Complete Guide to React Hooks Testing

This article explores how to test React Hooks and outlines an eight-step testing plan you could employ to test your own projects.

Hooks were introduced in React 16.8 in late 2018. They are functions that hook into a functional component and allow us to use state and component features like componentDidUpdate, componentDidMount, and more. This was not possible before.

Also, hooks allow us to reuse component and state logic across different components. This was tricky to do before. Therefore, hooks have been a game-changer.

In this article, we will explore how to test React Hooks. We will pick a sufficiently complex hook and work on testing it.

We expect that you are an avid React developer already familiar with React Hooks. In case you want to brush up your knowledge, you should check out our tutorial, and here’s the link to the official documentation.

The Hook We Will Use for Testing

For this article, we will use a hook that I wrote in my previous article, Stale-while-revalidate Data Fetching with React Hooks. The hook is called useStaleRefresh. If you haven’t read the article, don’t worry as I will recap that part here.

This is the hook we will be testing:

import { useState, useEffect } from "react";
const CACHE = {};

export default function useStaleRefresh(url, defaultValue = []) {
  const [data, setData] = useState(defaultValue);
  const [isLoading, setLoading] = useState(true);

  useEffect(() => {
    // cacheID is how a cache is identified against a unique request
    const cacheID = url;
    // look in cache and set response if present
    if (CACHE[cacheID] !== undefined) {
      setData(CACHE[cacheID]);
      setLoading(false);
    } else {
      // else make sure loading set to true
      setLoading(true);
      setData(defaultValue);
    }
    // fetch new data
    fetch(url)
      .then((res) => res.json())
      .then((newData) => {
        CACHE[cacheID] = newData;
        setData(newData);
        setLoading(false);
      });
  }, [url, defaultValue]);

  return [data, isLoading];
}

As you can see, useStaleRefresh is a hook that helps fetch data from a URL while returning a cached version of the data, if it exists. It uses a simple in-memory store to hold the cache.

It also returns an isLoading value that is true if no data or cache is available yet. The client can use it to show a loading indicator. The isLoading value is set to false when cache or fresh response is available.

A flowchart tracking the stale-while-refresh logic

At this point, I will suggest you spend some time reading the above hook to get a complete understanding of what it does.

In this article, we will see how we can test this hook, first using no test libraries (only React Test Utilities and Jest) and then by using react-hooks-testing-library.

The motivation behind using no test libraries, i.e., only a test runner Jest, is to demonstrate how testing a hook works. With that knowledge, you will be able to debug any issues that may arise when using a library that provides testing abstraction.

Defining the Test Cases

Before we begin testing this hook, let’s come up with a plan of what we want to test. Since we know what the hook is supposed to do, here’s my eight-step plan for testing it:

  1. When the hook is mounted with URL url1, isLoading is true and data is defaultValue.
  2. After an asynchronous fetch request, the hook is updated with data data1 and isLoading is false.
  3. When the URL is changed to url2, isLoading becomes true again and data is defaultValue.
  4. After an asynchronous fetch request, the hook is updated with new data data2.
  5. Then, we change the URL back to url1. The data data1 is instantly received since it is cached. isLoading is false.
  6. After an asynchronous fetch request, when a fresh response is received, the data is updated to data3.
  7. Then, we change the URL back to url2. The data data2 is instantly received since it is cached. isLoading is false.
  8. After an asynchronous fetch request, when a fresh response is received, the data is updated to data4.

The test flow mentioned above clearly defines the trajectory of how the hook will function. Therefore, if we can ensure this test works, we are good.

Test flow

Testing Hooks Without a Library

In this section, we will see how to test hooks without using any libraries. This will provide us with an in-depth understanding of how to test React Hooks.

To begin this test, first, we would like to mock fetch. This is so we can have control over what the API returns. Here is the mocked fetch.

function fetchMock(url, suffix = "") {
  return new Promise((resolve) =>
    setTimeout(() => {
      resolve({
        json: () =>
          Promise.resolve({
            data: url + suffix,
          }),
      });
    }, 200 + Math.random() * 300)
  );
}

This modified fetch assumes that the response type is always JSON and it, by default, returns the parameter url as the data value. It also adds a random delay of between 200ms and 500ms to the response.

