try catch blocks with async await

Question

I m digging into the node 7 async await feature and keep stumbling across code like this   x000D   x000D  function getQuote     x000D    let quote    Lorem ipsum dolor sit amet  consectetur adipiscing elit  sed do eiusmod tempor incididunt ut labore et dolore magna aliqua  Ut enim ad minim veniam  quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea commodo consequat  Duis aute irure dolor in reprehenderit in voluptate velit esse cillum dolore eu fugiat nulla pariatur  Excepteur sint occaecat cupidatat non proident  sunt in culpa qui officia deserunt mollit anim id est laborum    x000D    return quote  x000D    x000D   x000D  async function main     x000D    try   x000D      var quote   await getQuote    x000D      console log quote   x000D      catch  error    x000D      console error error   x000D      x000D    x000D   x000D  main    x000D   x000D   x000D    This seems to be the only possibility resolve reject or return throw with async await  however  v8 doesn t optimise code within try catch blocks    Are there alternatives

User · Answer

A cleaner alternative would be the following:

Due to the fact that every async function is technically a promise

You can add catches to functions when calling them with await

async function a(){
    let error;

    // log the error on the parent
    await b().catch((err)=>console.log('b.failed'))

    // change an error variable
    await c().catch((err)=>{error=true; console.log(err)})

    // return whatever you want
    return error ? d() : null;
}
a().catch(()=>console.log('main program failed'))

No need for try catch, as all promises errors are handled, and you have no code errors, you can omit that in the parent!!

Lets say you are working with mongodb, if there is an error you might prefer to handle it in the function calling it than making wrappers, or using try catches.

User · Answer

catching in this fashion  in my experience  is dangerous  Any error thrown in the entire stack will be caught  not just an error from this promise  which is probably not what you want    The second argument to a promise is already a rejection failure callback  It s better and safer to use that instead   Here s a typescript typesafe one-liner I wrote to handle this   function wait lt R  E gt  promise  Promise lt R gt     R   null  E   null      return  promise then  data  R    gt   data  null    err  E    gt   null  err   as any  as  R  E         Usage const  currUser  currUserError    await wait lt GetCurrentUser user  GetCurrentUser errors gt     apiClient getCurrentUser

User · Answer

Alternative Similar To Error Handling In Golang Because async await uses promises under the hood  you can write a little utility function like this  export function catchEm promise      return promise then data   gt   null  data        catch err   gt   err       Then import it whenever you need to catch some errors  and wrap your async function which returns a promise with it  import catchEm from  utility    async performAsyncWork       const  err  data    await catchEm asyncFunction arg1  arg2      if  err           handle errors     else          use data

User · Answer

Alternatives  An alternative to this   async function main       try       var quote   await getQuote        console log quote       catch  error        console error error           would be something like this  using promises explicitly   function main       getQuote   then  quote    gt        console log quote        catch  error    gt        console error error             or something like this  using continuation passing style   function main       getQuote  error  quote    gt        if  error          console error error         else         console log quote                   Original example  What your original code does is suspend the execution and wait for the promise returned by getQuote   to settle  It then continues the execution and writes the returned value to var quote and then prints it if the promise was resolved  or throws an exception and runs the catch block that prints the error if the promise was rejected   You can do the same thing using the Promise API directly like in the second example   Performance  Now  for the performance  Let s test it   I just wrote this code - f1   gives 1 as a return value  f2   throws 1 as an exception   function f1       return 1     function f2       throw 1      Now let s call the same code million times  first with f1     var sum   0  for  var i   0  i  lt  1e6  i        try       sum    f1        catch  e        sum    e        console log sum     And then let s change f1   to f2     var sum   0  for  var i   0  i  lt  1e6  i        try       sum    f2        catch  e        sum    e        console log sum     This is the result I got for f1     time node throw-test js  1000000  real    0m0 073s user    0m0 070s sys     0m0 004s   This is what I got for f2     time node throw-test js  1000000  real    0m0 632s user    0m0 629s sys     0m0 004s   It seems that you can do something like 2 million throws a second in one single-threaded process  If you re doing more than that then you may need to worry about it   Summary  I wouldn t worry about things like that in Node  If things like that get used a lot then it will get optimized eventually by the V8 or SpiderMonkey or Chakra teams and everyone will follow - it s not like it s not optimized as a principle  it s just not a problem   Even if it isn t optimized then I d still argue that if you re maxing out your CPU in Node then you should probably write your number crunching in C - that s what the native addons are for  among other things  Or maybe things like node native would be better suited for the job than Node js   I m wondering what would be a use case that needs throwing so many exceptions  Usually throwing an exception instead of returning a value is  well  an exception

User · Answer

An alternative to try-catch block is await-to-js lib  I often use it  For example  import to from  await-to-js    async function main callback        const  err quote    await to getQuote              if err     quote  return callback new Error  No Quote found          callback null quote       This syntax is much cleaner when compared to try-catch

User · Answer

async function main       var getQuoteError   var quote   await getQuote   catch err   gt    getQuoteError   err      if  getQuoteError  return console error err     console log quote      Alternatively instead of declaring a possible var to hold an error at the top you can do  if  quote instanceof Error                 Though that won t work if something like a TypeError or Reference error is thrown  You can ensure it is a regular error though with   async function main       var quote   await getQuote   catch err   gt        console error err             return new Error  Error getting quote           if  quote instanceOf Error  return quote    get out of here or do whatever    console log quote      My preference for this is wrapping everything in a big try-catch block where there s multiple promises being created can make it cumbersome to handle the error specifically to the promise that created it  With the alternative being multiple try-catch blocks which I find equally cumbersome

User · Answer

I d like to do this way     const sthError        gt  Promise reject  sth error     const test   opts   gt      return  async      gt            do sth     await sthError        return  ok           catch err   gt        console error err      error will be catched there            test   then ret   gt      console log ret         It s similar to handling error with co  const test   opts   gt      return co function              do sth     yield sthError        return  ok         catch err   gt        console error err

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