Introduction to Suspense


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Vue 3 has a new feature called “Suspense”. I find the naming to be a bit unintuitive - I had no idea what to expect when I first heard the name. I feel like a more descriptive name would be “LoadWithFallback” (although not as sexy). Anyway, today we will explore this new feature by building a simple app that shows some Covid19 (aka coronavirus) data.

Parsing the dataset

The dataset I am using is found here. I wrote some code to transform it - I wanted to get the total fatality count in each country. Let’s take a look at the code and test:

The test looks like this:

import { parseData, Data } from './parseResponse'

const data = require('./covid19.json') as Data

test('parses covid19.json', () => {
  const expected: Record<string, number> = {
    Afghanistan: 3
  }

  const actual = parseData(data)

  expect(actual.Afghanistan).toBe(3)
})

I like to delcare my expected value, up front, with typings before I even start writing the test. It makes it clear what the code does, and the compiler can help me out if I make a mistake.

The actual implementation looks like this:

interface Day {
  date: string
  confirmed: number
  deaths: number
  recovered: number
}

export type Data = Record<string, Day[]>

const parseData = (data: Data): Record<string, number> => {
  const map: Record<string, number> = {}

  for (const [key, days] of Object.entries(data)) {
    if (!map[key]) {
      map[key] = 0
    }

    for (const day of days) {
      map[key] += day.deaths
    }
  }

  return map
}

export { parseData }

Nothing too exciting - we loop over the data and sum up the fatalities.

Creating App.vue

We start off by defining App.vue, which contains the <Suspense> component. For now, let’s focus on the <template>:

<template>
  <div v-if="error">
    Error: {{ error }}
  </div>

  <Suspense v-else>
    <template #default>
      <DataTable />
    </template>

    <template #fallback>
      Fallback
    </template>
  </Suspense>
</template>

The first part is fairly simple - if there is an error, we render it. Next we have <Suspense>. When using <Suspense>, you need to provide templates. One is #default, which is a component with an async setup function. In this case, that will be <DataTable>. The other template is #fallback, which is displayed until the async setup function returns.

Before we get into any implementation details, let’s look at the test. <DataTable> will make a HTTP call to fetch the Covid 19 data, so we need to consider the loading state, error state and success state. App.spec.ts looks like this:

describe('App', () => {
  test('data has not loaded', async () => {
    const wrapper = mount(App)

    expect(wrapper.html()).toContain('Fallback')
  })

  test('data has loaded', async () => {
    const wrapper = mount(App)
    await flushPromises()

    expect(wrapper.html()).toContain('Afghanistan: 3')
  })

  test('an error occurred', async () => {
    const wrapper = mount(App)
    await flushPromises()

    expect(wrapper.html()).toContain('An error occurred')
  })
})

The tests are pretty standard. One caveat is flushPromises - we will need this to force the async setup function to resolve, letting us test the success and error states.

Implementing DataTable

The <DataTable> just makes a HTTP call with axios, then transforms the data using the parseData function we defined earlier.

<template>
  <div v-for="country of countries" :key="country.name">
    {{ country.name }}: {{ country.deaths }}
  </div>
</template>

<script lang="ts">
import { defineComponent } from 'vue'
import axios from 'axios'

import { parseData } from './parseResponse'

export default defineComponent({
  async setup() {
    const response = await axios.get('/api/covid19')
    const data = parseData(response.data)

    return { 
      countries: Object.keys(data).map(name => ({ name, deaths: data[name] })
    }
  }
})
</script>

Pretty standard stuff - nothing here should be new, as long as you have some exposure to the Composition API. Let’s use this in App.vue:

<template>
  <!-- Omitted for brevity -->
</template>

<script lang="ts">
import { defineComponent, ref } from 'vue'
import axios from 'axios'

import DataTable from './DataTable.vue'
import { parseData } from './parseResponse'

export default defineComponent({
  components: {
    DataTable
  },

  setup() {
    const error = ref<string | null>(null)

    return {
      error
    }
  }
})
</script>

Testing Suspense

We need to mock axios in the test - let’s do that. Add the following code to the top of App.spec.ts

import flushPromises from 'flush-promises'

import { mount } from '../src'
import App from './App.vue'

jest.mock('axios', () => ({
  get: () => {
    return { data: require('./covid19.json') }
  }
}))

describe('App', () => {
 // ..
})

Running this will get the first two tests - success and loading - to pass! Previously, you would have to have defined a loading variable, and did some v-if and v-else combination. <Suspense> makes this really clean.

The last test is pretty simple to get passing. We need some way to throw an error in the test. Update the axios mock, and the last test:

let mockShouldError = false 

jest.mock('axios', () => ({
  get: () => {
    if (mockShouldError) {
      throw new Error('ERROR!!!')
    }
    return { data: require('./covid19.json') }
  }
}))

describe('App', () => {
  // ...

  test('an error occurred', async () => {
    mockShouldError = true // <- added this
    const wrapper = mount(App)
    await flushPromises()

    expect(wrapper.html()).toContain('An error occurred')
  })
})

Setting mockShouldError will simulate the axios call erroring out. Finally, we need to handle the error in the component with <Suspense>, in this case App.vue. We do so with the onErrorCaptured hook:

<template>
  <!-- Omitted for brevity -->
</template>

<script lang="ts">
import { defineComponent, onErrorCaptured, ref } from 'vue'
import axios from 'axios'

import DataTable from './DataTable.vue'
import { parseData } from './parseResponse'

export default defineComponent({
  components: {
    DataTable
  },

  setup() {
    const error = ref<string | null>(null)
    onErrorCaptured(e => {
      error.value = 'An error occurred'
      return true
    })

    return {
      error
    }
  }
})
</script>

Make sure you return true - otherwise the error will not be captured, and bubble up to the top of your app.

Conclusion

This simple demo introduced the new <Suspense> component and onErrorCaptured hook, and showed how is simplifies the common use case of fetchign data asynchronously when a component is mounted.


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