3 patterns for writing async code in Javascript

Three useful patterns to make better use of async/await code in Javascript

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1. Use .catch function instead of handleError

Don’t: Pass handleError directly as a second parameter of .then. handleError does not catch error thrown by process:

getPromise().then(process, handleError)

Do: Pass handleError in .catch. .catch works for errors thrown by either getPromise or process:

getPromise().then(process).catch(handleError)

2. Use finally to execute some code after a promise fulfils or rejects

Don’t: Invoke the same code in .then and .catch

myPromise
.then(response => {
  doSomething(response)
  runFinalCode()
})
.catch(e => {
  returnError(e)
  runFinalCode()
})

Do: Make use of finally

myPromise
.then(response => {
  doSomething(response)
})
.catch(e => {
  returnError(e)
})
.finally(() => {
  runFinalCode()
})

3. Wrapping ephemeral resources inside an async function

This pattern builds on the previous one to make sure the resources are released when the function resolves or crashes.

const withTempDir = async (fn) => {
  const dir = await fs.mkdtemp(
    await fs.realpath(os.tmpdir()) + path.sep
  )
  try {
    return await fn(dir)
  } finally {
    fs.rmdir(dir, {recursive: true})
  }
}

await withTempDir((dir) => {
  // use dir to store temporary things
  // dir will always be delete when this function finishes or crashes
})