">
        <label for="password">Password:</label>
        <input id="password" type="password" ngControl="password">
        <br>
        <label for="confirmPassword">Confirm Password:</label>
        <input id="confirmPassword" type="password" ngControl="confirmPassword">
      </div>
    </form>
    <p>Valid?: {{form.valid}}</p>
    <pre>{{form.value | json}}</pre>
  `
})
export class App {
  form: ControlGroup
  constructor(fb: FormBuilder) {
    this.form = fb.group({
      name: ['', Validators.required],
      email: ['', Validators.required]
      matchingPassword: fb.group({
        password: ['', Validators.required],
        confirmPassword: ['', Validators.required]
      }, {validator: this.areEqual})
    });
  }

  areEqual(group: ControlGroup) {
    let val;
    let valid = true;

    for (name in group.controls) {
      if (val === undefined) {
        val = group.controls[name].value
      } else {
        if (val !== group.controls[name].value) {
          valid = false;
          break;
        }
      }
    }

    if (valid) {
      return null;
    }

    return {
      areEqual: true
    };
  }
}

Here's a working example: http://plnkr.co/edit/Zcbg2T3tOxYmhxs7vaAm?p=preview

Lots of digging in angular source but I've found a better way.

constructor(...) {
    this.formGroup = builder.group({
        first_name:        ['', Validators.required],
        matching_password: builder.group({
            password: ['', Validators.required],
            confirm:  ['', Validators.required]
        }, this.matchPassword)
    });

    // expose easy access to passworGroup to html
    this.passwordGroup = this.formGroup.controls.matching_password;
}

matchPassword(group): any {
    let password = group.controls.password;
    let confirm = group.controls.confirm;

    // Don't kick in until user touches both fields   
    if (password.pristine || confirm.pristine) {
      return null;
    }

    // Mark group as touched so we can add invalid class easily
    group.markAsTouched();

    if (password.value === confirm.value) {
      return null;
    }

    return {
      isValid: false
    };
}

HTML portion for password group

<div ng-control-group="matching_password" [class.invalid]="passwordGroup.touched && !passwordGroup.valid">
    <div *ng-if="passwordGroup.touched && !passwordGroup.valid">Passwords must match.</div>
    <div class="form-field">
        <label>Password</label>
        <input type="password" ng-control="password" placeholder="Your password" />
    </div>
    <div class="form-field">
        <label>Password Confirmation</label>
        <input type="password" ng-control="confirm" placeholder="Password Confirmation" />
    </div>
</div>

Here is another option that I was able to come up with that isn't dependent on an entire or sub ControlGroup but is tied directly to each Control.

The problem I had was the controls that were dependent on each other weren't hierarchically together so I was unable to create a ControlGroup. Also, my CSS was setup that each control would leverage the existing angular classes to determine whether to display error styling, which was more complicated when dealing with a group validation instead of a control specific validation. Trying to determine if a single control was valid was not possible since the validation was tied to the group of controls and not each individual control.

In my case I wanted a select box's value to determine if another field would be required or not.

This is built using the Form Builder on the component. For the select model instead of directly binding it to the request object's value I have bound it to get/set functions that will allow me to handle "on change" events for the control. Then I will be able to manually set the validation for another control depending on the select controls new value.

Here is the relevant view portion:

<select [ngFormControl]="form.controls.employee" [(ngModel)]="employeeModel">
  <option value="" selected></option>
  <option value="Yes">Yes</option>
  <option value="No">No</option>
</select>
...
<input [ngFormControl]="form.controls.employeeID" type="text" maxlength="255" [(ngModel)]="request.empID" />

The relevant component portion:

export class RequestComponent {
  form: ControlGroup;
  request: RequestItem;

  constructor(private fb: FormBuilder) {
      this.form = fb.group({
        employee: new Control("", Validators.required),
        empID: new Control("", Validators.compose([Validators.pattern("[0-9]{7}"]))
      });

  get employeeModel() {
    return this.request.isEmployee;
  }

  set employeeModel(value) {
    this.request.isEmployee = value;
    if (value === "Yes") {
      this.form.controls["empID"].validator = Validators.compose([Validators.pattern("[0-9]{7}"), Validators.required]);
      this.form.controls["empID"].updateValueAndValidity();
    }
    else {
      this.form.controls["empID"].validator = Validators.compose([Validators.pattern("[0-9]{7}")]);
      this.form.controls["empID"].updateValueAndValidity();
    }
  }
}

In my case I always had a pattern validation tied to the control so the validator is always set to something but I think you can set the validator to null if you don't have any validation tied to the control.

UPDATE: There are other methods of capturing the model change like (ngModelChange)=changeFunctionName($event) or subscribing to control value changes by using this.form.controls["employee"].valueChanges.subscribe(data => ...))