How can I use async/await at the top level?

Question

I have been going over async/await and after going over several articles, I decided to test things myself. However, I can't seem to wrap my head around why this does not work:

async function main() {  
    var value = await Promise.resolve('Hey there');
    console.log('inside: ' + value);
    return value;
}

var text = main();  
console.log('outside: ' + text);

The console outputs the following (node v8.6.0) :

> outside: [object Promise]

> inside: Hey there

Why does the log message inside the function execute afterwards? I thought the reason async/await was created was in order to perform synchronous execution using asynchronous tasks.

Is there a way could I use the value returned inside the function without using a .then() after main()?

Answer 1

I can't seem to wrap my head around why this does not work.

Because main returns a promise; all async functions do.

At the top level, you must either:

1. Use top-level await (ES2022, broadly supported in modern environments) that allows top-level use of await in a module.

   or

2. Use a top-level async function that never rejects (unless you want "unhandled rejection" errors).

   or

3. Use then and catch.

#1 top-level await in a module

You can use await at the top-level of a module (details). Your module won't finish loading until the promise you await settles (meaning any module waiting for your module to load won't finish loading until the promise settles). If the promise is rejected, your module will fail to load. Typically, top-level await is used in situations where your module won't be able to do its work until the promise is settled and won't be able to do it at all unless the promise is fulfilled, so that's fine:

const text = await main();
console.log(text);

If your module can continue to work even if the promise is rejected, you could wrap the top-level await in a try/catch:

// In a module, once the top-level `await` proposal lands
try {
    const text = await main();
    console.log(text);
} catch (e) {
    // Deal with the fact the chain failed
}
// `text` is not available here

when a module using top-level await is evaluated, it returns a promise to the module loader (like an async function does), which waits until that promise is settled before evaluating the bodies of any modules that depend on it.

You can't use await at the top level of a non-module script, only in modules.

#2 - Top-level async function that never rejects

(async () => {
    try {
        const text = await main();
        console.log(text);
    } catch (e) {
        // Deal with the fact the chain failed
    }
    // `text` is not available here
})();
// `text` is not available here, either, and code here is reached before the promise settles
// and before the code after `await` in the main function above runs

Notice the catch; you must handle promise rejections / async exceptions, since nothing else is going to; you have no caller to pass them on to (unlike with #1 above, where your "caller" is the module loader). If you prefer, you could do that on the result of calling it via the catch function (rather than try/catch syntax):

(async () => {
    const text = await main();
    console.log(text);
})().catch(e => {
    // Deal with the fact the chain failed
});
// `text` is not available here, and code here is reached before the promise settles
// and before the code after `await` in the main function above runs

...which is a bit more concise, though it somewhat mixes models (async/await and explicit promise callbacks), which I'd normally otherwise advise not to.

Or, of course, don't handle errors and just allow the "unhandled rejection" error.

#3 - then and catch

main()
    .then(text => {
        console.log(text);
    })
    .catch(err => {
        // Deal with the fact the chain failed
    });
// `text` is not available here, and code here is reached before the promise settles
// and the handlers above run

The catch handler will be called if errors occur in the chain or in your then handler. (Be sure your catch handler doesn't throw errors, as nothing is registered to handle them.)

Or both arguments to then:

main().then(
    text => {
        console.log(text);
    },
    err => {
        // Deal with the fact the chain failed
    }
);
// `text` is not available here, and code here is reached before the promise settles
// and the handlers above run

Again notice we're registering a rejection handler. But in this form, be sure that neither of your then callbacks throws any errors, since nothing is registered to handle them.

Answer 2

Top-Level await has moved to stage 3, so the answer to your question How can I use async/await at the top level? is to just use await:

const text = await Promise.resolve('Hey there');
console.log('outside: ' + text)

Of if you want a main() function: add await to the call to main() :

async function main() {
    var value = await Promise.resolve('Hey there');
    console.log('inside: ' + value);
    return value;
}

var text = await main();  
console.log('outside: ' + text)

Compatibility

• v8 since Oct 2019
  • the REPL in Chrome DevTools, Node.js and Safari web inspector
• Node v13.3+ behind the flag --harmony-top-level-await
• TypeScript 3.8+ (issue)
• Deno since Oct 2019
• [email protected]

Answer 3

2021 answer: you can now use top level await in the current stable version of node

Most of the answers above are a little out of date or very verbose, so here's a quick example for node 14 onwards.

