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      gt  outside   object Promise   gt  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

User · Answer

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

User · Answer

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

User · Answer

Since main   runs asynchronously it returns a promise  You have to get the result in then   method  And because then   returns promise too  you have to call process exit   to end the program    main       then         text    gt    console log  outside      text            err     gt    console log err             then      gt    process exit

User · Answer

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    Use a top-level async function that never rejects  unless you want  unhandled rejection  errors   or Use then and catch  or  Coming soon   Use top-level await  a proposal that has reached Stage 3 in the process that allows top-level use of await in a module     1 - Top-level async function that never rejects   async      gt        try           var text   await main            console log text         catch  e               Deal with the fact the chain failed               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  If you prefer  you could do that on the result of calling it via the catch function  rather than try catch syntax     async      gt        var text   await main        console log text        catch e   gt           Deal with the fact the chain failed          which is a bit more concise  I like it for that reason    Or  of course  don t handle errors and just allow the  unhandled rejection  error    2 - then and catch  main        then text   gt            console log text               catch err   gt               Deal with the fact the chain failed           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   gt            console log text              err   gt               Deal with the fact the chain failed            Again notice we re registering a rejection handler  But in this form  be sure that neither of your then callbacks doesn t throw any errors  nothing is registered to handle them    3 top-level await in a module  You can t use await at the top level of a non-module script  but the top-level await proposal  Stage 3  allows you to use it at the top level of a module  It s similar to using a top-level async function wrapper   1 above  in that you don t want your top-level code to reject  throw an error  because that will result in an unhandled rejection error  So unless you want to have that unhandled rejection when things go wrong  as with  1  you d want to wrap your code in an error handler      In a module  once the top-level  await  proposal lands try       var text   await main        console log text     catch  e           Deal with the fact the chain failed     Note that if you do this  any module that imports from your module will wait until the promise you re awaiting settles  when a module using top-level await is evaluated  it basically 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

User · Answer

i like this clever syntax to do async work from an entrypoint void async function main       await doSomeWork     await doMoreWork

User · Answer

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

User · Answer

Top-level await is a feature of the upcoming EcmaScript standard  Currently  you can start using it with TypeScript 3 8  in RC version at this time    How to Install TypeScript 3 8  You can start using TypeScript 3 8 by installing it from npm using  the following command     npm install typescript rc   At this time  you need to add the rc tag to install the latest typescript 3 8 version

User · Answer

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   Don t use await directly inside the function Don t use await  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      gt      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      gt  console log  Test       It would be interesting to see node add support for top-level await

User · Answer

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 add await 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   Or just  const text   await Promise resolve  Hey there    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 Webpack v5 0 0-alpha 15

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

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