Vue 2 plugin for Composition API

English | 中文 ・ Composition API Docs

⚠️ With the release of Vue 2.7, which has Composition API built-in, you no longer need this plugin. Thereby this plugin has entered maintenance mode and will only support Vue 2.6 or earlier. This project will reach End of Life by the end of 2022.

npm install @vue/composition-api
# or
yarn add @vue/composition-api

You must install @vue/composition-api as a plugin via Vue.use() before you can use the Composition API to compose your component.

import Vue from 'vue'
import VueCompositionAPI from '@vue/composition-api'

Vue.use(VueCompositionAPI)

// use the APIs
import { ref, reactive } from '@vue/composition-api'

💡 When you migrate to Vue 3, just replacing @vue/composition-api to vue and your code should just work.

Include @vue/composition-api after Vue and it will install itself automatically.

<script src="https://cdn.jsdelivr.net/npm/vue@2.6"></script>
<script src="https://cdn.jsdelivr.net/npm/@vue/composition-api@1.7.2"></script>

@vue/composition-api will be exposed to global variable window.VueCompositionAPI.

const { ref, reactive } = VueCompositionAPI

TypeScript version >4.2 is required

To let TypeScript properly infer types inside Vue component options, you need to define components with defineComponent

import { defineComponent } from '@vue/composition-api'

export default defineComponent({
  // type inference enabled
})

JSX is now officially supported on vuejs/jsx. You can enable it by following this document. A community maintained version can be found at babel-preset-vca-jsx by @luwanquan.

To support TSX, create a declaration file with the following content in your project.

// file: shim-tsx.d.ts
import Vue, { VNode } from 'vue';
import { ComponentRenderProxy } from '@vue/composition-api';

declare global {
  namespace JSX {
    interface Element extends VNode {}
    interface ElementClass extends ComponentRenderProxy {}
    interface ElementAttributesProperty {
      $props: any; // specify the property name to use
    }
    interface IntrinsicElements {
      [elem: string]: any;
    }
  }
}

Even if there is no definitive Vue 3 API for SSR yet, this plugin implements the onServerPrefetch lifecycle hook that allows you to use the serverPrefetch hook found in the classic API.

import { onServerPrefetch } from '@vue/composition-api'

export default {
  setup(props, { ssrContext }) {
    const result = ref()

    onServerPrefetch(async () => {
      result.value = await callApi(ssrContext.someId)
    })

    return {
      result,
    }
  }
}

@vue/composition-api supports all modern browsers and IE11+. For lower versions IE you should install WeakMap polyfill (for example from core-js package).

✅ Support     ❌ Not Supported

const a = {
  count: ref(0),
}
const b = reactive({
  list: [a], // `a.count` will not unwrap!!
})

// no unwrap for `count`, `.value` is required
b.list[0].count.value === 0 // true

const b = reactive({
  list: [
    {
      count: ref(0), // no unwrap!!
    },
  ],
})

// no unwrap for `count`, `.value` is required
b.list[0].count.value === 0 // true

const a = reactive({
  list: [
    reactive({
      count: ref(0),
    }),
  ]
})
// unwrapped
a.list[0].count === 0 // true

a.list.push(
  reactive({
    count: ref(1),
  })
)
// unwrapped
a.list[1].count === 1 // true

<template>
  <div ref="root"></div>
</template>

<script>
  export default {
    setup() {
      const root = ref(null)

      onMounted(() => {
        // the DOM element will be assigned to the ref after initial render
        console.log(root.value) // <div/>
      })

      return {
        root,
      }
    },
  }
</script>

export default {
  setup() {
    const root = ref(null)

    onMounted(() => {
      // the DOM element will be assigned to the ref after initial render
      console.log(root.value) // <div/>
    })

    return {
      root,
    }
  },
  render() {
    // with JSX
    return () => <div ref="root" />
  },
}

<template>
  <div :ref="el => root = el"></div>
</template>

<script>
  export default {
    setup() {
      const root = ref(null)

      return {
        root,
      }
    },
  }
</script>

export default {
  setup() {
    const root = ref(null)

    return () =>
      h('div', {
        ref: root,
      })

    // with JSX
    return () => <div ref={root} />
  },
}

⚠️ Warning: The SetupContext.refs won't exist in Vue 3.0. @vue/composition-api provide it as a workaround here.

