#はじめに

#テスト対象のコンポーネントの作成

rtl_lessonディレクトリの作成し、ターミナルにて下記のコマンドを実行し、アプリを作成する

$ npx create-react-app . --template redux

##1. 簡単なテスト
はじめに簡単なテストとして各要素が存在するかのテストを作成します。
そのため、テスト対象となるコンポーネントとして、srcディレクトリ直下にRender.jsファイルを作成する

src/Render.js

import React from "react";

const Render = () => {
  return (
    <div>
      <h1>React Testing Library Lesson</h1>
      <input type="text" />
      <button>Click1</button>
      <button>Click2</button>
      <p>Udemy</p>
      <span>@React</span>
    </div>
  );
};

export default Render;

src/Render.test.js

import React from "react";
import { render, screen } from "@testing-library/react";
import Render from "./Render";

describe("Rendering", () => {
  it("Should render all the elements correctly", () => {
    render(<Render />);
    expect(screen.getByRole("heading")).toBeTruthy();
    expect(screen.getByRole("textbox")).toBeTruthy();
    expect(screen.getAllByRole("button")[0]).toBeTruthy();
    expect(screen.getAllByRole("button")[1]).toBeTruthy();
    expect(screen.getByText("Udemy")).toBeTruthy();
    expect(screen.queryByText("Indeed")).toBeNull();
    expect(screen.getByTestId("copyright")).toBeTruthy();
  });
});

publicフォルダーのindex.htmlが大元になっている

##2. userEventとモック関数を使った簡単なテスト

src/App.js

import React from "react";
import logo from "./logo.svg";
import { Counter } from "./features/counter/Counter";
import "./App.css";
import RenderInput from "./RenderInput";　//追記行

function App() {
　　　　/* 以下の3行を追記 */
  const output = (text) => {
    console.log(text);　
  };　
  return (
    <div className="App">
      <header className="App-header">
        <img src={logo} className="App-logo" alt="logo" />
        <Counter />
        <RenderInput outputConsole={output} /> 　　　　　　　　　　　　　　　　　　　　　{/* 追記行 */}
        <p>
          Edit <code>src/App.js</code> and save to reload.
        </p>
        <span>
          <span>Learn </span>
          <a
            className="App-link"
            href="https://reactjs.org/"
            target="_blank"
            rel="noopener noreferrer">
            React
          </a>
          <span>, </span>
          <a
            className="App-link"
            href="https://redux.js.org/"
            target="_blank"
            rel="noopener noreferrer">
            Redux
          </a>
          <span>, </span>
          <a
            className="App-link"
            href="https://redux-toolkit.js.org/"
            target="_blank"
            rel="noopener noreferrer">
            Redux Toolkit
          </a>
          ,<span> and </span>
          <a
            className="App-link"
            href="https://react-redux.js.org/"
            target="_blank"
            rel="noopener noreferrer">
            React Redux
          </a>
        </span>
      </header>
    </div>
  );
}

export default App;

src/RenderInput.js

import React, { useState } from "react";

const RenderInput = ({ outputConsole }) => {
  const [input, setInput] = useState("");

  const outputValue = () => {
    if (input) {
      outputConsole(input);
    }
  };

  const updateValue = (e) => {
    setInput(e.target.value);
  };

  return (
    <>
      <input
        type="text"
        placeholder="Enter"
        value={input}
        onChange={updateValue}
      />
      <button onClick={outputValue}>Console</button>
    </>
  );
};

export default RenderInput;

src/RenderInput.test.js

import React from "react";
import { screen, render, cleanup } from "@testing-library/react";
import userEvent from "@testing-library/user-event";
import RenderInput from "./RenderInput";

afterEach(() => cleanup());

describe("Rendering", () => {
  it("Should render all the elements correctly", () => {
    render(<RenderInput />);
    expect(screen.getByRole("textbox")).toBeTruthy();
    expect(screen.getByRole("button")).toBeTruthy();
    expect(screen.getByPlaceholderText("Enter")).toBeTruthy();
  });
});

