How to know if a function is async?

Question

I have to pass a function to another function, and execute it as a callback. The problem is that sometimes this function is async, like:

async function() {
          


        

Answer 1

Theory

Native async functions may be identifiable when being converted to strings:

asyncFn[Symbol.toStringTag] === 'AsyncFunction'

Or by AsyncFunction constructor:

const AsyncFunction = (async () => {}).constructor;

asyncFn instanceof AsyncFunction === true

This won't work with Babel/TypeScript output, because asyncFn is regular function in transpiled code, it is an instance of Function or GeneratorFunction, not AsyncFunction. To make sure that it won't give false positives for generator and regular functions in transpiled code:

const AsyncFunction = (async () => {}).constructor;
const GeneratorFunction = (function* () => {}).constructor;

(asyncFn instanceof AsyncFunction && AsyncFunction !== Function && AsyncFunction !== GeneratorFunction) === true

Since native async functions were officially introduced to Node.js in 2017, the question likely refers to Babel implementation of async function, which relies on transform-async-to-generator to transpile async to generator functions, may also use transform-regenerator to transpile generator to regular functions.

The result of async function call is a promise. According to the proposal, a promise or a non-promise may be passed to await, so await callback() is universal.

There are only few edge cases when this may be needed. For instance, native async functions use native promises internally and don't pick up global Promise if its implementation was changed:

let NativePromise = Promise;
Promise = CustomPromiseImplementation;

Promise.resolve() instanceof Promise === true
(async () => {})() instanceof Promise === false;
(async () => {})() instanceof NativePromise === true;

This may affect function behaviour (this is a known problem for Angular and Zone.js promise implementation). Even then it's preferable to detect that function return value is not expected Promise instance instead of detecting that a function is async, because the same problem is applicable to any function that uses alternative promise implementation, not just async (the solution to said Angular problem is to wrap async return value with Promise.resolve).

Practice

From the outside, async function is just a function that unconditionally returns native promise, therefore it should be treated like one. Even if a function once was defined async, it can be transpiled at some point and become regular function.

A function that can return a promise

In ES6, a function that potentially returns a promise can be used with Promise.resolve (lets synchronous errors) or wrapped Promise constructor (handles synchronous errors):

Promise.resolve(fnThatPossiblyReturnsAPromise())
.then(result => ...);

new Promise(resolve => resolve(fnThatPossiblyReturnsAPromiseOrThrows()))
.then(result => ...);

In ES2017, this is done with await (this is how the example from the question is supposed to be written):

let result = await fnThatPossiblyReturnsAPromiseOrThrows();
...

A function that should return a promise

Checking if an object is a promise is a matter of a separate question, but generally it shouldn't be too strict or loose in order to cover corner cases. instanceof Promise may not work if global Promise was replaced, Promise !== (async () => {})().constructor. This can happen when Angular and non-Angular applications interface.

A function that requires to be async, i.e. to always return a promise should be called first, then returned value is checked to be a promise:

let promise = fnThatShouldReturnAPromise();
if (promise && typeof promise.then === 'function' && promise[Symbol.toStringTag] === 'Promise') {
  // is compliant native promise implementation
} else {
  throw new Error('async function expected');
}

TL;DR: async functions shouldn't be distinguished from regular functions that return promises. There is no reliable way and no practical reason to detect non-native transpiled async functions.

Answer 2

TL;DR

Short answer: Use instaceof after exposing AsyncFunction - see below.

Long answer: Don't do that - see below.

How to do it

You can detect whether a function was declared with the async keyword

When you create a function, it shows that it's a type Function:

> f1 = function () {};
[Function: f1]

You can test it with the instanceof operator:

> f1 instanceof Function
true

When you create an async function, it shows that it's a type AsyncFunction:

> f2 = async function () {}
[AsyncFunction: f2]

so one might expect that it can be tested with instanceof as well:

> f2 instanceof AsyncFunction
ReferenceError: AsyncFunction is not defined

Why is that? Because the AsyncFunction is not a global object. See the docs:

• https://developer.mozilla.org/Web/JavaScript/Reference/Global_Objects/AsyncFunction

even though, as you can see, it's listed under Reference/Global_Objects...

If you need easy access to the AsyncFunction then you can use my unexposed module:

• https://www.npmjs.com/package/unexposed

to get either a local variable:

const { AsyncFunction } = require('unexposed');

or to add a global AsyncFunction alongside other global objects:

require('unexposed').addGlobals();

and now the above works as expected:

> f2 = async function () {}
[AsyncFunction: f2]
> f2 instanceof AsyncFunction
true

Why you shouldn't do it

The above code will test whether the function was created with the async keyword but keep in mind that what is really important is not how a function was created but whether or not a function returns a promise.

Everywhere where you can use this "async" function:

const f1 = async () => {
  // ...
};

you could also use this:

const f2 = () => new Promise((resolve, reject) => {
});

even though it was not created with the async keyword and thus will not be matched with instanceof or with any other method posted in other answers.

Specifically, consider this:

const f1 = async (x) => {
  // ...
};

const f2 = () => f1(123);

The f2 is just f1 with hardcoded argument and it doesn't make much sense to add async here, even though the result will be as much "async" as f1 in every respect.

Summary

So it is possible to check if a function was created with the async keyword, but use it with caution because you when you check it then most likely you're doing something wrong.