Return from a promise then

Question

I have got a javascript code like this   function justTesting       promise then function output        return output   1           var test   justTesting      I have got always an undefined value for the var test  I think that it is because the promises are not resolved yet  there is a way to return a value from a promise

User · Answer

When you return something from a then   callback  it s a bit magic  If you return a value  the next then   is called with that value  However  if you return something promise-like  the next then   waits on it  and is only called when that promise settles  succeeds fails   Source  https   web dev promises  queuing-asynchronous-actions

User · Answer

To use a promise  you have to either call a function that creates a promise or you have to create one yourself   You don t really describe what problem you re really trying to solve  but here s how you would create a promise yourself    x000D   x000D  function justTesting input    x000D      return new Promise function resolve  reject    x000D             some async operation here x000D          setTimeout function     x000D                 resolve the promise with some value x000D              resolve input   10   x000D             500   x000D          x000D    x000D   x000D  justTesting 29  then function val    x000D        you access the value from the promise here x000D     log val   x000D      x000D   x000D     display output in snippet x000D  function log x    x000D      document write x   x000D    x000D   x000D   x000D    Or  if you already have a function that returns a promise  you can use that function and return its promise    x000D   x000D     function that returns a promise x000D  function delay t    x000D    return new Promise function resolve    x000D      setTimeout function     x000D        resolve    x000D         t   x000D        x000D    x000D   x000D  function justTesting input    x000D    return delay 100  then function     x000D      return input   10  x000D        x000D    x000D   x000D  justTesting 29  then function val    x000D       you access the value from the promise here x000D    log val   x000D      x000D   x000D     display output in snippet x000D  function log x    x000D    document write x   x000D    x000D   x000D   x000D

User · Answer

I prefer to use  await  command and async functions to get rid of confusions of promises   In this case I would write an asynchronous function first  this will be used instead of the anonymous function called under  promise then  part of this question    async function SubFunction output          Call to database   returns a promise  like an Ajax call etc       const response   await axios get  GetApiHost       api some endpoint          Return      return response       and then I would call this function from main function    async function justTesting        const lv result   await SubFunction output       return lv result   1      Noting that I returned both main function and sub function to async functions here

User · Answer

You cannot return value after resolving promise  Instead call another function when promise is resolved  function justTesting         promise then function output               instead of return call another function         afterResolve output   1              function afterResolve result           do something with result    var test   justTesting

User · Answer

What I have done here is that I have returned a promise from the justTesting function  You can then get the result when the function is resolved      new answer  function justTesting       return new Promise  resolve  reject    gt        if  true          return resolve  testing          else         return reject  promise failed                 justTesting      then res   gt         let test   res          do something with the output            catch err   gt        console log err           Hope this helps      old answer  function justTesting       return promise then function output        return output   1           justTesting   then  res    gt         var test   res         do something with the output

User · Answer

Promises don t  quot return quot  values  they pass them to a callback  which you supply with  then     It s probably trying to say that you re supposed to do resolve someObject   inside the promise implementation  Then in your then code you can reference someObject to do what you want

[javascript] Return from a promise then()

Examples related to javascript

Examples related to promise