README.md

use-safe-async-mount
_{The Asynchronous Functional Component Mounter}

The Problem

This is a useEffect hook with zero dependencies:

useEffect(() => {
  // Code that runs "on mount" (unfortunately)
}, [])

It is the most incorrectly [and dangerously] used part of the react functional component lifecycle.

Whenever using this hook (with the empty dependency array), you should ask yourself two questions:

1. Am I using this to just initialize a variable based on some synchronously computed value?

   • useEffect(() => { setValue(computeMyValue()) }, [])

2. Am I using this to just conditionally initialize a state variable based on props?

   • useEffect(() => { setValue(someProp ? "a" : "b") }, [])

If either of these situations describes you, there's a high chance you should be just be computing the value in-line or using useMemo. This saves you from having to deal with the initial render when your value (useState, useRef, var, etc.) is undefined.

Here's some situations that do fit in the empty useEffect hook:

• Making a network request
• Setting/removing an event listener
• UI-related analytics tracking

Async State

When a component's state depends on a value gathered from an async function, the common solution is to manually invoke it directly from the empty useEffect hook:

The Traditional Pattern

function ExampleComponent() {
  const [stateOne, setStateOne] = useState()

  useEffect(() => {
    someAsyncFunction().then(res => setStateOne(res))
    // OR
    const run = async () => {
      const res = await someAsyncFunction()
      setStateOne(res)
    }
    run();
  }, [])

  return (
    <>
        <h1>My Example Component</h1>
        {stateOne && <p>{stateOne}</p>}
    </>
  )
}

The Drawbacks

There's three negative implications to this solution:

1. When the component initially mounts, the variable is undefined. This requires more render logic.
2. The component may unmount during the asynchronous request. Setting a state variable on an unmounted component is a memory leak and will throw an error.
3. In TypeScript projects, your compiler won't recognize that your variable has been defined. That means !'s everywhere.

The Solution

Installation

npm i use-safe-async-mount

use-safe-async-mount solves these problems by acting as a true hook-based, type-safe componentWillMount implementation.

Example

import useSafeAsyncMount from 'use-safe-async-mount';

function ExampleComponent() {

  const { SafeRender } = useSafeAsyncMount(async isActive => {
    const res = await someAsyncFunction()
    if (isActive()) {
      // ^ This avoids setting component state after unmount
      
      // These values are defined and type-safe in the `SafeRender` component
      return { 
        stateOne: "Some value my component depends on",
        stateTwo: res
      }
    }
  })

  return (
    <>
      <h1>My Example Component</h1>
      <SafeRender>
        {({ stateOne, stateTwo }) => (
          <p>{stateOne}</p>
        )}
      </SafeRender>
    </>
  )
}

Example

Here's an interactive example and the associated source code.

Inspirations

• use-async-effect