Any difference between await Promise all and multiple await

Question

Is there any difference between   const  result1  result2    await Promise all  task1    task2        and  const t1   task1    const t2   task2     const result1   await t1  const result2   await t2    and  const  t1  t2     task1    task2     const  result1  result2     await t1  await t2

User · Answer

In case of await Promise all  task1    task2       task1    and  task2    will run parallel and will wait until both promises are completed  either resolved or rejected   Whereas in case of   const result1   await t1  const result2   await t2    t2 will only run after t1 has finished execution  has been resolved or rejected   Both t1 and t2 will not run parallel

User · Answer

You can check for yourself   In this fiddle  I ran a test to demonstrate the blocking nature of await  as opposed to Promise all which will start all of the promises and while one is waiting it will go on with the others

User · Answer

First difference - Fail Fast I agree with  zzzzBov s answer  but the  quot fail fast quot  advantage of Promise all is not the only difference  Some users in the comments have asked why using Promise all is worth it when it s only faster in the negative scenario  when some task fails   And I ask  why not  If I have two independent async parallel tasks and the first one takes a very long time to resolve but the second is rejected in a very short time  why leave the user to wait for the longer call to finish to receive an error message  In real-life applications we must consider the negative scenario  But OK - in this first difference you can decide which alternative to use  Promise all vs  multiple await  Second difference - Error Handling But when considering error handling  YOU MUST use Promise all  It is not possible to correctly handle errors of async parallel tasks triggered with multiple awaits  In the negative scenario you will always end with UnhandledPromiseRejectionWarning and PromiseRejectionHandledWarning  regardless of where you use try  catch  That is why Promise all was designed  Of course someone could say that we can suppress those errors using process on  unhandledRejection   err   gt      and process on  rejectionHandled   err   gt      but this is not good practice  I ve found many examples on the internet that do not consider error handling for two or more independent async parallel tasks at all  or consider it but in the wrong way - just using try  catch and hoping it will catch errors  It s almost impossible to find good practice in this  Summary TL DR  Never use multiple await for two or more independent async parallel tasks  because you will not be able to handle errors correctly  Always use Promise all   for this use case  Async  await is not a replacement for Promises  it s just a pretty way to use promises  Async code is written in  quot sync style quot  and we can avoid multiple thens in promises  Some people say that when using Promise all   we can t handle task errors separately  and that we can only handle the error from the first rejected promise  separate handling can be useful e g  for logging   This is not a problem - see  quot Addition quot  heading at the bottom of this answer  Examples Consider this async task    const task   function taskNum  seconds  negativeScenario      return new Promise  resolve  reject    gt        setTimeout     gt          if  negativeScenario          reject new Error  Task     taskNum     failed            else         resolve  Task     taskNum     succeed            seconds   1000            When you run tasks in the positive scenario there is no difference between Promise all and multiple awaits  Both examples end with Task 1 succeed  Task 2 succeed  after 5 seconds     Promise all alternative const run   async function          tasks run immediate in parallel and wait for both results   let  r1  r2    await Promise all       task 1  5  false       task 2  5  false          console log r1         r2      run       at 5th sec  Task 1 succeed  Task 2 succeed      multiple await alternative const run   async function          tasks run immediate in parallel   let t1   task 1  5  false     let t2   task 2  5  false        wait for both results   let r1   await t1    let r2   await t2    console log r1         r2      run       at 5th sec  Task 1 succeed  Task 2 succeed   However  when the first task takes 10 seconds and succeeds  and the second task takes 5 seconds but fails  there are differences in the errors issued     Promise all alternative const run   async function       let  r1  r2    await Promise all         task 1  10  false         task 2  5  true          console log r1         r2      run       at 5th sec  UnhandledPromiseRejectionWarning  Error  Task 2 failed      multiple await alternative const run   async function       let t1   task 1  10  false     let t2   task 2  5  true     let r1   await t1    let r2   await t2    console log r1         r2      run       at 5th sec  UnhandledPromiseRejectionWarning  Error  Task 2 failed     