runtime-type-validation.js

/* @flow */

import typify from "typify"

class Ok<A> {
  value: A
  constructor(value: A) {
    this.value = value
  }
}

class Err {
  msg: string
  constructor(msg: string) {
    this.msg = msg
  }
}

typify.record("results", {
  results: "array string",
})

type Results = {
  results: Array<string>
}

type ResultsResponse =
  | Ok<Results>
  | Err

function fetchItems() : Promise<Results> {
  return fetch("/api/items").then((res) => res.json())
}

function fetchItemsWithValidation() : Promise<ResultsResponse> {
  return fetchItems().then((data) => {
    if (typify.check("results", data)) {
      return new Ok(data)
    }
    return new Err("Invalid JSON")
  })
}

Good example!