How to access the value of a promise

Question

I m looking at this example from Angular s docs for  q but I think this probably applies to promises in general  The example below is copied verbatim from their docs with their comment included   promiseB   promiseA then function result      return result   1          promiseB will be resolved immediately after promiseA is resolved and its value    will be the result of promiseA incremented by 1   I m not clear how this works  If I can call  then   on the result of the first  then    chaining them  which I know I can  then promiseB is a promise object  of type Object  It is not a Number  So what do they mean by  its value will be the result of promiseA incremented by 1     Am I supposed to access that as promiseB value or something like that  How can the success callback return a promise AND return  result   1   I m missing something

User · Answer

I am a slow learner of javascript promises, by default all async functions return a promise, you can wrap your result as:

(async () => {
//Optional "await"
  await yourAsyncFunctionOrPromise()
    .then(function (result) {
      return result +1;
    })
    .catch(function (error) {
      return error;
    })()
})

"The await expression causes async function execution to pause until a Promise is settled (that is, fulfilled or rejected), and to resume execution of the async function after fulfillment. When resumed, the value of the await expression is that of the fulfilled Promise. If the Promise is rejected, the await expression throws the rejected value."

Read more about await and promises at MDN Web Docs

User · Answer

When a promise is resolved rejected  it will call its success error handler   var promiseB   promiseA then function result          do something with result       The then method also returns a promise  promiseB  which will be resolved rejected depending on the return value from the success error handler from promiseA   There are three possible values that promiseA s success error handlers can return that will affect promiseB s outcome   1  Return nothing -- gt  PromiseB is resolved immediately      and undefined is passed to the success handler of promiseB 2  Return a value -- gt  PromiseB is resolved immediately     and the value is passed to the success handler of promiseB 3  Return a promise -- gt  When resolved  promiseB will be resolved      When rejected  promiseB will be rejected  The value passed to    the promiseB s then handler will be the result of the promise   Armed with this understanding  you can make sense of the following   promiseB   promiseA then function result      return result   1        The then call returns promiseB immediately  When promiseA is resolved  it will pass the result to promiseA s success handler  Since the return value is promiseA s result   1  the success handler is returning a value  option 2 above   so promiseB will resolve immediately  and promiseB s success handler will be passed promiseA s result   1

User · Answer

promiseA s then function returns a new promise  promiseB  that is immediately resolved after promiseA is resolved  its value is the value of the what is returned from the success function within promiseA   In this case promiseA is resolved with a value - result and then immediately resolves promiseB with the value of result   1   Accessing the value of promiseB is done in the same way we accessed the result of promiseA   promiseB then function result           here you can use the result of promiseB         Edit December 2019  async await is now standard in JS  which allows an alternative syntax to the approach described above  You can now write   let result   await functionThatReturnsPromiseA    result   result   1    Now there is no promiseB  because we ve unwrapped the result from promiseA using await  and you can work with it directly   However  await can only be used inside an async function  So to zoom out slightly  the above would have to be contained like so   async function doSomething         let result   await functionThatReturnsPromiseA        return result   1

User · Answer

promiseA pram  then       result   gt            make sure promiseA function allready success and response        do something here    catch err   gt  console log err     gt            handle error with try catch

User · Answer

Parsing the comment a little differently than your current understanding might help      promiseB will be resolved immediately after promiseA is resolved   This states that promiseB is a promise but will be resolved immediately after promiseA is resolved  Another way of looking at this means that promiseA then   returns a promise that is assigned to promiseB      and its value will be the result of promiseA incremented by 1   This means that the value that promiseA resolved to is the value that promiseB will receive as its successCallback value   promiseB then function  val         val is now promiseA s result   1

User · Answer

pixelbits answer is correct and you should always use  then   to access the value of a promise in production code   However  there is a way to access the promise s value directly after it has been resolved by using the following unsupported internal node js binding   process binding  util   getPromiseDetails myPromise  1    WARNING  process binding was never meant to be used outside of nodejs core and the nodejs core team is actively looking to deprecate it  https   github com nodejs node pull 22004 https   github com nodejs node issues 22064

User · Answer

This example I find self-explanatory  Notice how await waits for the result and so you miss the Promise being returned   cryA   crypto subtle generateKey  name  ECDH   namedCurve  P-384    true    deriveKey    deriveBits    Promise    lt pending gt   cryB   await crypto subtle generateKey  name  ECDH   namedCurve  P-384    true    deriveKey    deriveBits     publicKey  CryptoKey  privateKey  CryptoKey

User · Answer

There are some good answer above and here is the ES6 Arrow function version  var something   async     gt       let result   await functionThatReturnsPromiseA       return result   1

User · Answer

then function of promiseB receives what is returned from  then function of promiseA   here promiseA is returning is a number  which will be available as number parameter in success function of promiseB  which will then be incremented by 1

User · Answer

You can easily do that using an async wait method in javascript   Below is an example retrieving a WebRTC promise value using a timeout    x000D   x000D  function await getipv4 timeout   1000    x000D      var t1   new Date    x000D      while  window ipv4    x000D          var stop   new Date   - t1  gt   timeout  x000D          if stop    x000D              console error  timeout exceeded for await getipv4     x000D              return false  x000D            x000D        x000D      return window ipv4  x000D    x000D   x000D  function async getipv4     x000D      var ipv4   null  x000D      var findIP   new Promise r  gt  var w window a new  w RTCPeerConnection  w mozRTCPeerConnection  w webkitRTCPeerConnection   iceServers      b     gt    a createDataChannel     a createOffer c  gt a setLocalDescription c b b  b  a onicecandidate c  gt  try c candidate candidate match    0-9  1 3     0-9  1 3   3   a-f0-9  1 4    a-f0-9  1 4   7   g  forEach r  catch e       x000D      findIP then ip   gt  window ipv4   ip   x000D      return await getipv4    x000D     x000D   x000D   x000D

User · Answer

Maybe this small Typescript code example will help   private getAccount id  Id    Account       let account   Account empty        this repository get id           then res   gt  account   res           catch e   gt  Notices results e        return account      Here the repository get id  returns a Promise lt Account gt   I assign it to the variable account within the then statement

User · Answer

In the Node REPL  to get a DB connection that was the value of a promise  I took the following approach   let connection try      async      gt        connection   await returnsAPromiseResolvingToConnection            catch err      console log err      The line with await would normally return a promise  This code can be pasted into the Node REPL or if saved in index js it can be run in Bash with   node -i -e     lt  index js     which leaves you in the Node REPL after running the script with access to the set variable  To confirm that the asynchronous function has returned  you can log connection for example  and then you re ready to use the variable  One of course wouldn t want to count on the asynchronous function being resolved yet for any code in the script outside the asynchronous function

[javascript] How to access the value of a promise?

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