describe("Input form onChange event", () => {
  it("Should update input value correctly", () => {
    render(<RenderInput />);
    const inputValue = screen.getByPlaceholderText("Enter");
    userEvent.type(inputValue, "test");
    expect(inputValue.value).toBe("test");
  });
});

describe("Console button conditionally triggered", () => {
  it("Should not trigger output function", () => {
    const outputConsole = jest.fn();
    render(<RenderInput outputConsole={outputConsole} />);
    userEvent.click(screen.getByRole("button"));
    expect(outputConsole).not.toHaveBeenCalled();
  });
  it("Should trigger output function", () => {
    const outputConsole = jest.fn();
    render(<RenderInput outputConsole={outputConsole} />);
    const inputValue = screen.getByPlaceholderText("Enter");
    userEvent.type(inputValue, "test");
    userEvent.click(screen.getByRole("button"));
    expect(outputConsole).toHaveBeenCalledTimes(1);
  });
});

##3. listに対する簡単なテスト

src/App.js

import React from "react";
import logo from "./logo.svg";
import { Counter } from "./features/counter/Counter";
import "./App.css";
import RenderInput from "./RenderInput";
import FrameworkList from "./FrameworkList"; //追記行

function App() {
  //以下のdata定数を追記
  const data = [
    {
      id: 1,
      item: "React",
    },
    {
      id: 2,
      item: "Angular",
    },
    {
      id: 3,
      item: "Vue",
    },
  ];

  const output = (text) => {
    console.log(text);
  };

  return (
    <div className="App">
      <header className="App-header">
        <img src={logo} className="App-logo" alt="logo" />
        <Counter />
        <RenderInput outputConsole={output} />
        /* Frameworkコンポーネントを追記 */
        <FrameworkList frameworks={data} />
        <p>
          Edit <code>src/App.js</code> and save to reload.
        </p>
        <span>
          <span>Learn </span>
          <a
            className="App-link"
            href="https://reactjs.org/"
            target="_blank"
            rel="noopener noreferrer">
            React
          </a>
          <span>, </span>
          <a
            className="App-link"
            href="https://redux.js.org/"
            target="_blank"
            rel="noopener noreferrer">
            Redux
          </a>
          <span>, </span>
          <a
            className="App-link"
            href="https://redux-toolkit.js.org/"
            target="_blank"
            rel="noopener noreferrer">
            Redux Toolkit
          </a>
          ,<span> and </span>
          <a
            className="App-link"
            href="https://react-redux.js.org/"
            target="_blank"
            rel="noopener noreferrer">
            React Redux
          </a>
        </span>
      </header>
    </div>
  );
}

export default App;

src/FrameworkList.js

import React from "react";

const FrameworkList = (props) => {
  if (!props.frameworks || !props.frameworks.length) {
    return <h1>No data!</h1>;
  }
  return (
    <>
      <ul>
        {props.frameworks.map(({ id, item }) => (
          <li key={id}>{item}</li>
        ))}
      </ul>
    </>
  );
};

export default FrameworkList;

src/FrameworkList.test.js

import React from "react";
import { screen, cleanup, render } from "@testing-library/react";
import FrameworkList from "./FrameworkList";
import { afterEach } from "@jest/globals";

afterEach(() => cleanup());

describe("Rendering the list with props", () => {
  it("Should render No data ! when no data propped", () => {
    render(<FrameworkList />);
    expect(screen.getByText("No data!")).toBeInTheDocument();
  });
  it("Should render list item correctly", () => {
    const dummyData = [
      { id: 1, item: "React dymmy" },
      { id: 2, item: "Angular dymmy" },
      { id: 3, item: "Vue dymmy" },
    ];
    render(<FrameworkList frameworks={dummyData} />);
    const frameworkItems = screen
      .getAllByRole("listitem")
      .map((item) => item.textContent);
    const dummyItems = dummyData.map((ele) => ele.item);
    expect(frameworkItems).toEqual(dummyItems);
    expect(screen.queryByText("No data!")).toBeNull();
  });
});

