Common Mistakes in

(And, more importantly... how to avoid them)

ue.js

@danielkelly_io

@danielkellyio

Alabama, USA

Daniel Kelly

Lead Instructor @ Vue School

Full Stack developer (10 + years)

Husband and Father

presentation Icons from fontawesome.com

Why this topic

?

  • We're all human and make mistakes
  • Simple mistakes can cause big headaches
  • Learn from the mistakes of others
  • It's a very practical topic

Why this topic?

?

Mistake # 1

index as :key

<li 
  v-for="(item, index) in items" 
  :key="index"
>...</li>

What's wrong with this? 🤔

index as :key

<li 
  v-for="(item, index) in items" 
  :key="index"
>...</li>

What's wrong with this? 🤔

❌

index as :key

Bug Manifests Under 2 Conditions

  • When the elements in the array change order
  • When a nested element or component has it's own state

It's tricky!

Let's look at an example!

index as :key

It's not just for loops!

Let's look at an example!

Mistake # 2

Non-reactive value as dep

const cookiesAccepted = computed(()=> 
  localStorage.getItem("cookiesAccepted")
)

What's wrong with this? 🤔

Non-reactive value as dep

const cookiesAccepted = computed(()=> 
  localStorage.getItem("cookiesAccepted")
)

What's wrong with this? 🤔

Non-reactive value as dep

const cookiesAccepted = computed(()=> 
  localStorage.getItem("cookiesAccepted")
) // 0

localStorage.setItem("cookiesAccepted", 1)

cookiesAccepted.value // 0

Changing the value of local storage doesn't update the value of the computed prop.

const accepted = 0;
const cookiesAccepted = computed(()=> accepted)

This is more obvious

Non-reactive value as dep

Often see this with browser API's:

  • localStorage
  • Date()
  • DOM element properties
    (like clientWidth, etc)
  • etc

Non-reactive value as dep

And, of course, usually these hide in more complex computed props

(nothing as obvious as our simple example)

What's the solution?

🤔

Reactive Deps

VueUse

VueUse

handy reactive wrapper around browser APIs

Let's look at an example!

Mistake # 3

Replacing Reactive State

let posts = reactive([
  'Post 1',
  'Post 2'
]);

function loadMore(){
  // fetch more posts, etc
  posts = newPosts;
}

What's wrong

with this? 🤔

Replacing Reactive State

let posts = reactive([
  'Post 1',
  'Post 2'
]);

function loadMore(){
  // fetch more posts, etc
  posts = newPosts;
}

What's wrong

with this? 🤔

👈 replacing reactive

kills reactivity

Replacing Reactive State

Replacing Reactive State

Let's look at some examples!

Moral of the story:

just use ref() everywhere!

I'm not the only one, 

it's the recommendation of the official Vue.js docs

I'm not the only one, 

it's the recommendation of the official Vue.js docs

Oh and one more reason to use ref() everywhere

default watcher depth is different between ref() and reactive()

Let's see an example of that

Mistake # 4

Mutating Props

<script setup lang="ts">
defineProps<{
  modelValue: string;
}>();
</script>

<template>
  <input id="username" v-model="modelValue" />
</template>

What's wrong with this? 🤔

Mutating Props

<script setup lang="ts">
defineProps<{
  modelValue: string;
}>();
</script>

<template>
  <input id="username" v-model="modelValue" />
</template>

What's wrong with this? 🤔

Mutating a prop

Let's look at some examples!

Common Vue.js Mistakes and How to Avoid Them

🔥 Free

https://vueschool.io/courses/common-vue-js-mistakes-and-how-to-avoid-them

 

  • Omitting the Key Directive on v-for
  • Prop Drilling
  • Watching Arrays the Wrong Way
  • Replacing Reactive State the Wrong Way
  • Unintentionally Mutating Props
  • Forgetting to Clean Up Your Manual Event Listeners
  • Expecting Changes to Non-Reactive Dependencies to Trigger Updates
  • Not Considering TypeScript
  • Destructuring Reactive Data
  • Calling Composables in the Wrong Place
  • Using v-html with User Provided Data
  • Unnecessary Manual DOM Manipulation

 

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https://vueschool.io/sales/blackfriday

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danielkelly_io

I'll tweet out the slides and the codebase after this

Thank you 🙏

Go code awesome things!

(with Vue 😉)