Make a file called runme.mjs:

import * as util from "util";
import { exec as lameExec } from "child_process";
const exec = util.promisify(lameExec);
const log = console.log.bind(console);

// Top level await works now
const { stdout, stderr } = await exec("ls -la");
log("Output:\n", stdout);
log("\n\nErrors:\n", stderr);

Run node runme.mjs

Output:
 total 20
drwxr-xr-x  2 mike mike 4096 Aug 12 12:05 .
drwxr-xr-x 30 mike mike 4096 Aug 12 11:05 ..
-rw-r--r--  1 mike mike  130 Aug 12 12:01 file.json
-rw-r--r--  1 mike mike  770 Aug 12 12:12 runme.mjs



Errors:

Answer 4

To give some further info on top of current answers:

The contents of a node.js file are currently concatenated, in a string-like way, to form a function body.

For example if you have a file test.js:

// Amazing test file!
console.log('Test!');

Then node.js will secretly concatenate a function that looks like:

function(require, __dirname, ... perhaps more top-level properties) {
  // Amazing test file!
  console.log('Test!');
}

The major thing to note, is that the resulting function is NOT an async function. So you cannot use the term await directly inside of it!

But say you need to work with promises in this file, then there are two possible methods:

1. Don't use await directly inside the function
2. Don't use await at all

Option 1 requires us to create a new scope (and this scope can be async, because we have control over it):

// Amazing test file!
// Create a new async function (a new scope) and immediately call it!
(async () => {
  await new Promise(...);
  console.log('Test!');
})();

Option 2 requires us to use the object-oriented promise API (the less pretty but equally functional paradigm of working with promises)

// Amazing test file!
// Create some sort of promise...
let myPromise = new Promise(...);

// Now use the object-oriented API
myPromise.then(() => console.log('Test!'));

It would be interesting to see node add support for top-level await!

Answer 5

You can now use top level await in Node v13.3.0

import axios from "axios";

const { data } = await axios.get("https://api.namefake.com/");
console.log(data);

run it with --harmony-top-level-await flag

node --harmony-top-level-await index.js

Answer 6

The actual solution to this problem is to approach it differently.

Probably your goal is some sort of initialization which typically happens at the top level of an application.

The solution is to ensure that there is only ever one single JavaScript statement at the top level of your application. If you have only one statement at the top of your application, then you are free to use async/await at every other point everwhere (subject of course to normal syntax rules)

Put another way, wrap your entire top level in a function so that it is no longer the top level and that solves the question of how to run async/await at the top level of an application - you don't.

This is what the top level of your application should look like:

import {application} from './server'

application();

Answer 7

Node -
You can run node --experimental-repl-await while in the REPL. I'm not so sure about scripting.

Deno -
Deno already has it built in.

Answer 8

Other solutions were lacking some important details for POSIX compliance:

You need to ...

• Report a 0 exit status on success and non-zero on fail.
• Emit errors to stderr output stream.

#!/usr/bin/env node

async function main() {
 // ... await stuff ... 
}

// POSIX compliant apps should report an exit status
main()
    .then(() => {
        process.exit(0);
    })
    .catch(err => {
        console.error(err); // Writes to stderr
        process.exit(1);
    });

If you're using a command line parser like commander, you may not need a main().

Example:

#!/usr/bin/env node

import commander from 'commander'

const program = new commander.Command();

program
  .version("0.0.1")
  .command("some-cmd")
  .arguments("<my-arg1>")
  .action(async (arg1: string) => {
    // run some async action
  });

program.parseAsync(process.argv)
  .then(() => {
    process.exit(0)
  })
  .catch(err => {
    console.error(err.message || err);
    if (err.stack) console.error(err.stack);
    process.exit(1);
  });

Answer 9

i like this clever syntax to do async work from an entrypoint

void async function main() {
  await doSomeWork()
  await doMoreWork()
}()

Answer 10

1. You need to add type in package.json

   "type": "module"
   

2. You are good to go.

   import axios from 'axios';
   const res = await axios.get('https://api.github.com/users/wesbos');
   console.log(res.data);
   

Remember if you change type of document then you must have to write code in ES6 way.

Answer 11

In NodeJS 14.8+, you can use top-level await module (#3 solution). You can rename also .js to .mjs (ES module) instead of .js (.cjs CommonJS).

Answer 12

Now with ECMAScript22, we can use await at the top-level module.

This is an example with ( await top-level ):

const response = await fetch("...");
console.log(response):

an other example without (await top-level )

  async function callApi() {
    const response = await fetch("...");
    console.log(response)      
}
callApi()