Assign value from successful promise resolve to external variable

Question

I have a pretty silly problem  Consider the following   vm feed   getFeed   then function data   return data       getFeed   returns a  q deferred promise  I am on angular  that resolves successfully   My goal is to set vm feed equal to the data value returned by the successful callback  As it is right now  the code simply assigns vm feed equal to the  promise object returned by getFeed     I know I could simply do  vm feed   data inside the resolved function but I want to understand why this code does not work as it is   PD  the promise resolves correctly and even after it has been resolved vm feed keeps being equal to the Promise  and not data  I copy the console log of vm feed after  10 seconds have elapsed   Promise    state  Object    state  Objectstatus 1 value  Object   That value property inside the Promise object contains the actual solution of the promise that I want to assign to vm feed  e i  data    thank you

User · Answer

This is one  trick  you can do since your out of an async function so can t use await keywork  Do what you want to do with vm feed inside a setTimeout  vm feed   getFeed   then function data   return data       setTimeout      gt           do you stuf here        after the time you promise will be revolved or rejected        if you need some of the values in here immediately out of settimeout        might occur an error if promise wore not yet resolved or rejected     console log  vm feed  vm feed       100

User · Answer

Your statement does nothing more than ask the interpreter to assign the value returned from then   to the vm feed variable  then   returns you a Promise  as you can see here  https   github com angular angular js blob master src ng q js L283   You could picture this by seeing that the Promise  a simple object  is being pulled out of the function and getting assigned to vm feed  This happens as soon as the interpreter executes that line   Since your successful callback does not run when you call then   but only when your promise gets resolved  at a later time  asynchronously  it would be impossible for then   to return its value for the caller  This is the default way Javascript works  This was the exact reason Promises were introduced  so you could ask the interpreter to push the value to you  in the form of a callback   Though on a future version that is being worked on for JavaScript  ES2016  a couple keywords will be introduced to work pretty much as you are expecting right now  The good news is you can start writing code like this today through transpilation from ES2016 to the current widely supported version  ES5    A nice introduction to the topic is available at  https   www youtube com watch v lil4YCCXRYc  To use it right now you can transpile your code through Babel  https   babeljs io docs usage experimental   by running with --stage 1     You can also see some examples here  https   github com lukehoban ecmascript-asyncawait

User · Answer

This could be updated to ES6 with arrow functions and look like   getFeed   then data   gt  vm feed   data     If you wish to use the async function  you could also do like that   async function myFunction        vm feed   await getFeed           do whatever you need with vm feed below

User · Answer

You could provide your function with the object and its attribute  Next  do what you need to do inside the function  Finally  assign the value returned in the promise to the right place in your object  Here s an example   let myFunction   function  vm  feed        getFeed   then  data   gt            vm feed    data            myFunction vm   feed     You can also write a self-invoking function if you want

User · Answer

The then   method returns a Promise  It takes two arguments  both are callback functions for the success and failure cases of the Promise  the promise object itself doesn t give you the resolved data directly  the interface of this object only provides the data via callbacks supplied  So  you have to do this like this   getFeed   then function data    vm feed   data       The then   function returns the promise with a resolved value of the previous then   callback  allowing you the pass the value to subsequent callbacks   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

[javascript] Assign value from successful promise resolve to external variable

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