Async await

async/await is a JavaScript syntactic sugar combining Promise and generator to create a powerful asynchronous mechanism.

You have to declare a function to be async before you can use await within the function.

// error
function foo() {
  return await doSomeAsyncWork()
}

when you add await to a statement, it will check if the result of the statement is an Promise. If it is, then the code will be blocked and wait for the promise to resolve.

Concurrent execution

Using async/await may negatively impact your code performance.

// bad, this function will not return until `p1` and `p2` are resolved
async function doMultipleThings() {
  const p1 = doSomeAsyncWork()
  const p2 = doSomeOtherAsyncWork()
  return await p1 + await p2 + 1
}

// good, no blocking
function doMultipleThings() {
  const p1 = doSomeAsyncWork()
  const p2 = doSomeOtherAsyncWork()
  return Promise.all([p1, p2]).then(([x, y]) => x + y + 1)
}

Why?

await will block the call until the Promise is resolved, it may slow down your code.

Return await

Should void return await.

// bad
async function foo() {
  return await doSomethingAsync()
}

// good
async function foo() {
  return doSomethingAsync()
}

// also good
async function tryDo() {
  try {
    return await doSomethingAsync()
  }
  catch (e) {
    // process the rejected value
  }
}

Why?

return await will cause your code to wait for the promise instead of propagating the promise to the caller. This is a common mistake made by novice and most likely is a mistake.

Why Not?

Sometimes you do want to wait for the promise. For example, you want to try-catch the rejected promise.

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