# Guidelines

Here are the guidelines for VueUse functions. You could also take them as a reference for authoring your own composable functions or apps.

You can also find some reasons for those design decisions and also some tips for writing composable functions with Anthony Fu's talk about VueUse:

• Composable Vue - at VueDay 2021
• 可组合的 Vue - at VueConf China 2021 (in Chinese)

# General

• Import all Vue APIs from "vue-demi"
• Use ref instead reactive whenever possible
• Use options object as arguments whenever possible to be more flexible for future extensions.
• Use shallowRef instead of ref when wrapping large amounts of data.
• Use configurableWindow (etc.) when using global variables like window to be flexible when working with multi-windows, testing mocks, and SSR.
• When involved with Web APIs that are not yet implemented by the browser widely, also outputs isSupported flag
• When using watch or watchEffect internally, also make the immediate and flush options configurable whenever possible
• Use tryOnUnmounted to clear the side-effects gracefully
• Avoid using console logs

Read also: Best Practice

# ShallowRef

Use shallowRef instead of ref when wrapping large amounts of data.

export function useFetch<T>(url: MaybeRef<string>) {
  // use `shallowRef` to prevent deep reactivity
  const data = shallowRef<T | undefined>()
  const error = shallowRef<Error | undefined>()

  fetch(unref(url))
    .then(r => r.json())
    .then(r => data.value = r)
    .catch(e => error.value = e)

  /* ... */
}

# Configurable Globals

When using global variables like window or document, support configurableWindow or configurableDocument in the options interface to make the function flexible when for scenarios like multi-windows, testing mocks, and SSR.

Learn more about the implementation: _configurable.ts

import { ConfigurableWindow, defaultWindow } from '../_configurable'

export function useActiveElement<T extends HTMLElement>(
  options: ConfigurableWindow = {}
) {
  const {
    // defaultWindow = isClient ? window : undefined
    window = defaultWindow
  } = options

  let el: T
  
  // skip when in Node.js environment (SSR)
  if (window) {
    window.addEventListener('blur', () => {
      el = window?.document.activeElement
    }, true)
  }

  /* ... */
}

Usage example:

// in iframe and bind to the parent window
useActiveElement({ window: window.parent })

# Watch Options

When using watch or watchEffect internally, also make the immediate and flush options configurable whenever possible. For example debouncedWatch

import { WatchOptions } from 'vue-demi'

// extend the watch options
export interface DebouncedWatchOptions extends WatchOptions {
  debounce?: number
}

export function debouncedWatch(
  source: any,
  cb: any,
  options: DebouncedWatchOptions = {},
): WatchStopHandle {
  return watch(
    source,
    () => /* ... */,
    options, // pass watch options
  )
}

# Controls

We use the controls option allowing users to use functions with a single return for simple usages, while being able to have more controls and flexibility when needed. Read more: #362.

# When to provide a controls option

• The function is more commonly used with single ref or
• Examples: useTimestamp useInterval

// common usage
const timestamp = useTimestamp()

// more controls for flexibility
const { timestamp, pause, resume } = useTimestamp({ controls: true })

Refer to useTimestamps source code for the implementation of proper TypeScript support.

# When NOT to provide a controls option

• The function is more commonly used with multiple returns
• Examples: useRafFn useRefHistory

const { pause, resume } = useRafFn(() => {})

# isSupported Flag

When involved with Web APIs that are not yet implemented by the browser widely, also outputs isSupported flag.

For example useShare

export function useShare(
  shareOptions: MaybeRef<ShareOptions> = {},
  options: ConfigurableNavigator = {}
) {
  const { navigator = defaultNavigator } = options
  const isSupported = navigator && 'canShare' in navigator

  const share = async(overrideOptions) => {
    if (isSupported) {
      /* ...implementation */ 
    }
  }

  return {
    isSupported,
    share,
  }
}

# Renderless Components

• Use render functions instead of Vue SFC
• Wrap the props in reactive to easily pass them as props to the slot
• Prefer to use the functions options as prop types instead of recreating them yourself
• Only wrap the slot in an HTML element if the function needs a target to bind to

import { defineComponent, reactive } from 'vue-demi'
import { useMouse, MouseOptions } from '@vueuse/core'

export const UseMouse = defineComponent<MouseOptions>({
  name: 'UseMouse',
  props: ['touch', 'resetOnTouchEnds', 'initialValue'] as unknown as undefined,
  setup(props, { slots }) {
    const data = reactive(useMouse(props))

    return () => {
      if (slots.default)
        return slots.default(data)
    }
  },
})

Sometimes a function may have multiple parameters, in that case, you maybe need to create a new interface to merge all the interfaces into a single interface for the component props.

import { useTimeAgo, TimeAgoOptions } from '@vueuse/core'

interface UseTimeAgoComponentOptions extends Omit<TimeAgoOptions<true>, 'controls'> {
  time: MaybeRef<Date | number | string>
}

export const UseTimeAgo = defineComponent<UseTimeAgoComponentOptions>({...})