How to Change the Value of a FluentUI Slider in a Jest Unit Test?

Ah, the elusive FluentUI slider – a mighty tool for adjusting values with ease, but a pesky creature to tame in the realm of Jest unit testing. Fear not, dear developer, for this article shall guide you through the mystical land of simulated user interactions and slider value manipulation. Buckle up, for we’re about to dive into the nitty-gritty of changing the value of a FluentUI slider in a Jest unit test!

Why Do We Need to Change the Value of a FluentUI Slider in a Jest Unit Test?

Before we delve into the “how,” let’s explore the “why.” In a Jest unit test, we aim to verify the functionality of our component under various conditions. When working with a FluentUI slider, we want to ensure that our component behaves correctly when the slider’s value changes. This might involve testing the following scenarios:

• The component updates correctly when the slider value increases or decreases.
• The component triggers the expected events when the slider value reaches specific thresholds.
• The component displays the correct values or labels based on the slider’s position.

To achieve these test scenarios, we need to simulate user interactions with the slider and manipulate its value programmatically. This is where the magic of Jest and FluentUI comes into play.

Understanding FluentUI Sliders and Their Event Model

Before we dive into the Jest unit test, let’s take a brief look at how FluentUI sliders work. FluentUI sliders are built on top of the React library and use a custom event model to handle user interactions. When a user interacts with the slider, it triggers events like `onChange`, `onMouseDown`, and `onMouseUp`. These events are crucial for updating the slider’s value and performing subsequent actions.

import { Slider } from '@fluentui/react';

const MySlider = () => {
  const [value, setValue] = useState(0);

  const handleChange = (event: React.SyntheticEvent<HTMLElement>, newValue: number) => {
    setValue(newValue);
  };

  return (
    <Slider
      min={0}
      max={100}
      value={value}
      onChange={handleChange}
    />
  );
};

In the above example, we’ve created a basic FluentUI slider with a controlled value using the `useState` hook. The `handleChange` function updates the slider’s value when the `onChange` event is triggered.

Simulating User Interactions with Jest and FluentUI

Now that we understand the event model of FluentUI sliders, let’s explore how to simulate user interactions in a Jest unit test. We’ll use the `fireEvent` function from `@testing-library/react` to trigger events on our slider component.

import React from 'react';
import { render, fireEvent, waitFor } from '@testing-library/react';
import { MySlider } from './MySlider';

describe('MySlider component', () => {
  it('updates the value when the slider is changed', () => {
    const { getByRole } = render(<MySlider />);
    const slider = getByRole('slider');

    fireEvent.change(slider, { target: { value: 50 } });
    expect(slider.value).toBe(50);
  });
});

In this example, we’ve rendered our `MySlider` component and retrieved the slider element using the `getByRole` function. We then use the `fireEvent.change` function to simulate a change event on the slider, passing an object with the `value` property set to 50. Finally, we assert that the slider’s value has been updated correctly.

But Wait, There’s More!

While the above example demonstrates how to simulate a change event, it doesn’t quite replicate the user interaction of dragging the slider thumb. To achieve this, we need to simulate a sequence of events, including `mousedown`, `mousemove`, and `mouseup`.

it('updates the value when the slider is dragged', () => {
  const { getByRole } = render(<MySlider />);
  const slider = getByRole('slider');
  const sliderThumb = slider.querySelector('.ms-Slider-thumb');

  fireEvent.mouseDown(sliderThumb, { clientX: 0 });
  fireEvent.mouseMove(sliderThumb, { clientX: 50 });
  fireEvent.mouseUp(sliderThumb);

  expect(slider.value).toBe(50);
});

In this updated example, we first retrieve the slider thumb element using the `querySelector` method. We then simulate a `mousedown` event on the thumb, followed by a `mousemove` event with the `clientX` property set to 50. Finally, we simulate a `mouseup` event to complete the drag gesture. The slider’s value should now be updated to 50.

Manually Updating the Slider Value in a Jest Unit Test

Sometimes, simulating user interactions might not be necessary or desirable. In such cases, we can manually update the slider value using the `setValue` function from `@fluentui/react`. This approach is particularly useful when testing the component’s behavior under different initial conditions.

import { MySlider } from './MySlider';
import { setValue } from '@fluentui/react';

describe('MySlider component', () => {
  it('renders with the correct initial value', () => {
    const { getByRole } = render(<MySlider />);
    const slider = getByRole('slider');

    setValue(slider, 75);
    expect(slider.value).toBe(75);
  });
});

In this example, we’ve rendered the `MySlider` component and retrieved the slider element using the `getByRole` function. We then use the `setValue` function to manually update the slider’s value to 75.

Conclusion

In conclusion, changing the value of a FluentUI slider in a Jest unit test requires a combination of simulated user interactions and manual value updates. By leveraging the `fireEvent` function from `@testing-library/react` and the `setValue` function from `@fluentui/react`, we can create comprehensive test suites that cover a wide range of scenarios. Remember to keep your tests concise, focused, and easy to maintain, and you’ll be well on your way to crafting robust and reliable components.

Additional Tips and Tricks

Here are some additional tips and tricks to keep in mind when testing FluentUI sliders:

• Use the `waitFor` function from `@testing-library/react` to wait for the slider’s value to update before making assertions.
• Test edge cases, such as the slider’s minimum and maximum values, to ensure your component behaves correctly under extreme conditions.
• Use the `debug` function from `@testing-library/react` to inspect the component’s DOM and debug any issues that arise during testing.
• Keep your test data and expected results organized using tables or objects to make your tests more readable and maintainable.

Scenario	Expected Result
Slider value increases by 10	Slider value is 10
Slider value decreases by 20	Slider value is -20
Slider value is set to 50	Slider value is 50

By following these guidelines and best practices, you’ll be able to craft robust and reliable unit tests for your FluentUI slider components, ensuring your application behaves correctly and delightfully for your users.

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