What is an unhandled promise rejection

Question

For learning Angular 2  I am trying their tutorial    I am getting an error like this    node 4796  UnhandledPromiseRejectionWarning  Unhandled promise rejection  r                                                                                                     ejection id  1   Error  spawn cmd ENOENT  1   node 4796  DeprecationWarning  Unhandled promise rejections are deprecated  In the future  promise rejections that are not handled will terminate the Node  js process with a non-zero exit code    I went through different questions and answers in SO but could not find out what an  Unhandled Promise Rejection  is   Can anyone simply explain me what it is and also what Error  spawn cmd ENOENT is  when it arises and what I have to check to get rid of this warning

User · Answer

DeprecationWarning  Unhandled promise rejections are deprecated    TLDR  A promise has resolve and reject  doing a reject without a catch to handle it is deprecated  so you will have to at least have a catch at top level

User · Answer

The origin of this error lies in the fact that each and every promise is expected to handle promise rejection i e  have a  catch        you can avoid the same by adding  catch      to a promise in the code as given below    for example  the function PTest   will either resolve or reject a promise based on the value of a global variable somevar  var somevar   false  var PTest   function          return new Promise function  resolve  reject            if  somevar     true              resolve            else             reject              var myfunc   PTest    myfunc then function           console log  Promise Resolved       catch function           console log  Promise Rejected          In some cases  the  unhandled promise rejection  message comes even if we have  catch     written for promises  It s all about how you write your code  The following code will generate  unhandled promise rejection  even though we are handling catch   var somevar   false  var PTest   function          return new Promise function  resolve  reject            if  somevar     true              resolve            else             reject              var myfunc   PTest    myfunc then function           console log  Promise Resolved           See the Difference here myfunc catch function           console log  Promise Rejected          The difference is that you don t handle  catch      as chain but as separate  For some reason JavaScript engine treats it as promise without un-handled promise rejection

User · Answer

Try not closing the connection before you send data to your database  Remove client close    from your code and it ll work fine

User · Answer

Promises can be  handled  after they are rejected  That is  one can call a promise s reject callback before providing a catch handler  This behavior is a little bothersome to me because one can write     var promise   new Promise function resolve    kjjdjf       this function does not exist           and in this case  the Promise is rejected silently  If one forgets to add a catch handler  code will continue to silently run without errors  This could lead to lingering and hard-to-find bugs   In the case of Node js  there is talk of handling these unhandled Promise rejections and reporting the problems  This brings me to ES7 async await  Consider this example   async function getReadyForBed       let teethPromise   brushTeeth      let tempPromise   getRoomTemperature          Change clothes based on room temperature   let temp   await tempPromise       Assume  changeClothes  also returns a Promise   if temp  gt  20        await changeClothes  warm        else       await changeClothes  cold           await teethPromise      In the example above  suppose teethPromise was rejected  Error  out of toothpaste   before getRoomTemperature was fulfilled  In this case  there would be an unhandled Promise rejection until await teethPromise   My point is this    if we consider unhandled Promise rejections to be a problem  Promises that are later handled by an await might get inadvertently reported as bugs  Then again  if we consider unhandled Promise rejections to not be problematic  legitimate bugs might not get reported   Thoughts on this   This is related to the discussion found in the Node js project here    Default Unhandled Rejection Detection Behavior  if you write the code this way   function getReadyForBed       let teethPromise   brushTeeth      let tempPromise   getRoomTemperature          Change clothes based on room temperature   return Promise resolve tempPromise       then temp   gt             Assume  changeClothes  also returns a Promise       if  temp  gt  20            return Promise resolve changeClothes  warm             else           return Promise resolve changeClothes  cold                         then teethPromise       then Promise resolve        since the async function returns nothing  ensure it s a resolved promise for  undefined   unless it s previously rejected     When getReadyForBed is invoked  it will synchronously create the final  not returned  promise - which will have the same  unhandled rejection  error as any other promise  could be nothing  of course  depending on the engine    I find it very odd your function doesn t return anything  which means your async function produces a promise for undefined   If I make a Promise right now without a catch  and add one later  most  unhandled rejection error  implementations will actually retract the warning when i do later handle it  In other words  async await doesn t alter the  unhandled rejection  discussion in any way that I can see   to avoid this pitfall please write the code this way   async function getReadyForBed       let teethPromise   brushTeeth      let tempPromise   getRoomTemperature          Change clothes based on room temperature   var clothesPromise   tempPromise then function temp           Assume  changeClothes  also returns a Promise     if temp  gt  20          return changeClothes  warm          else         return changeClothes  cold                     Note that clothesPromise resolves to the result of  changeClothes       due to Promise  chaining  magic           Combine promises and await them both   await Promise all teethPromise  clothesPromise       Note that this should prevent any unhandled promise rejection

User · Answer

I had faced a similar issue with NodeJS  where the culprit was a forEach loop  Note that forEach is a synchronous function  NOT Asynchronous   Therefore it just ignores the promise returned  The solution was to use a for-of loop instead  Code where I got the error   UnhandledPromiseRejectionWarning  Unhandled promise rejection  This error originated either by throwing inside of an async function without a catch block  or by rejecting a promise which was not handled with  catch    is as follows  permissionOrders forEach  async order    gt            const requestPermissionOrder   new RequestPermissionOrderSchema               item  order item              item desc  order item desc              quantity  order quantity              unit price  order unit price              total cost  order total cost              status  order status              priority  order priority              directOrder  order directOrder                      try               const dat order   await requestPermissionOrder save                res json dat order             catch err               res json   message   err                       Solution for the above issue is as follows  for  let order of permissionOrders           const requestPermissionOrder   new RequestPermissionOrderSchema               item  order item              item desc  order item desc              quantity  order quantity              unit price  order unit price              total cost  order total cost              status  order status              priority  order priority              directOrder  order directOrder                      try               const dat order   await requestPermissionOrder save                res json dat order             catch err               res json   message   err

User · Answer

When I instantiate a promise  I m going to generate an asynchronous function  If the function goes well then I call the RESOLVE then the flow continues in the RESOLVE handler  in the THEN  If the function fails  then terminate the function by calling REJECT then the flow continues in the CATCH  In NodeJs are deprecated the rejection handler  Your error is just a warning and I read it inside node js github  I found this   DEP0018  Unhandled promise rejections Type  Runtime Unhandled promise rejections are deprecated  In the future  promise rejections that are not handled will terminate the Node js process with a non-zero exit code

User · Answer

In my case was Promise with no reject neither resolve  because my Promise function threw an exception  This mistake cause UnhandledPromiseRejectionWarning message

User · Answer

This is when a Promise is completed with  reject   or an exception was thrown in an async executed code and no  catch   did handle the rejection   A rejected promise is like an exception that bubbles up towards the application entry point and causes the root error handler to produce that output   See also   https   developer mozilla org en docs Web JavaScript Reference Global Objects Promise reject https   developer mozilla org en-US docs Web JavaScript Reference Global Objects Promise https   developer mozilla org en-US docs Web JavaScript Reference Global Objects Promise catch

[javascript] What is an unhandled promise rejection?

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