Handle your promises with style 🎀

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npm install vue-promised
# or
yarn add vue-promised

If you are using Vue 2, you also need to install @vue/composition-api:

yarn add @vue/composition-api

When dealing with asynchronous requests like fetching content through API calls, you may want to display the loading state with a spinner, handle the error and even hide everything until at least 200ms have been elapsed so the user doesn't see a loading spinner flashing when the request takes very little time. This is quite some boilerplate, and you need to repeat this for every request you want:

<template>
  <div>
    <p v-if="error">Error: {{ error.message }}</p>
    <p v-else-if="isLoading && isDelayElapsed">Loading...</p>
    <ul v-else-if="!isLoading">
      <li v-for="user in data">{{ user.name }}</li>
    </ul>
  </div>
</template>

<script>
export default {
  data: () => ({
    isLoading: false,
    error: null,
    data: null,
    isDelayElapsed: false,
  }),

  methods: {
    fetchUsers() {
      this.error = null
      this.isLoading = true
      this.isDelayElapsed = false
      getUsers()
        .then((users) => {
          this.data = users
        })
        .catch((error) => {
          this.error = error
        })
        .finally(() => {
          this.isLoading = false
        })
      setTimeout(() => {
        this.isDelayElapsed = true
      }, 200)
    },
  },

  created() {
    this.fetchUsers()
  },
}
</script>

👉 Compare this to the version using Vue Promised that handles new promises.

That is quite a lot of boilerplate and it's not handling cancelling on going requests when fetchUsers is called again. Vue Promised encapsulates all of that to reduce the boilerplate.

Check the Changelog for breaking changes. v2 exposes the same Promised and a new usePromise function on top of that.

import { Promised, usePromise } from 'vue-promised'

Vue.component('Promised', Promised)
export default {
  setup() {
    const usersPromise = ref(fetchUsers())
    const promised = usePromise(usersPromise)

    return {
      ...promised,
      // spreads the following properties:
      // data, isPending, isDelayElapsed, error
    }
  },
}

Vue Promised also exposes the same API via a component named Promised. In the following examples, promise is a Promise but can initially be null. data contains the result of the promise. You can of course name it the way you want:

<template>
  <Promised :promise="usersPromise">
    <!-- Use the "pending" slot to display a loading message -->
    <template v-slot:pending>
      <p>Loading...</p>
    </template>
    <!-- The default scoped slot will be used as the result -->
    <template v-slot="data">
      <ul>
        <li v-for="user in data">{{ user.name }}</li>
      </ul>
    </template>
    <!-- The "rejected" scoped slot will be used if there is an error -->
    <template v-slot:rejected="error">
      <p>Error: {{ error.message }}</p>
    </template>
  </Promised>
</template>

<script>
export default {
  data: () => ({ usersPromise: null }),

  created() {
    this.usersPromise = this.getUsers()
  },
}
</script>

Note the pending slot will by default, display after a 200ms delay. This is a reasonable default to avoid layout shifts when API calls are fast enough. The perceived speed is also higher. You can customize it with the pendingDelay prop.

The pending slot can also receive the data that was previously available:

<Promised :promise="usersPromise">
  <template v-slot:pending="previousData">
    <p>Refreshing</p>
    <ul>
      <li v-for="user in previousData">{{ user.name }}</li>
    </ul>
  </template>
  <template v-slot="data">
    <ul>
      <li v-for="user in data">{{ user.name }}</li>
    </ul>
  </template>
</Promised>

Although, depending on the use case, this could create duplication and using a combined slot would be a better approach.

Sometimes, you need to customize how things are displayed rather than what is displayed. Disabling a search input, displaying an overlaying spinner, etc. Instead of using multiple slots, you can provide one single combined slot that will receive a context with all relevant information. That way you can customize the props of a component, toggle content with your own v-if but still benefit from a declarative approach:

<Promised :promise="promise">
  <template v-slot:combined="{ isPending, isDelayElapsed, data, error }">
    <pre>
      pending: {{ isPending }}
      is delay over: {{ isDelayElapsed }}
      data: {{ data }}
      error: {{ error && error.message }}
    </pre>
  </template>
</Promised>

This allows to create more advanced async templates like this one featuring a Search component that must be displayed while the searchResults are being fetched:

<Promised :promise="searchResults" :pending-delay="200">
  <template v-slot:combined="{ isPending, isDelayElapsed, data, error }">
    <div>
      <!-- data contains previous data or null when starting -->
      <Search :disabled-pagination="isPending || error" :items="data || []">
        <!-- The Search handles filtering logic with pagination -->
        <template v-slot="{ results, query }">
          <ProfileCard v-for="user in results" :user="user" />
        </template>
        <!--
          Display a loading spinner only if an initial delay of 200ms is elapsed
        -->
        <template v-slot:loading>
          <MySpinner v-if="isPending && isDelayElapsed" />
        </template>
        <!-- `query` is the same as in the default slot -->
        <template v-slot:noResults="{ query }">
          <p v-if="error" class="error">Error: {{ error.message }}</p>
          <p v-else class="info">No results for "{{ query }}"</p>
        </template>
      </Search>
    </div>
  </template>
</Promised>

• isPending: is true while the promise is in a pending status. Becomes false once the promise is resolved or rejected. It is reset to true when the promise prop changes.
• isRejected is false. Becomes true once the promise is rejected. It is reset to false when the promise prop changes.
• isResolved is false. Becomes true once the promise is resolved. It is reset to false when the promise prop changes.
• isDelayElapsed: is true once the pendingDelay is over or if pendingDelay is 0. Becomes false after the specified delay (200 by default). It is reset when the promise prop changes.
• data: contains the last resolved value from promise. This means it will contain the previous succesfully (non cancelled) result.
• error: contains last rejection or null if the promise was fullfiled.

There are different ways to provide a promise to Promised. The first one, is setting it in the created hook:

export default {
  data: () => ({ promise: null }),
  created() {
    this.promise = fetchData()
  },
}

But most of the time, you can use a computed property. This makes even more sense if you are passing a prop or a data property to the function returning a promise (fetchData in the example):

export default {
  props: ['id'],
  computed: {
    promise() {
      return fetchData(this.id)
    },
  },
}

You can also set the promise prop to null to reset the Promised component to the initial state: no error, no data, and pending:

export default {
  data: () => ({ promise: null }),
  methods: {
    resetPromise() {
      this.promise = null
    },
  },
}

usePromise returns an object of Ref representing the state of the promise.

const { data, error, isPending, isDelayElapsed } = usePromise(fetchUsers())

Signature:

function usePromise<T = unknown>(
  promise: Ref<Promise<T> | null | undefined> | Promise<T> | null | undefined,
  pendingDelay?: Ref<number | string> | number | string
): {
  isPending: Ref<boolean>
  isDelayElapsed: Ref<boolean>
  error: Ref<Error | null | undefined>
  data: Ref<T | null | undefined>
}

Promised will watch its prop promise and change its state accordingly.

All slots but combined can be used as scoped or regular slots.

MIT