If we want to change the response, we simply set the second argument suffix to a non-empty string value.

At this point, you might ask, why the delay? Why don’t we just return the response instantly? This is because we want to replicate the real world as much as possible. We can’t test the hook correctly if we return it instantly. Sure, we can reduce the delay to 50-100ms for faster tests, but let’s not worry about that in this article.

With our fetch mock ready, we can set it to the fetch function. We use beforeAll and afterAll for doing so because this function is stateless so we don’t need to reset it after an individual test.

// runs before any tests start running
beforeAll(() => {
  jest.spyOn(global, "fetch").mockImplementation(fetchMock);
});

// runs after all tests have finished
afterAll(() => {
  global.fetch.mockClear();
});

Then, we need to mount the hook in a component. Why? Because hooks are just functions on their own. Only when used in components can they respond to useState, useEffect, etc.

So, we need to create a TestComponent that helps us mount our hook.

// defaultValue is a global variable to avoid changing the object pointer on re-render
// we can also deep compare `defaultValue` inside the hook's useEffect
const defaultValue = { data: "" };

function TestComponent({ url }) {
  const [data, isLoading] = useStaleRefresh(url, defaultValue);
  if (isLoading) {
    return <div>loading</div>;
  }
  return <div>{data.data}</div>;
}

This is a simple component that either renders the data or renders a “Loading” text prompt if data is loading (being fetched).

Once we have the test component, we need to mount it on the DOM. We use beforeEach and afterEach to mount and unmount our component for each test because we want to start with a fresh DOM before each test.

let container = null;

beforeEach(() => {
  // set up a DOM element as a render target
  container = document.createElement("div");
  document.body.appendChild(container);
});

afterEach(() => {
  // cleanup on exiting
  unmountComponentAtNode(container);
  container.remove();
  container = null;
});

#testing #react #developer

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Complete Guide to React Hooks Testing
Autumn  Blick

Autumn Blick

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How native is React Native? | React Native vs Native App Development

If you are undertaking a mobile app development for your start-up or enterprise, you are likely wondering whether to use React Native. As a popular development framework, React Native helps you to develop near-native mobile apps. However, you are probably also wondering how close you can get to a native app by using React Native. How native is React Native?

In the article, we discuss the similarities between native mobile development and development using React Native. We also touch upon where they differ and how to bridge the gaps. Read on.

A brief introduction to React Native

Let’s briefly set the context first. We will briefly touch upon what React Native is and how it differs from earlier hybrid frameworks.

React Native is a popular JavaScript framework that Facebook has created. You can use this open-source framework to code natively rendering Android and iOS mobile apps. You can use it to develop web apps too.

Facebook has developed React Native based on React, its JavaScript library. The first release of React Native came in March 2015. At the time of writing this article, the latest stable release of React Native is 0.62.0, and it was released in March 2020.

Although relatively new, React Native has acquired a high degree of popularity. The “Stack Overflow Developer Survey 2019” report identifies it as the 8th most loved framework. Facebook, Walmart, and Bloomberg are some of the top companies that use React Native.

The popularity of React Native comes from its advantages. Some of its advantages are as follows:

  • Performance: It delivers optimal performance.
  • Cross-platform development: You can develop both Android and iOS apps with it. The reuse of code expedites development and reduces costs.
  • UI design: React Native enables you to design simple and responsive UI for your mobile app.
  • 3rd party plugins: This framework supports 3rd party plugins.
  • Developer community: A vibrant community of developers support React Native.

Why React Native is fundamentally different from earlier hybrid frameworks

Are you wondering whether React Native is just another of those hybrid frameworks like Ionic or Cordova? It’s not! React Native is fundamentally different from these earlier hybrid frameworks.

