@vue/composition-api

What is @vue/composition-api?

@vue/composition-api is a plugin that provides Vue 2.x users with the Composition API, which is a set of APIs that allows you to use Vue 3's reactivity system and component logic composition in Vue 2.x applications.

What are @vue/composition-api's main functionalities?

Reactive State

The `reactive` function is used to create a reactive state object. This allows you to manage state in a reactive manner, similar to Vue 3.

```javascript
import { reactive } from '@vue/composition-api';

export default {
  setup() {
    const state = reactive({ count: 0 });
    return { state };
  }
};
```

Computed Properties

The `computed` function is used to create computed properties that automatically update when their dependencies change.

```javascript
import { computed } from '@vue/composition-api';

export default {
  setup() {
    const state = reactive({ count: 0 });
    const doubleCount = computed(() => state.count * 2);
    return { state, doubleCount };
  }
};
```

Watchers

The `watch` function is used to perform side effects in response to reactive state changes.

```javascript
import { watch } from '@vue/composition-api';

export default {
  setup() {
    const state = reactive({ count: 0 });
    watch(() => state.count, (newValue, oldValue) => {
      console.log(`Count changed from ${oldValue} to ${newValue}`);
    });
    return { state };
  }
};
```

Lifecycle Hooks

The `onMounted` and `onUnmounted` functions are used to register lifecycle hooks within the setup function.

```javascript
import { onMounted, onUnmounted } from '@vue/composition-api';

export default {
  setup() {
    onMounted(() => {
      console.log('Component is mounted');
    });
    onUnmounted(() => {
      console.log('Component is unmounted');
    });
  }
};
```

Other packages similar to @vue/composition-api

vue-function-api

The `vue-function-api` package provides a similar Composition API for Vue 2.x. It allows you to use Vue 3's Composition API features in Vue 2.x applications. However, `@vue/composition-api` is the official plugin provided by the Vue team, making it more reliable and better supported.

vue-hooks

The `vue-hooks` package offers a way to use hooks in Vue components, similar to React hooks. While it provides some similar functionality to the Composition API, it is not as comprehensive or integrated as `@vue/composition-api`.

README

Vue Composition API

Vue Composition API

@vue/composition-api provides a way to use Vue 3.0's Composition api in Vue 2.x.

中文文档

---

Navigation

• Installation
• Usage
• TypeScript
  • TSX
• Limitations
• API
• Changelog

Installation

npm

npm install @vue/composition-api --save

yarn

yarn add @vue/composition-api

CDN

<script src="https://unpkg.com/@vue/composition-api/dist/vue-composition-api.umd.js"></script>

By using the global variable window.vueCompositionApi

Usage

You must install @vue/composition-api via Vue.use() before using other APIs:

import Vue from 'vue';
import VueCompositionApi from '@vue/composition-api';

Vue.use(VueCompositionApi);

After installing the plugin you can use the Composition API to compose your component.

TypeScript

This plugin requires TypeScript version >3.5.1. If you are using vetur, make sure to set vetur.useWorkspaceDependencies to true.

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

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

const Component = defineComponent({
  // type inference enabled
});

const Component = {
  // this will NOT have type inference,
  // because TypeScript can't tell this is options for a Vue component.
};

TSX

:rocket: An Example Repository with TS and TSX support is provided to help you start.

To support TSX, create a declaration file with 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 {
    // tslint:disable no-empty-interface
    interface Element extends VNode {}
    // tslint:disable no-empty-interface
    interface ElementClass extends ComponentRenderProxy {}
    interface ElementAttributesProperty {
      $props: any; // specify the property name to use
    }
    interface IntrinsicElements {
      [elem: string]: any;
    }
  }
}

Limitations

Ref Unwrap

Unwrap is not working with Array index.

Should not store ref as a direct child of Array:

const state = reactive({
  list: [ref(0)],
});
// no unwrap, `.value` is required
state.list[0].value === 0; // true

state.list.push(ref(1));
// no unwrap, `.value` is required
state.list[1].value === 1; // true

Should not use ref in a plain object when working with Array:

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

Should always use ref in a reactive when working with Array:

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

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

Using reactive will mutate the origin object

This is an limitation of using Vue.observable in Vue 2.

Vue 3 will return an new proxy object.

---

watch() API

onTrack and onTrigger are not available in WatchOptions.

---

Template Refs

:white_check_mark: Support     :x: Not Supported

:white_check_mark: String ref && return it from setup():

<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>

:white_check_mark: String ref && return it from setup() && Render Function / JSX:

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" />;
  },
};

:x: Function ref:

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

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

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

:x: Render Function / JSX in setup():

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

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

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

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

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

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" />;
  },
};

You may also need to augment the SetupContext when working with TypeScript:

import Vue from 'vue';
import VueCompositionApi from '@vue/composition-api';

Vue.use(VueCompositionApi);

declare module '@vue/composition-api/dist/component/component' {
  interface SetupContext {
    readonly refs: { [key: string]: Vue | Element | Vue[] | Element[] };
  }
}

SSR

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,
    };
  },
};

Changelog

0.4.0

• Refactor: rename createComponent to defineComponent (the createComponent function is still there but deprecated) #230
• Fix: correct the symbol check; fixes the compatibility issue in iOS 9 #218
• Fix: avoid accessing undeclared instance fields on type-level; fixes Vetur template type checking; fixes vue-router type compatibility #189
• Fix: onUnmounted should not be run on deactivated #217