Jest Mock Knex Insert Tutorial

When developing applications that interact with databases, it's crucial to write tests that ensure the correct functionality of these interactions. Jest, a popular JavaScript testing framework, provides robust tools for mocking dependencies, which is especially useful when testing database operations. In this tutorial, we'll explore how to use Jest to mock Knex, a SQL query builder for JavaScript, specifically focusing on mocking the `insert` method.

Introduction to Jest and Knex

Jest is a JavaScript testing framework developed by Facebook, known for its ease of use and robust set of features. Knex, on the other hand, is a “batteries included” SQL query builder for JavaScript. It supports PostgreSQL, MySQL, SQLite3, and Oracle.

When testing database interactions, mocking the database layer is essential to prevent actual data modifications and to ensure tests run independently and reliably. Jest provides the `jest.mock` function for creating mock implementations of modules.

Setting Up Your Project

To start, ensure you have a project set up with Jest and Knex installed. If not, you can initialize a new project and install the necessary dependencies:

npm init -y
npm install knex jest
npm install --save-dev @types/jest @types/knex

Create a simple database interaction module (`db.js`) that you want to test:

// db.js
const knex = require('knex')({
  client: 'sqlite3',
  connection: {
    filename: './test.db',
  },
  useNullAsDefault: true,
});

async function insertData(data) {
  try {
    const result = await knex('my_table').insert(data);
    return result;
  } catch (error) {
    throw error;
  }
}

module.exports = { insertData };

Mocking Knex with Jest

To test the insertData function without actually inserting data into a database, you’ll mock the Knex insert method. Create a test file (db.test.js) and use jest.mock to mock the Knex module:

// db.test.js
const db = require('./db');

jest.mock('knex', () => {
  return () => {
    return {
      insert: jest.fn().mockResolvedValueOnce([{ id: 1 }]),
    };
  };
});

describe('Database interactions', () => {
  it('should insert data', async () => {
    const data = { name: 'Test', email: 'test@example.com' };
    const result = await db.insertData(data);
    expect(result).toEqual([{ id: 1 }]);
  });
});

In this example, `jest.mock` is used to create a mock implementation of the Knex module. The `insert` method of the mock Knex instance is then mocked to return a resolved promise with a mock insert result.

Advanced Mocking Scenarios

Sometimes, you might need to mock different behaviors for the same method across different tests or even within the same test. Jest provides several ways to achieve this, including using mockReturnValueOnce for each test or resetting the mock implementation between tests.

it('should handle insert error', async () => {
  const error = new Error('Mocked error');
  knex.mockImplementationOnce(() => ({
    insert: jest.fn().mockRejectedValueOnce(error),
  }));

  await expect(db.insertData({})).rejects.toThrowError('Mocked error');
});

Testing Error Scenarios

Testing how your code handles errors is as important as testing the happy path. With Jest, you can easily mock error scenarios to ensure your application behaves as expected.

it('should handle database connection error', async () => {
  const error = new Error('Database connection error');
  jest.mock('knex', () => {
    return () => {
      throw error;
    };
  });

  await expect(db.insertData({})).rejects.toThrowError('Database connection error');
});

Best Practices for Mocking

When mocking dependencies, it’s essential to strike a balance between isolating the unit under test and ensuring the test remains realistic and relevant. Over-mocking can lead to tests that don’t accurately reflect real-world scenarios, while under-mocking can make tests fragile and prone to breaking due to changes in dependencies.

Keep your mocks as simple as possible, focusing on the specific behavior you're testing. Regularly review and refactor your tests and mocks to ensure they remain relevant and effective.