Information fetching in React the useful manner powered by TypeScript, io-ts & fp-ts


Over the previous few days, I’ve been engaged on a React utility. It’s a simple utility that doesn’t even require a database. Nevertheless, I didn’t need to embed all of the content material into the appliance’s JSX as a result of a few of will probably be up to date regularly. So I made a decision to make use of just a few easy JSON recordsdata to retailer the contents.

The applying is the web site for a convention, and I wished to construct a web page that appears as follows:


To generate a web page just like the one within the earlier picture I’ve saved the information within the following JSON file:

    { "startTime": "08:00", "title": "Registration & Breakfast", "minuteCount": 60 },
    { "startTime": "09:00", "title": "Keynote", "minuteCount": 25 },
    { "startTime": "09:30", "title": "Talk 1 (TBA)", "minuteCount": 25 },
    { "startTime": "10:00", "title": "Talk 2 (TBA)", "minuteCount": 25 },
    { "startTime": "10:30", "title": "Talk 3 (TBA)", "minuteCount": 25 },
    { "startTime": "10:55", "title": "Coffee Break", "minuteCount": 15 },
    { "startTime": "11:10", "title": "Talk 4 (TBA)", "minuteCount": 25 },
    { "startTime": "11:40", "title": "Talk 5 (TBA)", "minuteCount": 25 },
    { "startTime": "12:10", "title": "Talk 6 (TBA)", "minuteCount": 25 },
    { "startTime": "12:35", "title": "Lunch, Networking & Group Pic", "minuteCount": 80 },
    { "startTime": "14:00", "title": "Talk 7 (TBA)", "minuteCount": 25 },
    { "startTime": "14:30", "title": "Talk 8 (TBA)", "minuteCount": 25 },
    { "startTime": "15:00", "title": "Talk 9 (TBA)", "minuteCount": 25 },
    { "startTime": "15:25", "title": "Coffee Break", "minuteCount": 15 },
    { "startTime": "15:40", "title": "Talk 10 (TBA)", "minuteCount": 25 },
    { "startTime": "16:10", "title": "Talk 11 (TBA)", "minuteCount": 25 },
    { "startTime": "16:40", "title": "Talk 12 (TBA)", "minuteCount": 25 },
    { "startTime": "17:10", "title": "Closing Remarks", "minuteCount": 25 }

The issue

Whereas utilizing JSON recordsdata makes my life simpler, knowledge fetching in React is a really repetitive and tedious job. If that wasn’t dangerous sufficient, the information contained in an HTTP response might be utterly totally different from what we predict.

The kind-unsafe nature of fetch calls is especially harmful for TypeScript customers as a result of it compromises most of the advantages of TypeScript. So I made a decision to experiment a little bit bit to attempt to give you a pleasant automated resolution.

I’ve been studying rather a lot about useful programming and Class Concept over the previous few months as a result of I’ve been writing a e-book titled Fingers-On Practical Programming with TypeScript.

I’m not going to get an excessive amount of into Class Concept on this weblog put up. Nevertheless, I would like to clarify the fundamentals. Class Concept defines some sorts which can be significantly helpful when coping with unwanted side effects.

The Class Concept sorts enable us to specific potential issues utilizing the sort system and are helpful as a result of they drive our code to deal with unwanted side effects accurately at compilation time. For instance, the Both kind can be utilized to specific {that a} kind could be both a sort Left or one other kind Proper. The Both kind could be helpful after we need to categorical that one thing can go incorrect. For instance, a fetch name can return both an error (left) or some knowledge (proper).

A) Be sure that errors are dealt with

I wished to ensure that the return of my fetch calls are an Both occasion to make sure that we don’t attempt to entry the information with out first guaranteeing that the response will not be an error.

I’m fortunate as a result of I don’t should implement the Both kind. As a substitute I can merely use the implementation embody within the [fp-ts] open supply module. The Both kind is outlined by fp-ts as follows:

declare kind Both<L, A> = Left<L, A> | Proper<L, A>;

B) Be sure that knowledge is validated

The second drawback that I wished to unravel is that even when the request returns some knowledge, its format might be not what the appliance is anticipating. I wanted some runtime validation mechanism to validate the schema of the response. I’m fortunate as soon as extra as a result of as a substitute of implementing a runtime validation mechanism from scratch, I can use one other open supply library: [io-ts].

The answer

TL;DR This part explains the implementation particulars of the answer. Be at liberty to skip this half and soar into “The outcome” part if you’re solely within the ultimate shopper API.

The io-ts module permits us to declare a schema that can be utilized to carry out validation at runtime. We will additionally use io-ts to generate sorts from a given schema. Each of those options are showcased within the following code snippet:

import * as io from "io-ts";

export const ActivityValidator = io.kind({
    startTime: io.string,
    title: io.string,
    minuteCount: io.quantity

export const ActivityArrayValidator = io.array(ActivityValidator);

export kind IActivity = io.TypeOf<typeof ActivityValidator>;
export kind IActivityArray = io.TypeOf<typeof ActivityArrayValidator>;

We will use the decode technique to validate that some knowledge adheres to a schema. The validation outcome returned by decode is an Both occasion, which implies that we are going to both get a validation error (left) or some legitimate knowledge (proper).

