What are some examples of async validators in programming?

What are some examples of async validators in programming?

Async validators are functions that validate data asynchronously, often used in forms. For example, checking if a username is already taken by making an API call.

Can you give me a specific code example?

Sure! In Angular, you might use an async validator like this:

import { AbstractControl, ValidationErrors, AsyncValidatorFn } from '@angular/forms';

export function usernameValidator(service: UserService): AsyncValidatorFn {
  return (control: AbstractControl): Promise<ValidationErrors | null> => {
    return service.checkUsername(control.value).pipe(
      map(isTaken => (isTaken ? { usernameTaken: true } : null)),
      first()
    ).toPromise();
  };
}

What libraries or frameworks support async validators?

Many frameworks support async validators, including Angular, React (with libraries like Formik), and Vue.js. Each has its own way of implementing them.