Giorgio Polvara's Blog

How to Test Asynchronous Methods

I continue my series of posts on react-testing-library this time with a brief explanation on how to test asynchronous methods. The idea for this post comes from a person who contacted me on Twitter asking this:

[...] how would one test async methods loaded during componentdidMount?

In my personal experience 99% of the time an async method is going to fetch some data from the server. This is especially true if we call this method from componentDidMount. You can then understand how important it is to know how to test these methods properly.

Two notes before I start:

  • The question is about methods called in componentDidMount but the testing strategy is the same if you use Hooks;
  • If you are uncertain how to make asynchronous calls in the first place check my other post Fetching Asynchronous Data with React Hooks.

There are two things you want to test when it comes to asynchronous methods. The first is that the method itself got called and with the right parameters. The second is that after the call your application responds as it should. Let's see what this means in practice.

Imagine you have a small blog application written in React. In particular you have an Index component that shows the list of posts—kinda like what I have in my homepage.

Index will probably look something like this:

import React from "react";
import { fetchPosts } from "./api/posts";
import { useAsync } from "react-use"getResource;

function Index() {
  const posts = useAsync(fetchPosts, []);

  if (posts.loading) return "Loading...";
  if (posts.error) return "Something went wrong.";

  return (
    <>
      <h1>My Posts</h1>
      <ul>
        {posts.value.map(post => (
          <li key={post.id}>
            <a href={post.url}>{post.title}</a>
          </li>
        ))}
      </ul>
    </>
  );
}

You have probably seen code like this before and maybe even written some yourself. Here we're fetching a list of posts via the fetchPosts method. We're showing a loading message while this happens. Once the async method resolves, we either show an error or the list of posts.

If you are confused by the useAsync call refer to my previous post where I explain how it works.

If you prefer to avoid Hooks, this is the class implementation (note that the tests we're going to write are going to work with both implementations):

import React from "react";
import { fetchPosts } from "./api/posts";

class Index extends React.Component {
  state = { loading: true, error: null, posts: null };

  async componentDidMount() {
    try {
      this.setState({ loading: true });
      const posts = await fetchPosts();
      this.setState({ loading: false, posts });
    } catch (error) {
      this.setState({ loading: false, error });
    }
  }

  render() {
    if (this.state.loading) return "Loading...";
    if (this.state.error) return "Something went wrong.";

    return (
      <>
        <h1>My Posts</h1>
        <ul>
          {this.state.posts.map(post => (
            <li key={post.id}>
              <a href={post.url}>{post.title}</a>
            </li>
          ))}
        </ul>
      </>
    );
  }
}

A bit longer to type but it does the same thing the Hook version does.

Now to the tests!


The first thing we want to do is to mock the API calls. Why? I can give you two good reasons for this. The first one is that it will make our tests more reliable. If your test fetched the list of posts, it would need a server to call. This server might or might not be working making our tests failing randomly.

The second reason is speed. Performing a real API call takes time. It might not seem like much when you have just one test but as your codebase grows the slowness will show.

So, how do we mock the API call? In our case, the call happens in the fetchPosts method so we can mock it:

jest.mock("./api/posts");

You can read more about jest.mock in the official docs. What it does is to tell Jest to replace all the methods inside the ./api/posts module with a mock.

Now that we have the mock in place let's render the component and test that we see a loading message:

import React from "react";import { render } from "react-testing-library";import Index from "./Index";import "jest-dom/extend-expect";
jest.mock("./api/posts");

test("We show a list of posts", () => {  const { getByText } = render(<Index />);  expect(getByText("Loading...")).toBeInTheDocument();});

This was the first step. Now we need to make sure our async method gets called correctly:

import React from "react";
import { render } from "react-testing-library";
import Index from "./Index";
import "jest-dom/extend-expect";
import { fetchPosts } from "./api/posts";
jest.mock("./api/posts");

test("We show a list of posts", () => {
  const { getByText } = render(<Index />);
  expect(getByText("Loading...")).toBeInTheDocument();
  expect(fetchPosts).toHaveBeenCalledTimes(1);  expect(fetchPosts).toHaveBeenCalledWith();});

We're testing that fetchPosts has been called once and with no arguments. Note that, since we're using jest.mock, fetchPosts is not going to be the method we have in our codebase but a mock function.

OK, we're almost there. All is left to do is to return some data from fetchPosts and verify it appears in the DOM. This is exactly what we're going to do next:

import React from "react";
import { render, wait } from "react-testing-library";import Index from "./Index";
import "jest-dom/extend-expect";
import { fetchPosts } from "./api/posts";

jest.mock("./api/posts");

test("We show a list of posts", async () => {  const posts = [{ id: 1, title: "My post", url: "/1" }];  fetchPosts.mockResolvedValueOnce(posts);  const { getByText } = render(<Index />);
  expect(getByText("Loading...")).toBeInTheDocument();
  expect(fetchPosts).toHaveBeenCalledTimes(1);
  expect(fetchPosts).toHaveBeenCalledWith();
  await wait(() => expect(getByText("My Posts")).toBeInTheDocument());  posts.forEach(post => expect(getByText(post.title)).toBeInTheDocument());});

And that's it! We're returning some fake values using mockResolvedValueOnce. We then wait for the async method to resolve and for Index to rerender. To do that we use the wait method while checking that the title is rendered. After that we go post-by-post and make sure the title is on the page.


If you wanted to test that errors are rendered correctly it's just a matter of changing the mock:

import React from "react";
import { render, wait } from "react-testing-library";
import Index from "./Index";
import "jest-dom/extend-expect";
import { fetchPosts } from "./api/posts";

jest.mock("./api/posts");

test("We show an error message on failures", async () => {  fetchPosts.mockRejectedValueOnce("Error!");  const { getByText } = render(<Index />);
  expect(getByText("Loading...")).toBeInTheDocument();
  expect(fetchPosts).toHaveBeenCalledTimes(1);
  expect(fetchPosts).toHaveBeenCalledWith();
  await wait(() =>    expect(getByText("Something went wrong.")).toBeInTheDocument()  );});

To recap, these are the steps to test an asynchronous method:

  1. Mock the method with jest.mock and make it resolve to some data;
  2. Test the loading state;
  3. Test that the async method got called correctly;
  4. Test that the component rendered the data correctly.