If you really want to use template refs in this case, you can access vm.$refs via SetupContext.refs

export default {
  setup(initProps, setupContext) {
    const refs = setupContext.refs
    onMounted(() => {
      // the DOM element will be assigned to the ref after initial render
      console.log(refs.root) // <div/>
    })

    return () =>
      h('div', {
        ref: 'root',
      })

    // with JSX
    return () => <div ref="root" />
  },
}

reactive uses Vue.observable underneath which will mutate the original object.

💡 In Vue 3, it will return a new proxy object.

⚠️ Warning: set and del do NOT exist in Vue 3. We provide them as a workaround here, due to the limitation of Vue 2.x reactivity system.

In Vue 2, you will need to call set to track new keys on an object(similar to Vue.set but for reactive objects created by the Composition API). In Vue 3, you can just assign them like normal objects.

Similarly, in Vue 2 you will need to call del to ensure a key deletion triggers view updates in reactive objects (similar to Vue.delete but for reactive objects created by the Composition API). In Vue 3 you can just delete them by calling delete foo.bar.

import { reactive, set, del } from '@vue/composition-api'

const a = reactive({
  foo: 1
})

// add new reactive key
set(a, 'bar', 1)

// remove a key and trigger reactivity
del(a, 'bar')

watch(() => {
    /* ... */
}, {
  immediate: true,
  onTrack() {}, // not available
  onTrigger() {}, // not available
})

In Vue 3, createApp() is introduced to provide context(plugin, components, etc.) isolation between app instances. Due the design of Vue 2, in this plugin, we provide createApp() as a forward compatible API which is just an alias of the global.

const app1 = createApp(RootComponent1)
app1.component('Foo', Foo) // equivalent to Vue.component('Foo', Foo)
app1.use(VueRouter) // equivalent to Vue.use(VueRouter)

const app2 = createApp(RootComponent2)
app2.component('Bar', Bar) // equivalent to Vue.component('Bar', Bar)

createElement / h in Vue 2 is only accessable in render() function. To use it outside of render(), you can explicitly bind a component instance to it.

⚠️ Warning: This ability is provided as a workaround Vue 2, it's not part of the Vue 3 API.

import { h as _h } from '@vue/composition-api'

export default {
  setup() {
    const vm = getCurrentInstance()
    const h = _h.bind(vm)

    return () =>
      h('div', {
        ref: 'root',
      })
  },
}

💡 In Vue 3, it will return a new proxy object.

readonly() is provided as API alignment with Vue 3 on type-level only. Use isReadonly() on it or it's properties can not be guaranteed.

defineComponent({
  setup(props) {
    const { bar } = toRefs(props.foo) // it will `warn`

    // use this instead
    const { foo } = toRefs(props)
    const a = foo.value.bar
  }
})

Due to the difference in implementation, there is no such concept as a ReactiveEffect in @vue/composition-api. Therefore, effect is merely true to enable differentiating computed from refs:

function isComputed<T>(o: ComputedRef<T> | unknown): o is ComputedRef<T>
function isComputed(o: any): o is ComputedRef {
  return !!(isRef(o) && o.effect)
}

The following APIs introduced in Vue 3 are not available in this plugin.

• onRenderTracked
• onRenderTriggered
• isProxy

export default {
  data() {
    return {
      // will result { a: { value: 1 } } in template
      a: ref(1),
    }
  },
}

defineComponent({
  emits: {
    // has no effects
    submit: (eventOption) => {
      if (...) {
        return true
      } else {
        console.warn('Invalid submit event payload!')
        return false
      }
    }
  }
})

Due the limitation of Vue2's public API. @vue/composition-api inevitably introduces some performance overhead. Note that in most scenarios, this shouldn't be the source of performance issues.

You can check the benchmark results for more details.