React Native is very close to native. Consider the following aspects as described on the React Native website:

  • Access to many native platforms features: The primitives of React Native render to native platform UI. This means that your React Native app will use many native platform APIs as native apps would do.
  • Near-native user experience: React Native provides several native components, and these are platform agnostic.
  • The ease of accessing native APIs: React Native uses a declarative UI paradigm. This enables React Native to interact easily with native platform APIs since React Native wraps existing native code.

Due to these factors, React Native offers many more advantages compared to those earlier hybrid frameworks. We now review them.

#android app #frontend #ios app #mobile app development #benefits of react native #is react native good for mobile app development #native vs #pros and cons of react native #react mobile development #react native development #react native experience #react native framework #react native ios vs android #react native pros and cons #react native vs android #react native vs native #react native vs native performance #react vs native #why react native #why use react native

What are hooks in React JS? - INFO AT ONE

In this article, you will learn what are hooks in React JS? and when to use react hooks? React JS is developed by Facebook in the year 2013. There are many students and the new developers who have confusion between react and hooks in react. Well, it is not different, react is a programming language and hooks is a function which is used in react programming language.
Read More:- https://infoatone.com/what-are-hooks-in-react-js/

#react #hooks in react #react hooks example #react js projects for beginners #what are hooks in react js? #when to use react hooks

React-hooks-testing-library: Simple & Complete React Hooks Testing

react-hooks-testing-library 

Simple and complete React hooks testing utilities that encourage good testing practices.


Note about React 18 Support

As part of the changes for React 18, it has been decided that the renderHook API provided by this library will instead be included as official additions to both react-testing-library (PR) and react-native-testing-library (PR) with the intention being to provide a more cohesive and consistent implementation for our users.

Please be patient as we finalise these changes in the respective testing libraries.


The problem

You're writing an awesome custom hook and you want to test it, but as soon as you call it you see the following error:

Invariant Violation: Hooks can only be called inside the body of a function component.

You don't really want to write a component solely for testing this hook and have to work out how you were going to trigger all the various ways the hook can be updated, especially given the complexities of how you've wired the whole thing together.

The solution

The react-hooks-testing-library allows you to create a simple test harness for React hooks that handles running them within the body of a function component, as well as providing various useful utility functions for updating the inputs and retrieving the outputs of your amazing custom hook. This library aims to provide a testing experience as close as possible to natively using your hook from within a real component.

Using this library, you do not have to concern yourself with how to construct, render or interact with the react component in order to test your hook. You can just use the hook directly and assert the results.

When to use this library

  1. You're writing a library with one or more custom hooks that are not directly tied to a component
  2. You have a complex hook that is difficult to test through component interactions

When not to use this library

  1. Your hook is defined alongside a component and is only used there
  2. Your hook is easy to test by just testing the components using it

Example

useCounter.js

import { useState, useCallback } from 'react'

function useCounter() {
  const [count, setCount] = useState(0)

  const increment = useCallback(() => setCount((x) => x + 1), [])

  return { count, increment }
}

export default useCounter

useCounter.test.js

import { renderHook, act } from '@testing-library/react-hooks'
import useCounter from './useCounter'

test('should increment counter', () => {
  const { result } = renderHook(() => useCounter())

  act(() => {
    result.current.increment()
  })

  expect(result.current.count).toBe(1)
})

More advanced usage can be found in the documentation.

Installation

npm install --save-dev @testing-library/react-hooks

Peer Dependencies

react-hooks-testing-library does not come bundled with a version of react to allow you to install the specific version you want to test against. It also does not come installed with a specific renderer, we currently support react-test-renderer and react-dom. You only need to install one of them, however, if you do have both installed, we will use react-test-renderer as the default. For more information see the installation docs. Generally, the installed versions for react and the selected renderer should have matching versions:

npm install react@^16.9.0
npm install --save-dev react-test-renderer@^16.9.0

NOTE: The minimum supported version of react, react-test-renderer and react-dom is ^16.9.0.

API

See the API reference.

Issues

Looking to contribute? Look for the Good First Issue label.

🐛 Bugs

Please file an issue for bugs, missing documentation, or unexpected behavior.

