async-await.md

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.

• https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/async_function
• https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/async_function

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.