Complete Vue Interview Guide
Questions
Your Vue component is not updating. How do you debug a reactivity issue?
The Scenario
You are a frontend engineer at a social media company. You are building a new feature that allows users to like a post. When a user clicks the “like” button, the number of likes should be incremented.
However, the component is not updating. When a user clicks the “like” button, the number of likes is incremented in the component’s data, but the new value is not reflected in the template.
The Challenge
Explain what the reactivity system is in Vue and how it can be broken. What are the common causes of reactivity issues, and how would you debug this issue?
A junior engineer might not be aware of the reactivity system. They might try to debug the problem by manually updating the DOM, which would be a very bad practice. They might also not know how to use the Vue Devtools to debug reactivity issues.
A senior engineer would immediately suspect that the reactivity system is broken. They would be able to explain what the reactivity system is and how it can be broken. They would also have a clear plan for how to debug the issue, including using the Vue Devtools to inspect the component's data and see what is causing the reactivity issue.
Step 1: Understand the Reactivity System
The reactivity system is the core of Vue. It allows you to write declarative code that automatically updates the DOM when the data changes.
It works by tracking the dependencies between the component’s data and the template. When the data changes, Vue knows which parts of the template need to be updated, and it efficiently updates the DOM.
Step 2: The Common Causes of Reactivity Issues
| Cause | Example |
|---|---|
| Directly modifying an array by index | myArray[0] = 'new value' |
| Adding a new property to an object | myObject.newProperty = 'new value' |
Forgetting to use this | myVar = 'new value' instead of this.myVar = 'new value' |
Step 3: Debug the Issue
Here’s how you can debug a reactivity issue:
1. Use the Vue Devtools:
The Vue Devtools is a browser extension that allows you to inspect the component’s data and see what is causing the reactivity issue.
2. Use the $forceUpdate() method:
The $forceUpdate() method can be used to force a component to re-render. This can be useful for debugging, but you should not use it in production code.
3. Use the Vue.set() method:
The Vue.set() method can be used to add a new reactive property to an object.
import Vue from 'vue';
Vue.set(myObject, 'newProperty', 'new value');Step 4: Fix the Problem
Once you have identified the source of the problem, you can fix it by:
- Using the
Vue.set()method to add a new reactive property to an object. - Using the
Array.prototype.splice()method to modify an array. - Creating a new object or array and replacing the old one.
Practice Question
You are working with an array of objects and you want to add a new object to the array. Which of the following would be the most appropriate?