##4. useEffectを使ったテスト

$ npm install axios

src/useEffectRender.js

import React, { useState, useEffect } from "react";
import axios from "axios";

const UseEffectRender = () => {
  const [user, setUser] = useState(null);
  const fetchJSON = async () => {
    const res = await axios.get("https://jsonplaceholder.typicode.com/users/1");
    return res.data;
  };
  useEffect(() => {
    const fetchUser = async () => {
      const json_user = await fetchJSON();
      setUser(json_user);
    };
    fetchUser();
  }, []);

  return (
    <>
      {user ? (
        <p>
          I am {user.username} : {user.email}
        </p>
      ) : null}
    </>
  );
};

export default UseEffectRender;

src/useEffectRender.test.js

import React from "react";
import { screen, render } from "@testing-library/react";
import UseEffectRender from "./UseEffectRender";

describe("useEffect rendering", () => {
  it("Should render only after function resolved", async () => {
    render(<UseEffectRender />);
    expect(screen.queryByText(/I am/)).toBeNull();
    expect(await screen.findByText(/I am/)).toBeInTheDocument();
  });
});

##5. モックAPIを使ったテスト

$ npm install msw

src/MockServer.js

import React, { useState } from "react";
import axios from "axios";

const MockServer = () => {
  const [clicked, setClicked] = useState(false);
  const [username, setUsername] = useState("");
  const [error, setError] = useState("");

  const fetchUser = async () => {
    try {
      const res = await axios.get(
        "https://jsonplaceholder.typicode.com/users/1"
      );
      const { username } = res.data;
      setUsername(username);
      setClicked(true);
    } catch {
      setError("Fetching Failed!");
    }
  };
  const buttonText = clicked ? "Loaded" : "Start Fetch";
  return (
    <>
      <button onClick={fetchUser} disabled={clicked}>
        {buttonText}
      </button>
      {username && <h3>{username}</h3>}
      {error && <p data-testid="error">{error}</p>}
    </>
  );
};

export default MockServer;

src/MockServer.test.js

import React from "react";
import { screen, render, cleanup } from "@testing-library/react";
import userEvent from "@testing-library/user-event";

import { rest } from "msw";
import { setupServer } from "msw/node";
import MockServer from "./MockServer";

const server = setupServer(
  rest.get("https://jsonplaceholder.typicode.com/users/1", (req, res, ctx) => {
    return res(ctx.status(200), ctx.json({ username: "Bred dummy" }));
  })
);

beforeAll(() => server.listen());
afterEach(() => {
  server.resetHandlers();
  cleanup();
});
afterAll(() => server.close());

describe("Mocking API", () => {
  it("[Fetch success]Should display fetched data correctly and button disable", async () => {
    render(<MockServer />);
    userEvent.click(screen.getByRole("button"));
    expect(await screen.findByText("Bred dummy")).toBeInTheDocument();
    expect(screen.getByRole("button")).toHaveAttribute("disabled");
  });
  it("[Fetch failure]Should display error msg, no render heading and button abled", async () => {
    server.use(
      rest.get(
        "https://jsonplaceholder.typicode.com/users/1",
        (req, res, ctx) => {
          return res(ctx.status(404));
        }
      )
    );
    render(<MockServer />);
    userEvent.click(screen.getByRole("button"));
    expect(await screen.findByTestId("error")).toHaveTextContent(
      "Fetching Failed!"
    );
    expect(screen.queryByRole("heading")).toBeNull();
    expect(screen.getByRole("button")).not.toHaveAttribute("disabled");
  });
});

##6. ReduxToolKitを使用したテスト(Reducerのテスト)

app/store.js

import { configureStore } from "@reduxjs/toolkit";
import counterReducer from "../features/counter/counterSlice";
import customCounterReducer from "../features/customCounter/customCounterSlice";

export const store = configureStore({
  reducer: {
    counter: counterReducer,
    customCounter: customCounterReducer,
  },
});

src/customCounterSlice.js

import { createAsyncThunk, createSlice } from "@reduxjs/toolkit";
import axios from "axios";

const sleep = (msec) => {
  const start = new Date();
  while (new Date() - start < msec);
};

export const fetchDummy = createAsyncThunk("fetch/dummy", async (num) => {
  await sleep(2000);
  return num;
});

