How to reject in async await syntax

Question

How can I reject a promise that returned by an async await function  e g  Originally  foo id  string   Promise lt A gt      return new Promise  resolve  reject    gt        someAsyncPromise   then  value   gt resolve 200   catch  err   gt reject 400            Translate into async await  async foo id  string   Promise lt A gt      try      await someAsyncPromise        return 200      catch error     here goes if someAsyncPromise   rejected      return 400    this will result in a resolved promise           So  how could I properly reject this promise in this case

User · Answer

I know this is an old question  but I just stumbled across the thread and there seems to be a conflation here between errors and rejection that runs afoul  in many cases  at least  of the oft-repeated advice not to use exception handling to deal with anticipated cases  To illustrate  if an async method is trying to authenticate a user and the authentication fails  that s a rejection  one of two anticipated cases  and not an error  e g   if the authentication API was unavailable    To make sure I wasn t just splitting hairs  I ran a performance test of three different approaches to that  using this code   const iterations   100000   function getSwitch       return Math round Math random        1     function doSomething value      return  something done to     value toString       let processWithThrow   function        if  getSwitch          throw new Error  foo            let processWithReturn   function        if  getSwitch          return new Error  bar        else       return            let processWithCustomObject   function        if  getSwitch          return  type   rejection   message   quux        else       return  type   usable response   value   fnord            function testTryCatch limit      for  let i   0  i  lt  limit  i          try         processWithThrow          catch  e          const dummyValue   doSomething e                function testReturnError limit      for  let i   0  i  lt  limit  i          const returnValue   processWithReturn        if  returnValue instanceof Error          const dummyValue   doSomething returnValue                function testCustomObject limit      for  let i   0  i  lt  limit  i          const returnValue   processWithCustomObject        if  returnValue type      rejection           const dummyValue   doSomething returnValue                let start  end  start   new Date    testTryCatch iterations   end   new Date    const interval 1   end - start  start   new Date    testReturnError iterations   end   new Date    const interval 2   end - start  start   new Date    testCustomObject iterations   end   new Date    const interval 3   end - start   console log  with try catch    interval 1 ms  with returned Error    interval 2 ms  with custom object    interval 3 ms      Some of the stuff that s in there is included because of my uncertainty regarding the Javascript interpreter  I only like to go down one rabbit hole at a time   for instance  I included the doSomething function and assigned its return to dummyValue to ensure that the conditional blocks wouldn t get optimized out   My results were    with try catch  507ms  with returned Error  260ms  with custom object  5ms   I know that there are plenty of cases where it s not worth the trouble to hunt down small optimizations  but in larger-scale systems these things can make a big cumulative difference  and that s a pretty stark comparison   SO    while I think the accepted answer s approach is sound in cases where you re expecting to have to handle unpredictable errors within an async function  in cases where a rejection simply means  you re going to have to go with Plan B  or C  or D      I think my preference would be to reject using a custom response object

User · Answer

You can create a wrapper function that takes in a promise and returns an array with data if no error and the error if there was an error   function safePromise promise      return promise then data   gt    data    catch error   gt    null  error         Use it like this in ES7 and in an async function   async function checkItem       const   item  error     await safePromise getItem id      if  error    return null       handle error and return   return item     no error so safe to use item

User · Answer

It should probably also be mentioned that you can simply chain a catch   function after the call of your async operation because under the hood still a promise is returned   await foo   catch error   gt  console log error      This way you can avoid the try catch syntax if you do not like it

User · Answer

A better way to write the async function would be by returning a pending Promise from the start and then handling both rejections and resolutions within the callback of the promise  rather than just spitting out a rejected promise on error  Example   async foo id  string   Promise lt A gt        return new Promise function resolve  reject               execute some code here         if  success       let s say this is a boolean value from line above             return resolve success             else               return reject error      this can be anything  preferably an Error object to catch the stacktrace from this function                       Then you just chain methods on the returned promise   async function bar          try           var result   await foo  someID              use the result here       catch  error               handle error here          bar     Source - this tutorial    https   developer mozilla org en-US docs Web JavaScript Reference Global Objects Promise

User · Answer

I have a suggestion to properly handle rejects in a novel approach  without having multiple try-catch blocks   import to from    to    async foo id  string   Promise lt A gt        let err  result       err  result    await to someAsyncPromise        notice the to   here     if  err            return 400            return 200      Where the to ts function should be imported from   export default function to promise  Promise lt any gt    Promise lt any gt        return promise then data   gt            return  null  data          catch err   gt   err        Credits go to Dima Grossman in the following link

User · Answer

Your best bet is to throw an Error wrapping the value  which results in a rejected promise with an Error wrapping the value    catch  error        throw new Error 400      You can also just throw the value  but then there s no stack trace information    catch  error        throw 400     Alternately  return a rejected promise with an Error wrapping the value  but it s not idiomatic    catch  error        return Promise reject new Error 400        Or just return Promise reject 400    but again  then there s no context information   In your case  as you re using TypeScript and foo s return value is Promise lt A gt   you d use this  return Promise reject lt A gt  400   or Error       In an async await situation  that last is probably a bit of a semantic mis-match  but it does work  If you throw an Error  that plays well with anything consuming your foo s result with await syntax  try       await foo      catch  error           Here   error  would be an  Error   with stack trace  etc           Whereas if you used  throw 400   it would just be  400

User · Answer

This is not an answer over  T J  Crowder s one  Just an comment responding to the comment  And actually  if the exception is going to be converted to a rejection  I m not sure whether I am actually bothered if it s an Error  My reasons for throwing only Error probably don t apply    if your code is using async await  then it is still a good practice to reject with an Error instead of 400   try     await foo  a      catch  e         you would still want  e  to be an  Error  instead of  400

[javascript] How to reject in async/await syntax?

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