Using assertion signatures to validate data from external sources using the Yup validation/schema library

assertionSignatureValidation.tsx

import Yup from "yup";
import type { InferType } from "yup";

const UserSchema = Yup.object({
  name: Yup.string().required(),
  email: Yup.string().email().required(),
  fullname: Yup.string().required(),
  title: Yup.string().required(),
  gender: Yup.string().required().default("male"),
  rating: Yup.number().min(1).max(10).required(),
  birthdate: Yup.date().required(),
  is_active: Yup.boolean().default(false),
})

/* @HINT: extract typecript type from Yup Schema */
type User = InferType<typeof UserSchema> 

/* @NOTE: assertion signature function */
function isUser (user: unknown): user is User { 
  return UserSchema.isType(user)
}

const getAgeBandFor = (user: unknown) => {
  /* @HINT: assertion of `user` object passed in as argument */
  if (isUser(user)) { 

    const today = new Date();
    const age = user.birthdate.getFullYear() - today.getFullYear()

    switch(true) {
      case age >= 30 && age <= 39:
        return "millennial";
      case age >= 60 && age <= 89:
        return "boomer";
      case age >= 20 && age <= 29:
        return "gen-z";
      default:
        return "unknown"
    }
  }
}