Winjs promise return a value

Question

I need to return a value from a Winjs promise, something like this:

getString() {
        var str: string;
        new WinJS.Promise((completed, error) => { Test.getAString(completed, error); })
                         .done((s: string) => str = s);
        return str;
    }

The problem is str always returns undefined, but s has a value (ie "test") which I get in debug mode.

Thanks in advance to help me.

Answer 1

The reason why the str variable is undefined is because it is returned before the promise is fulfilled. I assume Test.getAString is asynchronous. It takes a little time before it completes. So return str; is executed before it is done.

A way to overcome this is to return the promise and, instead of handling the done-function here, handle the fulfillment when getting the string.

So you get something like:

getString().then((s:string)=> do something with the string.. );

function getString(){
    return new WinJS.Promise((completed, error) => { 
        Test.getAString(completed, error); 
    });
}