Understanding React.Suspense

This is a new feature that allows us to "stop" a render until we have finished a task (e.g. loading data from an API).

We are going to fetch a task when the Task component is mounted and save the result in a very simple cache. The interesting part to understand is that when we throw a promise, it is caught by Suspense and shows a loading state until it is resolved.

import React, { Suspense } from 'react'
import ReactDOM from 'react-dom'
import './styles.css'

function fetchFirstTask() {
  return fetch('https://jsonplaceholder.typicode.com/todos/1').then((response) => response.json())
}

let cache = null

function Task() {
  if (!cache) {
    throw fetchFirstTask().then((task) => (cache = task))
  }
  return (
    <div>
      {cache.completed ? '✅' : '⛔️'} {cache.title}
    </div>
  )
}

function App() {
  return (
    <div className="App">
      <h1>My task</h1>
      <Suspense fallback={<div>Loading...</div>}>
        <Task />
      </Suspense>
    </div>
  )
}

Sometimes we have a fast connection and the resource is loaded very quickly. In this case, it may not be necessary to show a loading state, so we can use maxDuration to avoid this weird blink.

<Suspense maxDuration={400} fallback={<div>Loading...</div>}>
  <Task />
</Suspense>