My first step was to wrap the fetch API, so it makes use of each fp-ts and io-ts to make sure that the response is and Both that represents an error (left) or some legitimate knowledge (proper). By doing this, the promise returned byfetch isn’t rejected. As a substitute, it’s at all times resolved as an Both occasion:

import { Both, Left, Proper } from "fp-ts/lib/Both";
import { Kind, Errors} from "io-ts";
import { reporter } from "io-ts-reporters";

export async perform fetchJson<T, O, I>(
    url: string,
    validator: Kind<T, O, I>,
    init?: RequestInit
): Promise<Both<Error, T>> {
    strive {
        const response = await fetch(url, init);
        const json: I = await response.json();
        const outcome = validator.decode(json);
        return outcome.fold<Both<Error, T>>(
            (errors: Errors) => {
                const messages = reporter(outcome);
                return new Left<Error, T>(new Error( a part of("n")));
            (worth: T) => {
                return new Proper<Error, T>(worth);
    } catch (err) {
        return Promise.resolve(new Left<Error, T>(err));

Then I created a React element named Distant that takes an Both occasion as one among its properties along with some rendering capabilities. The info could be both null | Error or some worth of kind T.

The loading perform is invoked when the information is null, the error is invoked when the information is an Error and the success perform is invoked when knowledge is a price of kind T:

import React from "react";
import { Both } from "fp-ts/lib/both";

interface RemoteProps<T>  null, T>;
  loading: () => JSX.Component,
  error: (error: Error) => JSX.Component,
  success: (knowledge: T) => JSX.Component

interface RemoteState {}

export class Distant<T> extends React.Part<RemoteProps<T>, RemoteState> {

  public render() {
    return (
          l => {
            if (l === null) {
              return this.props.loading();
            } else {
              return this.props.error(l);
          r => {
            return this.props.success(r);


export default Distant;

The above element is used to render an Both occasion, however it doesn’t carry out any knowledge fetching operations. As a substitute, I carried out a second element named Fetchable which takes an url and a validator along with some elective RequestInit configuration and a few rendering capabilities. The element makes use of the fetch wrapper and the validator to fetch some knowledge and validate it. It then passes the ensuing Both occasion to the Distant element:

import { Kind } from "io-ts";
import React from "react";
import { Both, Left } from "fp-ts/lib/Both";
import { fetchJson } from "./consumer";
import { Distant } from "./distant";

interface FetchableProps<T, O, I> {
    url: string;
    init?: RequestInit,
    validator: Kind<T, O, I>
    loading: () => JSX.Component,
    error: (error: Error) => JSX.Component,
    success: (knowledge: T) => JSX.Component

interface FetchableState<T>  null, T>;

export class Fetchable<T, O, I> extends React.Part<FetchableProps<T, O, I>, FetchableState<T>> {

    public constructor(props: FetchableProps<T, O, I>) {
        this.state = {
            knowledge: new Left<null, T>(null)

    public componentDidMount() {
        (async () => {
            const outcome = await fetchJson(
                knowledge: outcome

    public render() {
        return (


The outcome

I’ve launched all of the previous supply code as a module named react-fetchable. You’ll be able to set up the module utilizing the next command:

npm set up io-ts fp-ts react-fetchable

You’ll be able to then import the Fetchable element as follows:

import { Fetchable } from "react-fetchable";

At this level I can implement the web page that I described on the beguinning:

import React from "react";
import Container from "../../elements/container/container";
import Part from "../../elements/part/part";
import Desk from "../../elements/desk/desk";
import { IActivityArray, ActivityArrayValidator } from "../../lib/area/sorts";
import { Fetchable } from "react-fetchable";

interface ScheduleProps {}

interface ScheduleState {}

class Schedule extends React.Part<ScheduleProps, ScheduleState> {
  public render() {
    return (
        <Part title="Schedule">
            Lorem ipsum dolor sit amet, consectetur adipiscing elit,
            sed do eiusmod tempor incididunt ut labore et dolore magna aliqua.
            loading={() => <div>Loading...</div>}
            error={(e: Error) => <div>Error: {e.message}</div>}
            success={(knowledge: IActivityArray) => {
              return (
                  headers={["Time", "Activity"]}
                  rows={ => [`${a.startTime}`, a.title])}

export default Schedule;

I can go the URL /knowledge/schedule.json to the Fetchable element along with a validator ActivityArrayValidator. The element will then:

  1. Render Loading...
  2. Fetch the information
  3. Render a desk if the information is legitimate
  4. Render an error is the information can’t be loaded doesn’t adhere to the validator

I’m pleased with this resolution as a result of it’s type-safe, declarative and it solely takes just a few seconds to get it up and operating. I hope you have got discovered this put up attention-grabbing and that you just strive react-fetchable.

Additionally, if you’re fascinated by Practical Programming or TypeScript, please try my upcoming e-book Fingers-On Practical Programming with TypeScript.

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