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Mateo Tibaquirá for This is Angular

Posted on • Updated on • Originally published at

Parametrized Validators in Dynamic Forms

We are building the documentation of @myndpm/dyn-forms at and we've added support for a variety of custom functions like Validators, AsyncValidators, Matchers, Conditions and more.

The next crucial part of any form is validation, aka Validators and AsyncValidators, and we took some time to study a nice way to implement them and we picked the most declarative one:

createMatConfig('INPUT', {
  name: 'quantity',
  validators: ['required', ['min', 1] ],
  asyncValidators: ['myAsyncValidator'],
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Angular Validators

Angular provides default Validators that we're used to consume programatically in our Reactive Forms, some of them are Validator Functions (ValidatorFn) like Validators.required, and some others are Validator Factories ((args) => ValidatorFn) which builds a Validator based on a required parameter like Validators.minLength(4).

The definition of a Validator Function is:

(control: AbstractControl) => ValidationErrors | null
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it receives the control to be validated, and returns null if its value is valid, or an error object of the form { [error: string]: any }

Validator Factories are high-order functions that builds a Validator Function according some input parameters:

function minLength(minLength: number): ValidatorFn {
  return (control: AbstractControl) => {
    return (control.value && control.value.length < minLength)
      ? { minLength: true } // invalid
      : null; // valid
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as you can see, this is a very nice way to parametrize our Functions, so we defined the provisioning of Validators (and all the other handlers) with an id and a factory fn:

export interface DynControlValidator {
  id: string;
  fn: (...args: any[]) => ValidatorFn;
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The id will be the string that we will use in our Configuration Object. By default, @myndpm/dyn-forms provide the default Angular Validators with the same name as we know them: required, requiredTrue, email, pattern, minLength, maxLength, min and max.

The notation to use them in the Config Object is as follows:

// without parameters
validators: ['required'],

// with parameters as array
validators: ['required', ['min', 1] ],

// with parameters as object
validators: { required: null, minLength: 4 },

// with an inline ValidatorFn or ValidatorFn factory
validators: [myValidatorFn, myValidatorFactory(args)],
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supporting these different notations is unexpensive and can be useful for different kind of systems or developer tastes.

Custom Validators

As mentioned, all we need is to provide our ValidatorFn Factory with an id and a fn. So we can easily provide them in our module with a code like this:

import { AbstractControl, ValidatorFn } from '@angular/forms';
import { DynFormsModule } from '@myndpm/dyn-forms';
import { DynControlValidator } from '@myndpm/dyn-forms/core';

const validators: DynControlValidator[] = [
    id: 'email',
    fn: (): ValidatorFn => {
      return (control: AbstractControl) => {
        // implement my validator
        // to return { email: true } | null;

  imports: [
    DynFormsModule.forFeature({ validators, priority: 100 });
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note the priority parameter to override the default validators (which weight is 0); we will play with priorities in a further article.


Providing async validators works in the same way. You provide your fn with an id and use them in the Config Object:

createMatConfig('INPUT', {
  name: 'quantity',
  validators: ['required'],
  asyncValidators: ['myAsyncValidatorId'],
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and if you need to provide arguments to your AsyncValidator factory, you can use:

// single argument which can be an object
asyncValidators: [['myAsyncValidatorId', args]],
// your factory will receive fn(args)

// multiple arguments in array to be destructured
asyncValidators: [['myAsyncValidatorId', [arg1, arg2]]],
// your factory will receive fn(arg1, arg2)
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Custom Handlers

With this notation we added support for multiple kinds of functions that we require in the Dynamic Forms: Validators and AsyncValidators as we just saw, Matchers and Conditions to manipulate the controls under some special requirements, and also ParamFns to inject functions to the parameters of the DynControls too.

We will be digging into the conditional executions in the next chapter.
In the meantime, what do you think of this notation?

// PS. We are hiring!

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