react formik validate custom field on submit code example

Example: formik and yup

import React from 'react';
 import { Formik, Field, Form, ErrorMessage } from 'formik';
 import * as Yup from 'yup';
 
 const SignupForm = () => {
   return (
     <Formik
       initialValues={{ firstName: '', lastName: '', email: '' }}
       validationSchema={Yup.object({
         firstName: Yup.string()
           .max(15, 'Must be 15 characters or less')
           .required('Required'),
         lastName: Yup.string()
           .max(20, 'Must be 20 characters or less')
           .required('Required'),
         email: Yup.string().email('Invalid email address').required('Required'),
       })}
       onSubmit={(values, { setSubmitting }) => {
         setTimeout(() => {
           alert(JSON.stringify(values, null, 2));
           setSubmitting(false);
         }, 400);
       }}
     >
       <Form>
         <label htmlFor="firstName">First Name</label>
         <Field name="firstName" type="text" />
         <ErrorMessage name="firstName" />
         <label htmlFor="lastName">Last Name</label>
         <Field name="lastName" type="text" />
         <ErrorMessage name="lastName" />
         <label htmlFor="email">Email Address</label>
         <Field name="email" type="email" />
         <ErrorMessage name="email" />
         <button type="submit">Submit</button>
       </Form>
     </Formik>
   );
 };

export default SignupForm;