Testing React Hooks with Jest: A Comprehensive Guide

Suppose we have a react hook like this

import { useState } from 'react';

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

  const increment = () => setCount((prevCount) => prevCount + 1);
  const decrement = () => setCount((prevCount) => prevCount - 1);
  const reset = () => setCount(initialValue);

  return { count, increment, decrement, reset };
}

I hope you have Jest & RTL already setup if not you can install it like this

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

To test this hook, we will use @testing-library/react's renderHook method along with Jest's describe and it syntax.

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

describe('useCounter', () => {
  it('should initialize with the default value', () => {
    const { result } = renderHook(() => useCounter());

    expect(result.current.count).toBe(0);
  });

  it('should initialize with a custom value', () => {
    const { result } = renderHook(() => useCounter(10));

    expect(result.current.count).toBe(10);
  });

  it('should increment the count', () => {
    const { result } = renderHook(() => useCounter());

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

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

  it('should decrement the count', () => {
    const { result } = renderHook(() => useCounter(1));

    act(() => {
      result.current.decrement();
    });

    expect(result.current.count).toBe(0);
  });

  it('should reset the count to initial value', () => {
    const { result } = renderHook(() => useCounter(10));

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

    expect(result.current.count).toBe(10);
  });
});

Explanation of the Test Structure

describe: This block groups related tests together. It's helpful for organizing tests, especially when you have multiple hooks or functions to test.
it: Each it block represents an individual test case. The string description should clearly explain what behavior the test is checking.
renderHook: This function from React Testing Library is used to render your custom hook in a test environment. It returns an object containing the result, which represents the current state and methods returned by the hook.
act: The act function ensures that all updates to the hook's state are applied before you make assertions. It’s essential when interacting with state-changing functions like increment, decrement, and reset.

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