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Youngho Chaa cha cha

Handle Variables and Dynamic Values with `jest.fn()`

javascript, mock, jest2 min read

When it comes to testing React applications with Jest, mocking dependencies is a common practice to ensure reliable and isolated tests. However, there are scenarios where you might need to mock dependencies that rely on variables or dynamic values.

The Challenge: Mocking Dependencies with Variables

Let's say you have a component that uses the useParams hook from the react-router-dom library to access route parameters. You want to mock this dependency in your tests, but you also need to provide a dynamic value for the nodeId parameter based on your test data. Here's an example:

// nodeId is not available yet
jest.mock('react-router-dom', () => ({
useParams: () => ({ nodeId })

The issue with this approach is that jest.mock() is executed at the beginning of the test file, before any variables or constants are defined. As a result, nodeId is not available when the mocking function is executed, leading to potential undefined behaviour or errors.

The Solution: Using jest.fn() and mockReturnValue()

To overcome this challenge, we can leverage the power of jest.fn() and mockReturnValue(). Instead of directly mocking the useParams hook with a specific value, we can create a mock function and set its return value dynamically within our tests. Here's how you can do it:

import { useParams } from 'react-router-dom';
jest.mock('react-router-dom', () => ({
useParams: jest.fn()
describe('ActivityReviewGraph', () => {
useParams.mockReturnValue({ accountUid: nodesId });
// ...

In this example, we first import the useParams hook from react-router-dom. Then, we mock the react-router-dom module using jest.mock(), but instead of providing a specific implementation for useParams, we assign jest.fn() to it. This creates a mock function that we can control later in our tests.

Within the describe block for our component, we use useParams.mockReturnValue({ accountUid: nodesId }) to set the return value of the mocked useParams function based on the value of the nodesId variable.

By following this approach, we can easily mock dependencies that rely on dynamic values or variables without running into issues related to the execution order of jest.mock().

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