Unique Method for Yup Validation

uniqueValidationMethod.js

/*
 * Author: @carlosagsmendes
 * Source: https://github.com/jquense/yup/issues/345#issuecomment-487320558
 */

import yup from 'yup';

yup.addMethod(yup.array, 'unique', function (message, mapper = a => a) {
  return this.test('unique', message, function (list) {
    return list.length === new Set(list.map(mapper)).size;
  });
});

const validationSchema = yup.object().shape({
  people: yup
    .array()
    .of(
      yup.object().shape({
        phone: yup.number(),
        firstName: yup.string().max(10),
        lastName: yup.string().min(2)
      })
    )
    .unique('duplicate phone', a => a.phone)
    .required("Must have friends")
    .min(3, 'Minimum of 3 friends'),
 // these constraints are shown if and only if inner constraints are satisfied
});