how to assign the returned value of a promise to a variable?

Question

EDITED as comment to duplicate I quote from: [How do I return the response from an asynchronous call?

Promises are containers for future values. When the promise receives the value (it is resolved) or when it is cancelled (rejected), it notifies all of its "listeners" who want to access this value.

This question is about how to return the value contained in the promise. The answer was useful to me, because it clarified that it is not possible to return the value, rather to access the value within the promise function.

Other useful sources about the subject, here:

• How do I return the response from an asynchronous call?
• http://www.html5rocks.com/en/tutorials/es6/promises/
• jQuery deferreds and promises - .then() vs .done().

Below the original question:

---

Could you please help in understanding how to get the value from a promise and differences between these two examples ?

//I have a simple ajax call like:

var fetch = function(start_node, end_node) {
var apiEndpoint = 'localhost/nodes/';
var loadurl = apiEndpoint+start_node+'/'+end_node;
return $.ajax({
    url: loadurl,
    type: 'GET',
    dataType: 'json',
    jsonpCallback: 'json'

  });

};
// Then I processed results in something like:
    var getResult = function(data) {
      // do smtg with data
      var result = {'myobject' : result_from_data}
      return result
    }

And finally I want to assign it results.

The following works, but I think it wastes the concept of the promise since result is assigned to a global variable declared before it:

var r;  
fetch('val1','val2')
.then(function(data){
  r = getResult(data);
})

Instead the following assigns the promise function to res.

var res = fetch('val1','val2')
.done(function(data){
  return getResult(data);
})

Could you clarify how to pass the resulting 'myobject' to the variable res, and not the promise itself?

I also tried:

var res = $.when(fetch('val1','val2'))
.done(function(data){
  return getResult(data);
})

but no success.

Solution

You have to use the global variable trick, or accept use save-as-a-promise trick.

var getStuff = $.when(req1,req2).then(function(data1,data2) { return data1.concat(data2); });

//the variable getStuff is now a promise and any .then chained 
//to it will have data1.concat(data2) passed to it as an argument

getStuff
  .then(function(data1Data2) {
    console.log(data1Data2);
  });

//the next time you want to use it, you have to use the same promise-interface with .then
getStuff
  .then(function(data1Data2) {
    console.log(data1Data2);
  });

Comments

[user305883]

Thank you! So the only way it to use the variable inside the promise itself.

[user305883]

Thank you! So it is possible to access the value of the promise, only inside the promise itself. A little further: @Adam, I run the getStuff .then(function(data1Data2) { console.log(data1Data2); }); in the browser console, and it returns both the value and the promise itself. (e.g.: ` Object {'myobject' : result_from_data}, Object {}` Could you explain why?

[Adam]

It doesn't return both the value and the promise itself - that statement returns the promise (which the browser console logs) and it also console.log's the data which is why you see two logs. Your two options are, as you said - use a global variable, or simply store the promise and always access it's data via a .then

[agm1984]

I do it like this: 1) Put your code inside a function that returns a promise, promiseFunction() 2) create an async function and put this inside: const varName = await promiseFunction().then((done) => { return done; }).catch((err) => { throw new Error(`RADICAL ERROR: ${err}`); });