See Bugs

💡 Feature Requests

Please file an issue to suggest new features. Vote on feature requests by adding a 👍. This helps maintainers prioritize what to work on.

See Feature Requests

❓ Questions

For questions related to using the library, you can raise issue here, or visit a support community:

Read The Docs 

Author: Testing-library
Source Code: https://github.com/testing-library/react-hooks-testing-library 
License: MIT license

#react #javascript #typescript #hooks #testing 

Hayden Slater

1599277908

Validating React Forms With React-Hook-Form

Validating inputs is very often required. For example, when you want to make sure two passwords inputs are the same, an email input should in fact be an email or that the input is not too long. This is can be easily done using React Hook From. In this article, I will show you how.

Required Fields

The most simple, yet very common, validation is to make sure that an input component contains input from the user. React Hook Form basic concept is to register input tags to the form by passing register() to the tag’s ref attribute. As we can see here:

#react-native #react #react-hook-form #react-hook

The Ugly Side of React Hooks

In this post, I will share my own point of view about React Hooks, and as the title of this post implies, I am not a big fan.

Let’s break down the motivation for ditching classes in favor of hooks, as described in the official React’s docs.

Motivation #1: Classes are confusing

we’ve found that classes can be a large barrier to learning React. You have to understand how "this"_ works in JavaScript, which is very different from how it works in most languages. You have to remember to bind the event handlers. Without unstable syntax proposals, the code is very verbose […] The distinction between function and class components in React and when to use each one leads to disagreements even between experienced React developers._

Ok, I can agree that

thiscould be a bit confusing when you are just starting your way in Javascript, but arrow functions solve the confusion, and calling a_stage 3_feature that is already being supported out of the box by Typescript, an “unstable syntax proposal”, is just pure demagoguery. React team is referring to theclass fieldsyntax, a syntax that is already being vastly used and will probably soon be officially supported

class Foo extends React.Component {
  onPress = () => {
    console.log(this.props.someProp);
  }

  render() {
    return <Button onPress={this.onPress} />
  }
}

As you can see, by using a class field arrow function, you don’t need to bind anything in the constructor, and

this will always point to the correct context.

And if classes are confusing, what can we say about the new hooked functions? A hooked function is not a regular function, because it has state, it has a weird looking

this(aka_useRef_), and it can have multiple instances. But it is definitely not a class, it is something in between, and from now on I will refer to it as aFunclass. So, are those Funclasses going to be easier for human and machines? I am not sure about machines, but I really don’t think that Funclasses are conceptually easier to understand than classes. Classes are a well known and thought out concept, and every developer is familiar with the concept ofthis, even if in javascript it’s a bit different. Funclasses on the other hand, are a new concept, and a pretty weird one. They feel much more magical, and they rely too much on conventions instead of a strict syntax. You have to follow somestrict and weird rules, you need to be careful of where you put your code, and there are many pitfalls. Telling me to avoid putting a hook inside anifstatement, because the internal mechanism of hooks is based on call order, is just insane! I would expect something like this from a half baked POC library, not from a well known library like React. Be also prepared for some awful naming like useRef (a fancy name forthis),useEffect ,useMemo,useImperativeHandle(say whatt??) and more.

The syntax of classes was specifically invented in order to deal with the concept of multiple instances and the concept of an instance scope (the exact purpose of

this ). Funclasses are just a weird way of achieving the same goal, using the wrong puzzle pieces. Many people are confusing Funclasses with functional programming, but Funclasses are actually just classes in disguise. A class is a concept, not a syntax.

Oh, and about the last note:

The distinction between function and class components in React and when to use each one leads to disagreements even between experienced React developers

Until now, the distinction was pretty clear- if you needed a state or lifecycle methods, you used a class, otherwise it doesn’t really matter if you used a function or class. Personally, I liked the idea that when I stumbled upon a function component, I could immediately know that this is a “dumb component” without a state. Sadly, with the introduction of Funclasses, this is not the situation anymore.

#react #react-hooks #javascript #reactjs #react-native #react-hook #rethinking-programming #hackernoon-top-story