at 10th sec  PromiseRejectionHandledWarning  Promise rejection was handled asynchronously  rejection id  1     at 10th sec  UnhandledPromiseRejectionWarning  Error  Task 2 failed   We should already notice here that we are doing something wrong when using multiple awaits in parallel  Let s try handling the errors     Promise all alternative const run   async function       let  r1  r2    await Promise all       task 1  10  false       task 2  5  true          console log r1         r2      run   catch err   gt    console log  Caught error   err          at 5th sec  Caught error Error  Task 2 failed   As you can see  to successfully handle errors  we need to add just one catch to the run function and add code with catch logic into the callback  We do not need to handle errors inside the run function because async functions do this automatically - promise rejection of the task function causes rejection of the run function  To avoid a callback we can use  quot sync style quot   async  await   try  catch  try   await run      catch err      but in this example it s not possible  because we can t use await in the main thread - it can only be used in async functions  because nobody wants to block main thread   To test if handling works in  quot sync style quot  we can call the run function from another async function or use an IIFE  Immediately Invoked Function Expression  MDN    async function        try        await run         catch err         console log  Caught error   err               This is the only correct way to run two or more async parallel tasks and handle errors  You should avoid the examples below  Bad Examples    multiple await alternative const run   async function       let t1   task 1  10  false     let t2   task 2  5  true     let r1   await t1    let r2   await t2    console log r1         r2       We can try to handle errors in the code above in several ways    try   run      catch err    console log  Caught error   err         at 5th sec  UnhandledPromiseRejectionWarning  Error  Task 2 failed     at 10th sec  UnhandledPromiseRejectionWarning  Error  Task 2 failed     at 10th sec  PromiseRejectionHandledWarning  Promise rejection was handled       nothing got caught because it handles sync code but run is async  run   catch err   gt    console log  Caught error   err          at 5th sec  UnhandledPromiseRejectionWarning  Error  Task 2 failed     at 10th sec  Caught error Error  Task 2 failed     at 10th sec  PromiseRejectionHandledWarning  Promise rejection was handled asynchronously  rejection id  1       huh  We see firstly that the error for task 2 was not handled and later that it was caught  Misleading and still full of errors in console  it s still unusable this way    async function     try   await run      catch err    console log  Caught error   err               at 5th sec  UnhandledPromiseRejectionWarning  Error  Task 2 failed     at 10th sec  Caught error Error  Task 2 failed     at 10th sec  PromiseRejectionHandledWarning  Promise rejection was handled asynchronously  rejection id  1       the same as above  User  Qwerty in his deleted answer asked about this strange behavior where an error seems to be caught but are also unhandled  We  catch error the because run   is rejected on the line with the await keyword and can be caught using try  catch when calling run    We also get an unhandled error because we are calling an async task function synchronously  without the await keyword   and this task runs and fails outside the run   function  It is similar to when we are not able to handle errors by try  catch when calling some sync function which calls setTimeout  function test       setTimeout function          console log causesError           0       try      test       catch e          this will never catch error          Another poor example  const run   async function       try       let t1   task 1  10  false       let t2   task 2  5  true       let r1   await t1      let r2   await t2        catch  err        return new Error err         console log r1         r2      run   catch err   gt    console log  Caught error   err          at 5th sec  UnhandledPromiseRejectionWarning  Error  Task 2 failed     at 10th sec  PromiseRejectionHandledWarning  Promise rejection was handled asynchronously  rejection id  1        quot only quot  two errors  3rd one is missing  but nothing is caught  Addition  handling separate task errors and also first-fail error  const run   async function       let  r1  r2    await Promise all       task 1  10  true  catch err   gt    console log  Task 1 failed     throw err          task 2  5  true  catch err   gt    console log  Task 2 failed     throw err             console log r1         r2      run   catch err   gt    console log  Run failed  does not matter which task             at 5th sec  Task 2 failed     at 5th sec  Run failed  does not matter which task      at 10th sec  Task 1 failed       note that in this example I rejected both tasks to better demonstrate what happens  throw err is used to fire final error

