rules-use-with-caution.md

Priority D Rules: Use with Caution {#priority-d-rules-use-with-caution}

::: warning Note This Vue.js Style Guide is outdated and needs to be reviewed. If you have any questions or suggestions, please open an issue. :::

Some features of Vue exist to accommodate rare edge cases or smoother migrations from a legacy code base. When overused however, they can make your code more difficult to maintain or even become a source of bugs. These rules shine a light on potentially risky features, describing when and why they should be avoided.

Element selectors with scoped {#element-selectors-with-scoped}

Element selectors should be avoided with scoped.

Prefer class selectors over element selectors in scoped styles, because large numbers of element selectors are slow.

::: details Detailed Explanation To scope styles, Vue adds a unique attribute to component elements, such as data-v-f3f3eg9. Then selectors are modified so that only matching elements with this attribute are selected (e.g. button[data-v-f3f3eg9]).

The problem is that large numbers of element-attribute selectors (e.g. button[data-v-f3f3eg9]) will be considerably slower than class-attribute selectors (e.g. .btn-close[data-v-f3f3eg9]), so class selectors should be preferred whenever possible. :::

Bad

<template>
  <button>×</button>
</template>

<style scoped>
button {
  background-color: red;
}
</style>

Good

<template>
  <button class="btn btn-close">×</button>
</template>

<style scoped>
.btn-close {
  background-color: red;
}
</style>

Implicit parent-child communication {#implicit-parent-child-communication}

Props and events should be preferred for parent-child component communication, instead of this.$parent or mutating props.

An ideal Vue application is props down, events up. Sticking to this convention makes your components much easier to understand. However, there are edge cases where prop mutation or this.$parent can simplify two components that are already deeply coupled.

The problem is, there are also many simple cases where these patterns may offer convenience. Beware: do not be seduced into trading simplicity (being able to understand the flow of your state) for short-term convenience (writing less code).

Bad

app.component('TodoItem', {
  props: {
    todo: {
      type: Object,
      required: true
    }
  },

  template: '<input v-model="todo.text">'
})

app.component('TodoItem', {
  props: {
    todo: {
      type: Object,
      required: true
    }
  },

  methods: {
    removeTodo() {
      this.$parent.todos = this.$parent.todos.filter(
        (todo) => todo.id !== vm.todo.id
      )
    }
  },

  template: `
    <span>
      {{ todo.text }}
      <button @click="removeTodo">
        ×
      </button>
    </span>
  `
})

Good

app.component('TodoItem', {
  props: {
    todo: {
      type: Object,
      required: true
    }
  },

  emits: ['input'],

  template: `
    <input
      :value="todo.text"
      @input="$emit('input', $event.target.value)"
    >
  `
})

app.component('TodoItem', {
  props: {
    todo: {
      type: Object,
      required: true
    }
  },

  emits: ['delete'],

  template: `
    <span>
      {{ todo.text }}
      <button @click="$emit('delete')">
        ×
      </button>
    </span>
  `
})

Bad

<script setup>
defineProps({
  todo: {
    type: Object,
    required: true
  }
})
</script>

<template>
  <input v-model="todo.text" />
</template>

<script setup>
import { getCurrentInstance } from 'vue'

const props = defineProps({
  todo: {
    type: Object,
    required: true
  }
})

const instance = getCurrentInstance()

function removeTodo() {
  const parent = instance.parent
  if (!parent) return

  parent.props.todos = parent.props.todos.filter((todo) => {
    return todo.id !== props.todo.id
  })
}
</script>

<template>
  <span>
    {{ todo.text }}
    <button @click="removeTodo">×</button>
  </span>
</template>

Good

<script setup>
defineProps({
  todo: {
    type: Object,
    required: true
  }
})

const emit = defineEmits(['input'])
</script>

<template>
  <input :value="todo.text" @input="emit('input', $event.target.value)" />
</template>

<script setup>
defineProps({
  todo: {
    type: Object,
    required: true
  }
})

const emit = defineEmits(['delete'])
</script>

<template>
  <span>
    {{ todo.text }}
    <button @click="emit('delete')">×</button>
  </span>
</template>