export const fetchJSON = createAsyncThunk("fetch/api", async () => {
  const res = await axios.get("https://jsonplaceholder.typicode.com/users/1");
  const { username } = res.data;
  return username;
});

const initialState = {
  mode: 0,
  value: 0,
  username: "",
};

export const customCounterSlice = createSlice({
  name: "customCounter",
  initialState,
  reducers: {
    increment: (state) => {
      switch (state.mode) {
        case 0:
          state.value += 1;
          break;
        case 1:
          state.value += 100;
          break;
        case 2:
          state.value += 10000;
          break;
        default:
          break;
      }
    },
    decrement: (state) => {
      state.value -= 1;
    },
    incrementByAmount: (state, action) => {
      switch (state.mode) {
        case 0:
          state.value += action.payload;
          break;
        case 1:
          state.value += 100 * action.payload;
          break;
        case 2:
          state.value += 10000 * action.payload;
          break;
        default:
          break;
      }
    },
  },
  extraReducers: (builder) => {
    builder.addCase(fetchDummy.fulfilled, (state, action) => {
      state.value = 100 + action.payload;
    });
    builder.addCase(fetchDummy.rejected, (state, action) => {
      state.value = 100 - action.payload;
    });
    builder.addCase(fetchJSON.fulfilled, (state, action) => {
      state.username = action.payload;
    });
    builder.addCase(fetchJSON.rejected, (state, action) => {
      state.username = "anonymous";
    });
  },
});

export const { increment, decrement, incrementByAmount } =
  customCounterSlice.actions;

export const selectCount = (state) => state.customCounter.value;
export const selectUsername = (state) => state.customCounter.username;

export default customCounterSlice.reducer;

非同期関数をテストするために、fetchDummy関数はsleep関数の引数に受けった値分だけ待機して値を返している

src/Reducer.test.js

import reducer, {
  increment,
  incrementByAmount,
  fetchDummy,
} from "../src/features/customCounter/customCounterSlice";

describe("Reducer of ReduxToolKit", () => {
  describe("increment action", () => {
    let initialState = {
      mode: 0,
      value: 1,
    };
    it("Should increment by 1 with mode 0", () => {
      const action = { type: increment.type };
      const state = reducer(initialState, action);
      expect(state.value).toEqual(2);
    });
    it("Should increment by 100 with mode 1", () => {
      initialState.mode = 1;
      const action = { type: increment.type };
      const state = reducer(initialState, action);
      expect(state.value).toEqual(101);
    });
    it("Should increment by 10000 with mode 2", () => {
      initialState.mode = 2;
      const action = { type: increment.type };
      const state = reducer(initialState, action);
      expect(state.value).toEqual(10001);
    });
  });
  describe("incrementByAmount action", () => {
    let initialState = {
      mode: 0,
      value: 1,
    };
    it("Should increment by payload value with mode 0", () => {
      const action = { type: incrementByAmount.type, payload: 2 };
      const state = reducer(initialState, action);
      expect(state.value).toEqual(3);
    });
    it("Should increment by 100 * payload value with mode 1", () => {
      initialState.mode = 1;
      const action = { type: incrementByAmount.type, payload: 2 };
      const state = reducer(initialState, action);
      expect(state.value).toEqual(201);
    });
    it("Should increment by 10000 * payload value with mode 2", () => {
      initialState.mode = 2;
      const action = { type: incrementByAmount.type, payload: 2 };
      const state = reducer(initialState, action);
      expect(state.value).toEqual(20001);
    });
  });
  describe("extraReducers", () => {
    const initialState = {
      mode: 0,
      value: 0,
    };
    it("Should output 100 + payload when fulfilled", () => {
      const action = { type: fetchDummy.fulfilled.type, payload: 5 };
      const state = reducer(initialState, action);
      expect(state.value).toEqual(105);
    });
    it("Should output 100 - payload when rejected", () => {
      const action = { type: fetchDummy.rejected.type, payload: 5 };
      const state = reducer(initialState, action);
      expect(state.value).toEqual(95);
    });
  });
});

