How to return data from promise

One of the fundamental principles behind a promise is that it's handled asynchronously. This means that you cannot create a promise and then immediately use its result synchronously in your code (e.g. it's not possible to return the result of a promise from within the function that initiated the promise).

What you likely want to do instead is to return the entire promise itself. Then whatever function needs its result can call .then() on the promise, and the result will be there when the promise has been resolved.

Here is a resource from HTML5Rocks that goes over the lifecycle of a promise, and how its output is resolved asynchronously:
https://web.dev/promises/

You have to return a promise instead of a variable. So in your function just return:

return relationsManagerResource.GetParentId(nodeId)

And later resolve the returned promise. Or you can make another deferred and resolve theParentId with it.

I also don't like using a function to handle a property which has been resolved again and again in every controller and service. Seem I'm not alone :D

Don't tried to get result with a promise as a variable, of course no way. But I found and use a solution below to access to the result as a property.

Firstly, write result to a property of your service:

app.factory('your_factory',function(){
    var theParentIdResult = null;
    var factoryReturn = {  
        theParentId: theParentIdResult,
        addSiteParentId : addSiteParentId
    };
    return factoryReturn;
    function addSiteParentId(nodeId) {   
         var theParentId = 'a';
         var parentId = relationsManagerResource.GetParentId(nodeId)
             .then(function(response){                               
                 factoryReturn.theParentIdResult = response.data;
                 console.log(theParentId);  // #1
             });                    
    }        
})

Now, we just need to ensure that method addSiteParentId always be resolved before we accessed to property theParentId. We can achieve this by using some ways.

• Use resolve in router method:

  resolve: {
      parentId: function (your_factory) {
           your_factory.addSiteParentId();
      }
  }
  

then in controller and other services used in your router, just call your_factory.theParentId to get your property. Referce here for more information: http://odetocode.com/blogs/scott/archive/2014/05/20/using-resolve-in-angularjs-routes.aspx

• Use run method of app to resolve your service.

  app.run(function (your_factory) { your_factory.addSiteParentId(); })
  

• Inject it in the first controller or services of the controller. In the controller we can call all required init services. Then all remain controllers as children of main controller can be accessed to this property normally as you want.

Chose your ways depend on your context depend on scope of your variable and reading frequency of your variable.