Watchers for side effects in Vue - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a watcher in Vue?

A watcher in Vue is a special function that 'watches' a reactive value or expression and runs code when that value changes. It's used to perform side effects like fetching data or updating something outside the component.

beginner

How do you define a watcher using the Composition API in Vue 3?

You use the watch function by passing the reactive source and a callback function that runs when the source changes. For example:

watch(() => count.value, (newVal, oldVal) => { console.log(newVal) })

intermediate

Why use watchers for side effects instead of computed properties?

Computed properties are for deriving values based on reactive data and are cached. Watchers are for running code that causes side effects, like API calls or manual DOM updates, which should not be done inside computed properties.

intermediate

What does the immediate option do in a Vue watcher?

The immediate option makes the watcher callback run right away when the watcher is created, not just when the watched value changes later. This is useful to run side effects on initial setup.

intermediate

How can you stop a watcher in Vue?

When you call watch, it returns a stop function. Calling this function stops the watcher from running in the future, which helps avoid memory leaks or unwanted side effects.

What is the main purpose of a watcher in Vue?

A To run code when reactive data changes

B To create new components

C To style elements

D To define routes

Watchers run code when reactive data changes, often for side effects.

Which Vue API function is used to create a watcher in the Composition API?

A watchEffect

B watch

C computed

D ref

watch is used to watch reactive sources and run side effect code.

What does setting immediate: true do in a watcher?

A Prevents the watcher from running

B Runs the watcher only once

C Runs the watcher callback immediately on setup

D Delays the watcher callback

The immediate option runs the watcher callback right away when created.

Which of these is NOT a good use case for watchers?

A Calculating a value based on other data

B Updating the page title

C Fetching data when a value changes

D Logging changes to the console

Calculations should be done with computed properties, not watchers.

How do you stop a watcher from running in Vue?

A Remove the component

B Set the watcher to null

C Use <code>unwatch()</code> function

D Call the stop function returned by watch

The watch function returns a stop function to stop the watcher.

Explain how watchers help manage side effects in Vue applications.

Think about when and why you want to run extra code after data changes.

Describe how to create a watcher using Vue 3 Composition API and how to stop it.

Focus on the syntax and lifecycle of a watcher.