##7. Integrationテストの作成

src/App.js

import React from "react";
import logo from "./logo.svg";
import { Counter } from "./features/counter/Counter";
import "./App.css";
import RenderInput from "./RenderInput";
import FrameworkList from "./FrameworkList";
import UseEffectRender from "./UseEffectRender";
import MockServer from "./MockServer";
import Redux from "./Redux";

function App() {
  const data = [
    {
      id: 1,
      item: "React",
    },
    {
      id: 2,
      item: "Angular",
    },
    {
      id: 3,
      item: "Vue",
    },
  ];

  const output = (text) => {
    console.log(text);
  };

  return (
    <div className="App">
      <header className="App-header">
        <img src={logo} className="App-logo" alt="logo" />
        <Counter />
        <RenderInput outputConsole={output} />
        <FrameworkList frameworks={data} />
        <UseEffectRender />
        <MockServer />
        <Redux />
        <p>
          Edit <code>src/App.js</code> and save to reload.
        </p>
        <span>
          <span>Learn </span>
          <a
            className="App-link"
            href="https://reactjs.org/"
            target="_blank"
            rel="noopener noreferrer">
            React
          </a>
          <span>, </span>
          <a
            className="App-link"
            href="https://redux.js.org/"
            target="_blank"
            rel="noopener noreferrer">
            Redux
          </a>
          <span>, </span>
          <a
            className="App-link"
            href="https://redux-toolkit.js.org/"
            target="_blank"
            rel="noopener noreferrer">
            Redux Toolkit
          </a>
          ,<span> and </span>
          <a
            className="App-link"
            href="https://react-redux.js.org/"
            target="_blank"
            rel="noopener noreferrer">
            React Redux
          </a>
        </span>
      </header>
    </div>
  );
}

export default App;

src/Redux.js

import React, { useState } from "react";
import { useSelector, useDispatch } from "react-redux";
import {
  selectCount,
  increment,
  decrement,
  incrementByAmount,
} from "./features/customCounter/customCounterSlice";

const Redux = () => {
  const [number, setNumber] = useState(0);
  const count = useSelector(selectCount);
  const dispatch = useDispatch();

  return (
    <>
      <h3>Redux Integration Test</h3>
      <div>
        <button onClick={() => dispatch(increment())}>+</button>
        <span data-testid="count-value">{count}</span>
        <button onClick={() => dispatch(decrement())}>-</button>
        <button onClick={() => dispatch(incrementByAmount(number | 0))}>
          IncrementByAmount
        </button>
        <input
          type="text"
          placeholder="Enter"
          value={number}
          onChange={(e) => setNumber(e.target.value)}></input>
      </div>
    </>
  );
};

export default Redux;

src/Redux.test.js

import React from "react";
import { render, screen, cleanup } from "@testing-library/react";
import userEvent from "@testing-library/user-event";
import { Provider } from "react-redux";
import Redux from "./Redux";
import { configureStore } from "@reduxjs/toolkit";
import customCounterReducer from "./features/customCounter/customCounterSlice";

afterEach(() => {
  cleanup();
});