User · Answer

Generally  using Promise all   runs requests  quot async quot  in parallel  Using await can run in parallel OR be  quot sync quot  blocking  test1 and test2 functions below show how await can run async or sync  test3 shows Promise all   that is async  jsfiddle with timed results  - open browser console to see test results Sync behavior  Does NOT run in parallel  takes  1800ms  const test1   async      gt      const delay1   await Promise delay 600     runs 1st   const delay2   await Promise delay 600     waits 600 for delay1 to run   const delay3   await Promise delay 600     waits 600 more for delay2 to run     Async behavior  Runs in paralel  takes  600ms  const test2   async      gt      const delay1   Promise delay 600     const delay2   Promise delay 600     const delay3   Promise delay 600     const data1   await delay1    const data2   await delay2    const data3   await delay3    runs all delays simultaneously    Async behavior  Runs in parallel  takes  600ms  const test3   async      gt      await Promise all     Promise delay 600      Promise delay 600      Promise delay 600       runs all delays simultaneously     TLDR  If you are using Promise all   it will also  quot fast-fail quot  - stop running at the time of the first failure of any of the included functions

User · Answer

Note  This answer just covers the timing differences between await in series and Promise all  Be sure to read  mikep s comprehensive answer that also covers the more important differences in error handling    For the purposes of this answer I will be using some example methods   res ms  is a function that takes an integer of milliseconds and returns a promise that resolves after that many milliseconds  rej ms  is a function that takes an integer of milliseconds and returns a promise that rejects after that many milliseconds   Calling res starts the timer  Using Promise all to wait for a handful of delays will resolve after all the delays have finished  but remember they execute at the same time  Example  1 const data   await Promise all  res 3000   res 2000   res 1000                                                                                                     delay 1    delay 2    delay 3       ms ------1---------2---------3                                 O delay 1                       O           delay 2             O                     delay 3                                    O Promise all   x000D   x000D  async function example       const start   Date now     let i   0   function res n        const id     i     return new Promise  resolve  reject    gt          setTimeout      gt            resolve           console log  res    id  called after   n  milliseconds   Date now   - start           n                const data   await Promise all  res 3000   res 2000   res 1000      console log  Promise all finished   Date now   - start     example   x000D   x000D   x000D   This means that Promise all will resolve with the data from the inner promises after 3 seconds  But  Promise all has a  quot fail fast quot  behavior  Example  2 const data   await Promise all  res 3000   res 2000   rej 1000                                                                                                     delay 1    delay 2    delay 3       ms ------1---------2---------3                                 O delay 1                       O           delay 2             X                     delay 3                X                     Promise all   x000D   x000D  async function example       const start   Date now     let i   0   function res n        const id     i     return new Promise  resolve  reject    gt          setTimeout      gt            resolve           console log  res    id  called after   n  milliseconds   Date now   - start           n                  function rej n        const id     i     return new Promise  resolve  reject    gt          setTimeout      gt            reject           console log  rej    id  called after   n  milliseconds   Date now   - start           n                  try       const data   await Promise all  res 3000   res 2000   rej 1000        catch  error        console log  Promise all finished   Date now   - start         example   x000D   x000D   x000D   If you use async-await instead  you will have to wait for each promise to resolve sequentially  which may not be as efficient  Example  3 const delay1   res 3000  const delay2   res 2000  const delay3   rej 1000   const data1   await delay1 const data2   await delay2 const data3   await delay3     ms ------1---------2---------3                                 O delay 1                       O           delay 2             X                     delay 3                                    X await   x000D   x000D  async function example       const start   Date now     let i   0   function res n        const id     i     return new Promise  resolve  reject    gt          setTimeout      gt            resolve           console log  res    id  called after   n  milliseconds   Date now   - start           n                  function rej n        const id     i     return new Promise  resolve  reject    gt          setTimeout      gt            reject           console log  rej    id  called after   n  milliseconds   Date now   - start           n                  try       const delay1   res 3000      const delay2   res 2000      const delay3   rej 1000       const data1   await delay1     const data2   await delay2     const data3   await delay3     catch  error        console log  await finished   Date now   - start         example   x000D   x000D   x000D

[javascript] Any difference between await Promise.all() and multiple await?

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