DEV Community

Andrew Chaa
Andrew Chaa

Posted on

4

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

Jest

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', () => ({
  ...jest.requireActual('react-router-dom'),
  useParams: () => ({ nodeId })
}));


Enter fullscreen mode Exit fullscreen mode

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', () => ({
  ...jest.requireActual('react-router-dom'),
  useParams: jest.fn()
}));

describe('ActivityReviewGraph', () => {
  useParams.mockReturnValue({ accountUid: nodesId });
  // ...
})


Enter fullscreen mode Exit fullscreen mode

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().

Image of Timescale

Timescale – the developer's data platform for modern apps, built on PostgreSQL

Timescale Cloud is PostgreSQL optimized for speed, scale, and performance. Over 3 million IoT, AI, crypto, and dev tool apps are powered by Timescale. Try it free today! No credit card required.

Try free

Top comments (0)

👋 Kindness is contagious

Explore a sea of insights with this enlightening post, highly esteemed within the nurturing DEV Community. Coders of all stripes are invited to participate and contribute to our shared knowledge.

Expressing gratitude with a simple "thank you" can make a big impact. Leave your thanks in the comments!

On DEV, exchanging ideas smooths our way and strengthens our community bonds. Found this useful? A quick note of thanks to the author can mean a lot.

Okay