describe("Redux Integration Test", () => {
  let store;
  beforeEach(() => {
    store = configureStore({
      reducer: {
        customCounter: customCounterReducer,
      },
    });
  });
  it("Should display value with increment by 1 per click", () => {
    render(
      <Provider store={store}>
        <Redux />
      </Provider>
    );
    userEvent.click(screen.getByText("+"));
    userEvent.click(screen.getByText("+"));
    userEvent.click(screen.getByText("+"));
    expect(screen.getByTestId("count-value")).toHaveTextContent(3);
  });
  it("Should display value with decrement by 1 per click", () => {
    render(
      <Provider store={store}>
        <Redux />
      </Provider>
    );
    userEvent.click(screen.getByText("-"));
    userEvent.click(screen.getByText("-"));
    expect(screen.getByTestId("count-value")).toHaveTextContent(-2);
  });
  it("Should display value with incrementByAmount", () => {
    render(
      <Provider store={store}>
        <Redux />
      </Provider>
    );
    userEvent.click(screen.getByText("-"));
    userEvent.click(screen.getByText("-"));
    expect(screen.getByTestId("count-value")).toHaveTextContent(2);
  });
  it("Should display value with incrementByAmount", () => {
    render(
      <Provider store={store}>
        <Redux />
      </Provider>
    );
    userEvent.type(screen.getByPlaceholderText("Enter"), "30");
    userEvent.click(screen.getByText("IncrementByAmount"));
    expect(screen.getByTestId("count-value")).toHaveTextContent(30);
  });
});

```src/ReduxAsync.js
import React from "react";
import { useSelector, useDispatch } from "react-redux";
import {
  selectCount,
  selectUsername,
  fetchDummy,
  fetchJSON,
} from "./features/customCounter/customCounterSlice";

const ReduxAsync = () => {
  const count = useSelector(selectCount);
  const username = useSelector(selectUsername);
  const dispatch = useDispatch();

  return (
    <div>
      <span data-testid="count-value">{count}</span>
      <button onClick={() => dispatch(fetchDummy(5))}>fetchDummy</button>
      {username && <h1>{username}</h1>}
      <button onClick={() => dispatch(fetchJSON())}>fetchJSON</button>
    </div>
  );
};

export default ReduxAsync;

src/ReducxAsync.test.js

import React from "react";
import { render, screen, cleanup } from "@testing-library/react";
import userEvent from "@testing-library/user-event";
import { rest } from "msw";
import { setupServer } from "msw/node";
import { Provider } from "react-redux";
import { configureStore } from "@reduxjs/toolkit";
import customCounterReducer from "../src/features/customCounter/customCounterSlice";
import ReduxAsync from "./ReduxAsync";

const server = setupServer(
  rest.get("https://jsonplaceholder.typicode.com/users/1", (req, res, ctx) => {
    return res(ctx.status(200), ctx.json({ username: "Bred dummy" }));
  })
);

beforeAll(() => server.listen());
let store;
beforeEach(() => {
  store = configureStore({
    reducer: {
      customCounter: customCounterReducer,
    },
  });
});
afterEach(() => {
  server.resetHandlers();
  cleanup();
});
afterAll(() => server.close());

describe("ReduxAsync Test", () => {
  it("Should display value with 100 + payload", async () => {
    render(
      <Provider store={store}>
        <ReduxAsync />
      </Provider>
    );
    expect(screen.getByTestId("count-value")).toHaveTextContent(0);
    userEvent.click(screen.getByText("fetchDummy"));
    expect(await screen.findByTestId("count-value")).toHaveTextContent(105);
  });
});

describe("Redux Async API Mocking", () => {
  it("[Fetch success] Should display username in h3 tag", async () => {
    render(
      <Provider store={store}>
        <ReduxAsync />
      </Provider>
    );
    expect(screen.queryByText("Bred dummy")).toBeNull();
    userEvent.click(screen.getByText("fetchJSON"));
    expect(await screen.findByText("Bred dummy")).toBeInTheDocument();
  });
  it("[Fetch failed] Should display username in h3 tag", async () => {
    server.use(
      rest.get(
        "https://jsonplaceholder.typicode.com/users/1",
        (req, res, ctx) => {
          return res(ctx.status(404));
        }
      )
    );
    render(
      <Provider store={store}>
        <ReduxAsync />
      </Provider>
    );
    expect(screen.queryByText("Bred dummy")).toBeNull();
    userEvent.click(screen.getByText("fetchJSON"));
    expect(await screen.findByText("anonymous")).toBeInTheDocument();
  });
});

##8. Custom Hooksを使用したテスト

$ npm i @testing-library/react-hooks 
$ npm i react-test-renderer

src/App.js

import React from "react";
import logo from "./logo.svg";
import { Counter } from "./features/counter/Counter";
import "./App.css";
import RenderInput from "./RenderInput";
