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How do you create a custom validator in reactive forms?

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01

Understand the problem

Explain custom validators.

formsvalidators
02

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03

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A validator is a function taking a control and returning null (valid) or an error object: (c) => c.value === 'x' ? null : { invalid: true }. Attach it in the FormControl config. Async validators return an Observable/Promise of the error object.

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04

Read the code

A factory + cross-field validator
import { AbstractControl, ValidationErrors, ValidatorFn, FormGroup, FormControl } from '@angular/forms';

// parameterized (factory) validator
export function forbidden(word: string): ValidatorFn {
  return (c: AbstractControl): ValidationErrors | null =>
    c.value === word ? { forbidden: { word } } : null;
}

// cross-field validator on the group
export const matchPasswords: ValidatorFn = (g: AbstractControl) =>
  g.get('pw')?.value === g.get('confirm')?.value ? null : { mismatch: true };

const form = new FormGroup({
  name: new FormControl('', [forbidden('admin')]),
  pw: new FormControl(''),
  confirm: new FormControl(''),
}, { validators: matchPasswords });
05

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