Component testing with Vue Test Utils - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is Vue Test Utils used for?

Vue Test Utils is a library that helps you test Vue components by letting you mount them and interact with their output and behavior in a simple way.

beginner

How do you mount a Vue component using Vue Test Utils?

You use the mount() function from Vue Test Utils and pass your component to it. This creates a wrapper that lets you test the component's output and behavior.

beginner

What is a wrapper in Vue Test Utils?

A wrapper is an object returned by mount() that represents the mounted component. It lets you find elements, trigger events, and check the component's state.

intermediate

How can you simulate a user clicking a button in a Vue component test?

You find the button element in the wrapper using find() and then call trigger('click') on it to simulate a click event.

intermediate

Why is it important to test emitted events in Vue components?

Testing emitted events ensures that your component communicates correctly with its parent or other parts of the app, which is key for interaction and data flow.

Which function from Vue Test Utils is used to mount a component for testing?

A mount()

B render()

C create()

D build()

The mount() function is used to mount a Vue component so you can test it.

What does the wrapper returned by mount() allow you to do?

A Compile Vue templates

B Write CSS styles

C Deploy the app

D Find elements and trigger events

The wrapper lets you find elements inside the component and trigger events to test behavior.

How do you check if a Vue component emitted a specific event in a test?

A Use <code>wrapper.emitted()</code>

B Use <code>wrapper.find()</code>

C Use <code>wrapper.trigger()</code>

D Use <code>wrapper.setProps()</code>

The emitted() method returns an object with all events emitted by the component.

Which method simulates a user clicking a button in Vue Test Utils?

A emit('click')

B trigger('click')

C fire('click')

D click()

You call trigger('click') on the element wrapper to simulate a click.

Why should you avoid testing implementation details in Vue component tests?

A Because it makes tests run faster

B Because implementation details are always public

C Because tests should focus on what the user sees and does

D Because Vue Test Utils does not allow it

Testing user-visible behavior makes tests more stable and meaningful.

Explain how to write a basic test for a Vue component using Vue Test Utils.

Think about mounting, interacting, and checking results.

Describe how you would test that a Vue component emits an event when a button is clicked.

Focus on event emission and user interaction.

Practice

1. What is the primary purpose of using mount() from Vue Test Utils in component testing?

easy

A. To compile Vue components into production code

B. To deploy Vue components to a server

C. To style Vue components with CSS

D. To create a full instance of a Vue component for testing its behavior

Solution

1. Step 1: Understand the role of mount()

   mount() creates a real instance of a Vue component in a test environment.

2. Step 2: Identify what mount() is NOT for

   It is not used for compiling, styling, or deploying components.

3. Final Answer:

   To create a full instance of a Vue component for testing its behavior -> Option D

4. Quick Check:

   mount() creates component instance [OK]

Hint: Remember: mount() means create component instance [OK]

Common Mistakes:

• Confusing mount() with build or deploy tools
• Thinking mount() styles components
• Assuming mount() compiles code

2. Which of the following is the correct way to import mount from Vue Test Utils in a test file?

easy

A. import { mount } from '@vue/test-utils';

B. import mount from 'vue-test-utils';

C. const mount = require('vue-test-utils');

D. import { mount } from 'vue';

Solution

1. Step 1: Check the official import syntax

   The correct import uses named import from '@vue/test-utils'.

2. Step 2: Identify incorrect import styles

   import mount from 'vue-test-utils'; uses default import incorrectly, C uses CommonJS require which is outdated here, A imports from wrong package.

3. Final Answer:

   import { mount } from '@vue/test-utils'; -> Option A

4. Quick Check:

   Correct import syntax = import { mount } from '@vue/test-utils'; [OK]

Hint: Use named import from '@vue/test-utils' [OK]

Common Mistakes:

• Using default import instead of named import
• Using require() instead of import
• Importing from wrong package

3. Given this test code snippet:

const wrapper = mount({ template: '{{ count }}', data() { return { count: 0 } } });
await wrapper.find('button').trigger('click');
console.log(wrapper.text());

What will be logged to the console?

medium

A. 0

B. click

C. 1

D. undefined

Solution

1. Step 1: Understand the component behavior

   The button shows the current count, starting at 0, and increments count on click.

2. Step 2: Analyze the test actions

   The test triggers a click event on the button, increasing count from 0 to 1, then logs the button text.

3. Final Answer:

   1 -> Option C

4. Quick Check:

   Click increments count, text shows 1 [OK]

Hint: Click triggers count++ so text updates to 1 [OK]

Common Mistakes:

• Assuming text stays 0 after click
• Confusing event name with text output
• Forgetting to await trigger for update

4. What is wrong with this test code snippet?

const wrapper = mount(MyComponent);
wrapper.find('button').trigger('click');
expect(wrapper.text()).toContain('Clicked');

medium

A. Missing await before trigger to wait for DOM update

B. Incorrect selector used in find()

C. Missing import of mount from Vue Test Utils

D. Using expect() incorrectly without a matcher

Solution

1. Step 1: Identify asynchronous DOM update

   Triggering events may update DOM asynchronously, so awaiting trigger is needed.

2. Step 2: Check the code for await usage

   The code calls trigger() without await, so the expect() runs before DOM updates.

3. Final Answer:

   Missing await before trigger to wait for DOM update -> Option A

4. Quick Check:

   Await trigger() for DOM update [OK]

Hint: Always await trigger() to ensure DOM updates before assertions [OK]

Common Mistakes:

• Not awaiting trigger causing test to fail
• Assuming find() selector is wrong without checking
• Forgetting to import mount (not shown here)
• Misusing expect() matcher syntax

5. You want to test a component that emits a custom event submit with a payload object when a form is submitted. Which Vue Test Utils method correctly checks the event and its payload?

hard

A. expect(wrapper.find('form').trigger('submit')).toBeTruthy();

B. expect(wrapper.emitted().submit[0]).toEqual([{ name: 'John' }]);

C. expect(wrapper.text()).toContain('submit');

D. expect(wrapper.props().submit).toBeDefined();

Solution

1. Step 1: Understand event emission checking

   Vue Test Utils stores emitted events in wrapper.emitted(), accessible by event name and call index.

2. Step 2: Verify correct assertion for event payload

   expect(wrapper.emitted().submit[0]).toEqual([{ name: 'John' }]); checks the first submit event payload matches the expected object, which is correct.

3. Final Answer:

   expect(wrapper.emitted().submit[0]).toEqual([{ name: 'John' }]); -> Option B

4. Quick Check:

   Use emitted() to check event and payload [OK]

Hint: Use wrapper.emitted() to check events and payloads [OK]

Common Mistakes:

• Checking text instead of emitted events
• Testing props instead of events
• Not accessing emitted event payload array