import FrameworkList from "./FrameworkList";
import UseEffectRender from "./UseEffectRender";
import MockServer from "./MockServer";
import Redux from "./Redux";
import ReduxAsync from "./ReduxAsync";
import CustomHooks from "./CustomHooks";

function App() {
  const data = [
    {
      id: 1,
      item: "React",
    },
    {
      id: 2,
      item: "Angular",
    },
    {
      id: 3,
      item: "Vue",
    },
  ];

  const output = (text) => {
    console.log(text);
  };

  return (
    <div className="App">
      <header className="App-header">
        <img src={logo} className="App-logo" alt="logo" />
        <Counter />
        <RenderInput outputConsole={output} />
        <FrameworkList frameworks={data} />
        <UseEffectRender />
        <MockServer />
        <Redux />
        <ReduxAsync />
        <CustomHooks />
        <p>
          Edit <code>src/App.js</code> and save to reload.
        </p>
        <span>
          <span>Learn </span>
          <a
            className="App-link"
            href="https://reactjs.org/"
            target="_blank"
            rel="noopener noreferrer">
            React
          </a>
          <span>, </span>
          <a
            className="App-link"
            href="https://redux.js.org/"
            target="_blank"
            rel="noopener noreferrer">
            Redux
          </a>
          <span>, </span>
          <a
            className="App-link"
            href="https://redux-toolkit.js.org/"
            target="_blank"
            rel="noopener noreferrer">
            Redux Toolkit
          </a>
          ,<span> and </span>
          <a
            className="App-link"
            href="https://react-redux.js.org/"
            target="_blank"
            rel="noopener noreferrer">
            React Redux
          </a>
        </span>
      </header>
    </div>
  );
}

export default App;

src/customHooks.js

import React from "react";
import { useCounter } from "./useCounter";

const CustomHooks = () => {
  const { count, increment, decrement, double, triple, reset } = useCounter(3);
  return (
    <>
      <p>{count}</p>
      <button onClick={increment}>Increment</button>
      <button onClick={decrement}>Decrement</button>
      <button onClick={triple}>triple</button>
      <button onClick={double}>double</button>
      <button onClick={reset}>reset</button>
    </>
  );
};

export default CustomHooks;

src/useCounter.js

import { useState } from "react";

export const useCounter = (initialCount) => {
  const [count, setCount] = useState(initialCount);

  const increment = () => {
    setCount((count) => count + 1);
  };
  const decrement = () => {
    setCount((count) => count - 1);
  };
  const double = () => {
    setCount((count) => count * 2);
  };
  const triple = () => {
    setCount((count) => count * 3);
  };
  const reset = () => {
    setCount(0);
  };
  return { count, increment, decrement, double, triple, reset };
};

src/useCounter.test.js

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

afterEach(() => cleanup());

describe("useCounter custom Hook", () => {
  it("Should increment by 1", () => {
    const { result } = renderHook(() => useCounter(3));
    expect(result.current.count).toBe(3);
    act(() => {
      result.current.increment();
    });
    expect(result.current.count).toBe(4);
  });
  it("Should decrement by 1", () => {
    const { result } = renderHook(() => useCounter(3));
    expect(result.current.count).toBe(3);
    act(() => {
      result.current.decrement();
    });
    expect(result.current.count).toBe(2);
  });
  it("Should double by 3", () => {
    const { result } = renderHook(() => useCounter(3));
    expect(result.current.count).toBe(3);
    act(() => {
      result.current.double();
    });
    expect(result.current.count).toBe(6);
  });
  it("Should triple by 3", () => {
    const { result } = renderHook(() => useCounter(3));
    expect(result.current.count).toBe(3);
    act(() => {
      result.current.triple();
    });
    expect(result.current.count).toBe(9);
  });
  it("Should reset to 0", () => {
    const { result } = renderHook(() => useCounter(3));
    expect(result.current.count).toBe(3);
    act(() => {
      result.current.reset();
    });
    expect(result.current.count).toBe(0);
  });
});

customHooksのテストをする場合はrenderHook()を使用する
customHooks内の関数を実行する場合はact()内で関数を実行して使用する

#tips

create-react-appコマンドで作成するとデフォルトで, React Testing LibraryとJestが導入されている
Roleの確認
本記事の作成時のJestのバージョンは26.2
テスト実行時にテストタイトルも表示したい場合はpackage.json内のtests項目を編集"test": "react-scripts test --env=jsdom --verbose"
afterEachはテスト（it）が終了した直後にrenderによって取得したコンポーネントをアンマウントしてくれる
jsonplaceholder
たぶんこれが一番分かりやすいと思います React + Redux のフロー図解